spazzyone Posted October 17, 2006 Report Share Posted October 17, 2006 Dude that is such an awsome mock up and shows your idea so clearly that even i understant it kudos for taking the time to do that Link to comment Share on other sites More sharing options...
psw Posted October 17, 2006 Author Report Share Posted October 17, 2006 Oh, so many questions, bancika/Branislav 1>>> I begain quite interested in psw's driver/pickup combo. I could dissasemble (crappy) squier neck pickup and wind both driver and pickup there. 2>>> My question is: pickup coil will be under driver coil. Does it affect pickup performace. Does magnet distance from string count or coil distance? 3>>> Also, maybe it wouldn't be a bad idea to try dual coil design as psw suggested, with coils one below another. My question is, which winding direction should I follow for both coils? 4>>> When I unwind pickup can I use same wire for winding pickup "part" of driver/pickup? Since pickup is crappy, can a wire be one of the reasons? 5>>> As I understood, I'll need additional DPST switch for turning off pickup with sustainer is active, right? So it makes: one SPST for sustainer On/Off, one DPDT for polarity swap and DPST I mentioned? 1>>> Building a driver on top of, or on the top part of a crappy pickup is to be recommended...if nothing else, you get the magnet, a neat cover to hide it under so it will look good, and a really good way of mounting it and adjusting it's height. 4>>> However, it will modifiy your pickup irreversibly and likely destroy it if any attempt to unwind it is made. Pickup wire is very thin and difficult to work with, very hard to impossible to unwind (certainly by hand) and impossible to wind back without kinking and breaking (we are probably talking about 9,000 winds). It is not easy to buy, and certainly not in small amounts should you wish to wind it new, and you wont be able to do it without some kind of pickup winder. That is one reason mine is built on top of the pickup coil as in the pictorial... PSW's Pickup/driver winding Pictorial The easiest way to get the pickup wire off, is to cut it off...but you do get tiny wire everywhere...so there goes your pickup...still if you don't mind (and another crappy pickup can be found on ebay anyway) perhaps that is the way to go for now, and forget about the neck pickup for your first go. Most have found that they have needed to have a couple of goes at winding a driver...the first one works, but they could always have done it a little better. Fortunately, the driver wire is cheap and easy to work with (I just bougth a reel of 0.2mm for A$7 locally, enough for perhaps 3 drivers) and if you use something like PVA glue for the potting, should come off alright so you can reuse everything else. Primal made his driver this way...perhaps he could share some light on it but basically you "block up" the bottom part, leaving only the 3-4mm for the driver coil on the top part... 2>>> Actually, mine got better...but that is because I didn't touch it, the pickup coil is further away now the driver coil is on top but mostly because the little poles were replaced by a substantial piece of steel through both the pickup and driver coils! But...it can be subjective...I compared my modified coil to another from the same guitar and it is a thicker sound perhaps...more body to the sound (very cheap single coil) However...if you were to take off the pickup windings, wind on a driver and get what you could back on there...if you have a winder...it would not sound good. Pickup winding is a specialised art and is far harder to do, and takes equipment and specialist supplies (like the wire, etc) and is a whole different subject. 3>>> The stacked coil concept has not been tried but I do think it is a good one...a bit trickier to do though. Basically you have two 4 ohm coils on top of one another. One is "wound" the reverse of the other (as in a humbucker) so one pushes while the other pulls in response to the signal. Now, I haven't had time to fully think this through yet...normally you would have these coils on different ends of the magnet (reverse polarity) so the effect upon the string is the same...hmmm More thought required! One saviour is that you can wind them the same way, and join the ends to achieve the same effect as if you had physically wound it backwards...or, you could connect them together so they are not reversed and effectively made a single coil with a join in the middle. That way if the stacked coil didn't turn out so good, it would still work like my single coil design... But all this is a lot of work...two coils, stepping into the unknown and all. Perhaps you should try for a simple single coil like mine, get it working then change elements as you go (drivers, circuits, etc) to make improvements and as developments come forward. 5>>> Switching is complex and depends on the guitar. A guitar with only one pickup in the bridge, just requires the circuit to be turned on (SPST) but other guitars require bypassing of the controls and all other pickups...on my strat I needed a 4PDT $witch... It also mean't a complete rewire of the instrument to get this all happening effectively... I'm beginer in electronics and utter beginer in sustainer theory, but I've been thinking a little about AGC "thing" and maybe someone can find it useful. In order to make string response more even maybe we could split whole frequency range on 6 zones and we'll assume that if tone of given frequency from zone 4 is played on 4th string. Zones could be determined statisticaly. I know that not all notes from one zone will be played on that note, but I consider that 50% hits is an improvement. With some combination of high pass/low pass filters input signals could be separated into 6 signals (one for each zone) and then set their level separately, mix them and then send to power amp. That way, we could amplify more notes that we assumed are played on thinner strings. What do you think about it?... I don't have a clue if any of this is possible, it's just how I see it And probably something like this has been discussed before. After all, this is "ideas" thread, little brainstorming can't hurt anyone. Quite right, this is a brainstorming thread... I will leave this for now and others to comment, I don't think it is a practical solution although it does remind me a little of the way some have tackled the phase problem, and probably quite complex. Btw, is 0.15mm wire ok for driver? No...sorry, I tried this and a range of wires but this is not suitable. 0.2mm is ideal and a proven formula. Other guages may not be able to transform enough power, result in too few winds for the same resistance...it may not seem that different but 1.5mm it is 25% thinner... There is also the resonance of the coil to consider also, while this might work to drive the lower strings ok, getting those higher strings working may be a problem. Thinner wire provides more resistance to the current we are putting into it so will effect the amount of power and the speed it works at...not desirable. Thicker wire is also no better (although I think I have used 0.22, maybe 0.25 ok.) Still a lot of this comes down to trial and error...I've made a lot of errors and I contribute here to help you learn from mine...hopefully. That said, I have not tried everything, and what I have tried and failed at may have been undermined by one little overlooked factor, so I do encourage experiment. Summary...In your case I would scrap the pickup and wind a driver onto the top half of the bobbin. However, the next best would be to leave the pickup alone and build the driver on top of the pickup. It would probably be a good idea to build a test coil anyway, before doing anything and holding it over the strings. All the modifications, $witches and stuff could be done later once you have proven that you have something going that you are happy with, you would be sure that the circuit is working and had a first go a coil winding. If your pickup has a ceramic magnet under it, you could use that or get some small magnets from a craft shop or something. Getting something working is not expensive...getting it into the guitar and stuff can be harder...you don't need any switches to get it working or to create the harmonic effect (just unplug the battery and reverse the driver wires (or flip the magnet ) by hand). Too many people jump right into the installation ideas and exactly how they want things to go instead of simply getting the basic thing to go. It is even possible with a couple of tries to wind a coil only 3mm thick that would fit next to the neck pickup on a strat, on top of the pickguard!!! And, you can try all kinds of varitaions if you want to get into the driver design side of things without taking your guitar apart all the time Hope that helps you, and other's thinking about doing this project...good questions... pete Link to comment Share on other sites More sharing options...
psw Posted October 17, 2006 Author Report Share Posted October 17, 2006 Dude that is such an awsome mock up and shows your idea so clearly that even i understant it kudos for taking the time to do that +1 as always... Just a quickie. I'll edit later. I know it's a picture rule violation, but it's only 25 kB Yes...as is quoting it...hahahaha Question though...so that is wire through the core? What holds the top down...this is the critical thing, especially when you push on the sides with those clamps! Otherwise, on the right track...making it of wood or whatever would make it easier to make and hold while winding! The other thing, and you may have missed the post, I have since had some reservations about the coreless paper bobbined coil machine. There is a lot of pressure in the windings, especially at the ends and I am worried that when the core is removed it may try to bend in on itself. An exoskeleton of glued paper could actually be quite strong and my concerns unwarranted...of course using epoxy would probably work ok, but there are problems with that as we know...may end up gluing the whole jigamathingie together with coil!!! hope the new job is going OK...glad to see you back... pete Link to comment Share on other sites More sharing options...
psw Posted October 17, 2006 Author Report Share Posted October 17, 2006 hello, to all me has been happened an idea that I have not seen that it has been proven, and it is that, that pasaria if a harvesting upon the one becomes of the center that has the guitar to use it with the conductor, totally independent of the circuit of the guitar? And to use it like in ebow, everything totally independent, podria to occur him around 1000 returns for the harvesting with fine thread, more or less like in ebow and using our conductors, or somebody has proven it? Yes...an ebow has the pickup and the driver separate from the guitar. On that DIY ebow, he used 1000 turns of pickup wire to make the pickup...but this is for one string. I am having a little trouble with the language translation, I'm sorry. I gather that you are suggesting separate pickup and driver for a sustainer system, independant of the guitar. I have tried it, as has col recently, and we have similar problems anyway. Part of the problem is that with the ebow the coils are very small, but with ours we are driving all the strings...so more power and bigger coils. The result is more EMI radiation and more distance between the pickup and the driver is required. The original ebow patent described a version of six pickups and drivers (six built in ebows) with an ingenious keyboard to trigger them independantly, but as far as I know it has never been tested and is not really practical. Ebow patent link The easiest way is to use the guitar's existing pickups and simply build a driver for a sustainer. Multiple drivers cause multiple problems and would still leak into the signal chain through the guitar's pickups...even the ebow itself can suffer from such problems I believe. hello, bancika The thread for the coil that this giving better result is of 0.2 mm to 0.24. Greetings. Yes...see my loooong post above! PSW, in previous post you asked me for the tin that I use to cover the conductor and to isolate EMI, or rather to lead it, I have of decirte that is the one of tins of pepsi and they are not of aluminum since they are atraidas by the magnet, so, I think that iron or steel would seran of some alloy with, I am not safe, greetings and I animate because we are obtaining it. There are lots of sources of steel if you look around that could be layered to make a core or a cover...very good ideas! pete Link to comment Share on other sites More sharing options...
onelastgoodbye Posted October 18, 2006 Report Share Posted October 18, 2006 That's (iron) wire through the core yeah. I've made a test model that uses the same dimensions as in the above animation (2 x 2 mm core, 5mm overall coil width). the bobbins are clamped by a bolt + nut, like shown somewhere in my previous posts. You just make 2 loops in the wire, slide those over the bolt and tighten with the nut. Might not seem so, but that arrangement can exert quite a bit of force. In any case, it clamps tighter than my first jig, so i wouldn't worry too much about it not being strong enough. You need a small drill bit for the holes (1mm diameter), but i don't think those are that hard to find? Now the epoxy thing, that should provided you let it fully dry. IMO, Even the 5 min stuff needs about 24 hours to properly cure. If you open the jig too soon, the coil gets damaged. I now also tape (electrician's vinyl tape or clear cling film) up the bobbin and sides to prevent the epoxy from sticking to the aluminium; It's just a slight mechanical stick, but better safe than sorry. the stick is more because of bad quality epoxy and the fact the aluminium isn't perfectly smooth. In other news, I've got my hands on some very elaborate FEA (finite element analysis) software. Sort of like femm on steroids. It can do fluid mechanics, forces, temperatures, magnetic field shape and change, EMI analysis on coils and pcb's, hysteresis losses, calculate cut-off frequency,....all in 3D from cad data. Basically, it could provide scientific proof or disproof of all our theories. Problem is...this stuf is REALLY complicated mad professor complicated. you could build a lot of drivers in the time it takes to get it to work, and as we're almost there anyway...it's probably not worth it. Kinda sad because i was looking forward to using it, but it really is over my head for now. Link to comment Share on other sites More sharing options...
curtisa Posted October 18, 2006 Report Share Posted October 18, 2006 The biggest problem so far (not just with this circuit): It works ok within a reasonable band of input levels, say 50mV to 300mV, which is plenty for us after the sustain has started to function. However, stronger signals from about 600mV up to 2.5 volts (which is something like peak for a good hot passive humbucker) are big enough so that with the VCA of the compressor at mimimum gain doesn't attenuate them - 1v - 2.5v come through strong enough that there might be issues with the system never relaxing down to the level where the compressor can do its business... Got a diagram of your alterations so we can debug it? Another more subtle but possibly more tricky issue is that of distortion of the waveform... This compressor produces a nice undistorted output with levels below the threshold, but at the point about half way between no compression and maximum attenuation, there quite a lot of asymetry and some DC offset in the signal. Problem here is that this is exactly where the equilibrium will be in our system. It will always settle at a point where there is just enough power in the driver to keep the string(s) going at the same level... somewhere between max and minimum, probably closer to minimum for all but the most stubborn string/fret combos... basically at the point of most asymetry and dc offset :-| Again, got a schematic? The sidechain of the THATcorp app note I mentioned earlier (here if you can't be bothered trying to find it again) is a pretty standard setup for opamp based feed-forward designs. I've used it myself on at least one other compressor that worked fine, and I know of at least one other high-end compressor that uses a very similar setup. Referring to page 2 figure 1, if you omit the 2252 RMS detector and just use the components based around OA2 and OA3 you should be able to easily generate a constantly varying DC voltage based on the AC input signal that is dependent on the Threshold, Compression and Makeup Gain controls (omitting the RMS detector will make the compressor a peak-detecting type rather than RMS detecting - differences are possibly not too important in our application). I simulated the above circuit earlier today and I can get it to work quite well for a variety of input voltages. Based on past experience, the real-life performance is quite good aswell. Implementing a circuit like this into the LM13700 should at least be a good starting point, though I would expect a bit of tweaking is required to get the correct operation. I still have to check out that 1176LN the gain circuit look good, but the control circuit looks a bit complicated . And the FET based circuits I've tried so far have proved difficult to hack. Yes, the detector circuit of the 1176 is waaaaay too complicated for our use, but that gain reduction element is ridiculously simple and effective - a few resistors and one FET. It's possible that you could marry the THATcorp sidechain with the 1176 gain reduction element (with a bit of fiddling of course) and have a highly effective compressor that's smaller than the LM13700 chip alone (and probably a lot kinder on battery consumption too!). Cheers, Curtis Link to comment Share on other sites More sharing options...
psw Posted October 18, 2006 Author Report Share Posted October 18, 2006 Hey there Tim In other news, I've got my hands on some very elaborate FEA (finite element analysis) software. Sort of like femm on steroids. It can do fluid mechanics, forces, temperatures, magnetic field shape and change, EMI analysis on coils and pcb's, hysteresis losses, calculate cut-off frequency,....all in 3D from cad data. Basically, it could provide scientific proof or disproof of all our theories. Problem is...this stuf is REALLY complicated mad professor complicated. you could build a lot of drivers in the time it takes to get it to work, and as we're almost there anyway...it's probably not worth it. Kinda sad because i was looking forward to using it, but it really is over my head for now. Yes...even FEMM can do a lot more, and I have seen others...it really is too difficult for us and more so interpruting the results... In the end it comes back to the real world and what we can make, not just imagine (though there is nothing wrong with that... and as we're almost there anyway... That is what I love about your posts, so optimistic!!! Yes, I think you could be right, and I wish I could prove it so, though I don't know where there is. I think you suggested we'd be there at page 150...so we must be close, right Perhaps we should set some goals... I've just been sitting here playing my guitar...yes it would be nice to have a choice of pickups, yes it would be nice to have absolutely no "fizz" in the clean harmonic mode, better battery consumption would be a big plus, and no "pop" on turn off is really getting to me...but it does work, even in this basic form, and pretty well too... Other than the "pop" (does anyone else have this problem?), the basic thin pickup/driver is a really worthwhile thing. Now...where I am seeking to go (so I'll know when I am there) is a popless mid coil pickup driver. I know we used to be shooting for the moon with no mod installations, but this will be close enough...at least for now, and for me. I don't think at the moment, or I have yet to be convinced that, admiral as it is, the compressor refinements will address this goal. The driver is still the key in all this... I think the split blade bi-lateral concept has some really interesting features. The pickup blades of the rail produce an interesting shielding that may be effective in driver only designs too. For a stand alone driver, as long as the magnetics were right, I think the stacked coils too have some appeal...it could still be made very thin. In fact, the 3mm thin coil (or a 2x1.5mm stack) are as thin as the hex designs. So...as for the overall project...we will know we are there when this becomes as easy and as failsafe as it can be...supply of components, ease of construction, certainty of circuits, clear installation and troubleshooting...now that would be a nice place for it to be... Then perhaps I'd make these things...or perhaps move on to something else...perhaps I'd just settle down, play guitar and get a life!... pete Link to comment Share on other sites More sharing options...
col Posted October 18, 2006 Report Share Posted October 18, 2006 (edited) got a schematic? I do now I've fiddled some more, and although I've not built and tested this, it works ok as a simulation. The thing I'm most unhappy about is the power efficiency - seems like it uses a LOT of juice... maybe it will be better in real life Its built from bits grabbed from elsewhere, and I don't totally understand all of it, so there may be obvious errors (and not so obvious ones) The idea when this is used in conjunction with driver and pickup is that the pot on the input amp controls the output level and the (trimmer) pot before the LM386 will set the maximum power available, I figure you would set this so that there's no clipping when you have the input pot at the highest setting you want to use... EDIT: oops.. the value for the pot on the input amp should be 100k It's possible that you could marry the THATcorp sidechain with the 1176 gain reduction element (with a bit of fiddling of course) and have a highly effective compressor that's smaller than the LM13700 chip alone (and probably a lot kinder on battery consumption too!). That sounds like a very nice idea - unfortunately I've had no luck trying to tweak FET based compressor circuits. Is that something you could hack together ? @Pete I've only tried switching the system on and off while playing since I started using the dual driver - haven't noticed any obvious pops - I'll try to make some on purpose next time I fire the thing up cheers Col Edited October 18, 2006 by col Link to comment Share on other sites More sharing options...
psw Posted October 18, 2006 Author Report Share Posted October 18, 2006 Virtual Sustainer Corp The thing I'm most unhappy about is the power efficiency - seems like it uses a LOT of juice... maybe it will be better in real life That's very funny...hahahaha It has been suggested by people who should know, and I saw recently in the hoover/osbourne patent I think...that the pop on switch off is caused by a "backlash" of current from the driver... That is why it appears that commonly used methods in stomp boxes don't seem to work on this... Now...I had thought that perhaps then a dual coil, reverse wound driver might address that in that there would be a backlash in opposite directions from each coil... Otherwise...a BTL amp might address the problem by not having the driver connected to ground directly... In the patent I think there is some kind of delay to allow this backlash to disapate...haven't worked out what is going on there...the schematic goes for 7 pages! A lot of things were put up in that patent though, but a lot may simply be ideas...there never appeared in the "real world"... pete Link to comment Share on other sites More sharing options...
curtisa Posted October 19, 2006 Report Share Posted October 19, 2006 (edited) Ok, first glance assessment is all I can give it as I don't have much time to spend on it today: So you've got your virtual earth buffer in the top left which gives our psuedo 0V supply - cool. Input buffer at bottom left has variable gain which feeds both the side chain and VCA. Input impedance is a little low for connection to a pickup (1M and 8K2 in parallel). Ditch the 8K2 series resistor on pin 5 of the LM358 and run direct into the non-inverting input. Next stage is your normal/harmonic phase inverter, feeding the VCA which is wired up pretty much like the app note, but with the gainset and linearising diode resistors scaled to reflect the lower supply voltage? In the side chain is another buffer, but it doesn't look quite right to my thinking - you've got the diodes connected outside the feedback loop of the opamp. It won't rectify small signals (less than the forward voltage drop of the 1N4148 diodes) properly like that. I'm not sure the biasing of the BC337 trasistor is right - are you trying to steer current away from the LM13700 gain control pin when the input signal exceeds the threshold? I think the way you have the transistor biasing set up it will always be conducting (base is 0.6V higner than the emitter) and conducting even more when the control signal appears - probably why you're experiencing such high power consumption? It's also possible that the transistor is being run saturated meaning that no matter how much extra control signal you apply it will never reduce the gain any further - the transistor is already conducting as much as it can - possibly why you're getting such bad compression performance for high input signals. Also, I'm assuming the variable resistor "R" is your threshold control, but it controls the strength of the signal entering the VCA and the sidechain - as you wind down the threshold you're also reducing the signal that the VCA sees. Threshold should only affect the sidechain signal - the signal entering the VCA needs to be uneffected by this control. Have another look at the THATcorp appnote to see how they implement the Threshold control into the side chain, and also how they rectify the input signal for the control voltage. That's just about as simple and effective as it gets. Cheers, Curtis. Edited October 19, 2006 by curtisa Link to comment Share on other sites More sharing options...
col Posted October 19, 2006 Report Share Posted October 19, 2006 Ok, first glance assessment is all I can give it as I don't have much time to spend on it today: So you've got your virtual earth buffer in the top left which gives our psuedo 0V supply - cool. Input buffer at bottom left has variable gain which feeds both the side chain and VCA. Input impedance is a little low for connection to a pickup (1M and 8K2 in parallel). Ditch the 8K2 series resistor on pin 5 of the LM358 and run direct into the non-inverting input. I'll try that. I'm not sure the biasing of the BC337 trasistor is right - are you trying to steer current away from the LM13700 gain control pin when the input signal exceeds the threshold? I think the way you have the transistor biasing set up it will always be conducting (base is 0.6V higner than the emitter) and conducting even more when the control signal appears - probably why you're experiencing such high power consumption? It's also possible that the transistor is being run saturated meaning that no matter how much extra control signal you apply it will never reduce the gain any further - the transistor is already conducting as much as it can - possibly why you're getting such bad compression performance for high input signals. You're probably right about the transistor biasing, I lifted that and the diodes from an old circuit in a book, but I had to use different transistors and diodes, so I just fiddled about with the values until I got a response. I don't think its related to the power drain, that seems to be shared out between the op amps - it may just be inaccuracy in the models... I switched to different op-amp models because I discovered they weren't clipping on high signals, and that sorted out the 'poor compression at high levels' issue, but not the power drain. I will stry swaping in the That corp. rectifier circuit to see what happens. Also, I'm assuming the variable resistor "R" is your threshold control, but it controls the strength of the signal entering the VCA and the sidechain - as you wind down the threshold you're also reducing the signal that the VCA sees. Threshold should only affect the sidechain signal - the signal entering the VCA needs to be uneffected by this control. The way I have the threshold setup is chosen for the way I think the sustainer should function... When the gain of the input amp is increased, that lowers the effective threshold, and at the same time amplifies the output signal. This means that (ideally) no matter where the threshold is set, any signal below that threshold will be drivven driven equally hard until the threshold is reached... If this was not the case, then reducing the threshold would cause the output level to be lower meaning that the sustainer would use less force with a low threshold... remember that this is not about making a compressor, it's about driving strings quickly to a desired level using feedback... we should use the threshold to tell the system when to stop pushing, not how hard to push (hmm, the 'how hard to push' knob might be useful as a response control... to change the speed of the 'swell' effects) In simple terms you tell the circuit what the maximum force it can use is with the output (trim)pot. you tell it what output level you want the sustain to be at using the input pot. any time the signal is below the desired sustain level, the system ideally applies maximum force until the desired sustain level is reached, then eases off applying just enough force to keep the output at that level. Have another look at the THATcorp appnote to see how they implement the Threshold control into the side chain, and also how they rectify the input signal for the control voltage. That's just about as simple and effective as it gets. Will the compression setting pot/amp from the That circuit work if I just lift it from their circuit along with the rectifier, or is it dependant on other components, e.g. gain control? Also, am I correct in my assumption that I can just ditch their threshold control and use an input amp as in my circuit ? (I guess that depends on the threshold detector and ratio control combo having linear response relative to input level) Anyhow, I'll try out some combinations Thanks Col Link to comment Share on other sites More sharing options...
bancika Posted October 19, 2006 Report Share Posted October 19, 2006 Yo! Another thought. What about using relays for switching? With three relays and only one DPDT on/off/on switch one could make switching sustainer on and off, phase switching and if you add another relay pickup switching (provided neck pickup exists). One pole would control DPDT relay for phase and second pole for other delay (either SPST or better) that will shut battery supply down when switch is in middle position. Pickup relay would be added in parallel with that relay. Relays aren't much more expensive than switches, but it's neat to have as few switches sticking out of pickuard as possible (at least to me) What do you think? Cheers PS Sorry if it has been discussed before Link to comment Share on other sites More sharing options...
Primal Posted October 19, 2006 Report Share Posted October 19, 2006 I think its been discussed before, and the result was that relays eat too much power when in use. 9v batteries already have a short enough lifespan, no need to make it even shorter! Link to comment Share on other sites More sharing options...
psw Posted October 19, 2006 Author Report Share Posted October 19, 2006 (edited) OK...I think I've got the idea that the C & C (col and curtis) Music Factory are trying to grapple with...tell me if I am right, cause I think a few readers eyes are glazing over when they see the schematic...I know mine do... ------- First...let's illustrate the problem... I have oft complained about the set up on my POS guitar...It has had a pretty low action and 10 gauge strings...but the frets and neck are not really built for that, and it had got itself so low lately that that it was fretting out on some of the high strings and bends... This occured in part because I leave the thing leaning on the desk here for instant play...and it is a POS guitar... Anyway...so I finally decided to do something about it with my limited tools...taking a screwdriver I adjusted the neck tilt to give me a higher action, and retuned...no problem... The sustainer had been giving a fairly even response but careful listeners will have heard whith a lot of sustain there is string rattle (that at times can be appealing) as the vibrating string crashes into the frets... Now...the thing rings true but it has accentuated the problem that these guys are trying to address... A higher action means that when a string is pushed to a fret, the string is also pushed closer to the driver. Higher fretted notes are pushed very close, while lower frets, not much closer at all. -------- A compressor will amplify quieter sounding notes up to a given level and perhaps squash notes above a level (limiting) so as all notes come out the same, or similar level...sounds like what we need, and there are plenty of compressor circuits about...however... What a compressor does with a sustainer is lift the signal up to a certain level and no more...it senses it's own output and says, that's enough. What we need is something that is sensing what the string is actually doing, not what it has done to it. It is listening to the effect it is having on the signal, not the effect on the actual string's vibration...that is what we are looking for in this application. Hence...remember that this is not about making a compressor, it's about driving strings quickly to a desired level using feedback... we should use the threshold to tell the system when to stop pushing, not how hard to push So...we want a compressor that is going to adjust the level of the drive signal in response to what the string is actually doing and so compensating for things like the distance from the driver caused by fretting notes at different points along a string, thickness and ease of drive of a string, tightness of a string, picking strength, etc... We don't want it to make all drive even (as a compressor seeks to do) as some strings (for various reasons) require an extra push! Is that the idea? Maybe someone would like to clarify the intention some more, or put it more succinctly... Taking it back to the basic intentions may enable more to contribute and spur other ideas as to how we can obtain the effect. Now that my guitar is set up with a higher action, the effect is a lot more noticable than it's previously flat action. Such a circuit would be able to compensate for any action and address the drive issues for different string types, high string response and probably the differences that occur with driving the higher harmonics... So...fortunately col has been persistant in his assessment of the situation. In a typical compressor application (eg a stompbox) where we are dealing only with the signal (not a physical string in the feedback chain) it would make little difference...but such is the enigma of the sustainer that it is only here that we need deal with these issues...we are dealing not with what the circuit sounds like, but what it is actually doing to the string. As a result there are not going to be ready made compressor circuits that will do this, nor will people who are not engaged with the sustainer readily understand the difference... Good on you both...with a little more thought about this, perhaps I and others could bring something to the table... pete Edited October 19, 2006 by psw Link to comment Share on other sites More sharing options...
col Posted October 19, 2006 Report Share Posted October 19, 2006 A compressor will amplify quieter sounding notes up to a given level and perhaps squash notes above a level (limiting) so as all notes come out the same, or similar level...sounds like what we need, and there are plenty of compressor circuits about...however... What a compressor does with a sustainer is lift the signal up to a certain level and no more...it senses it's own output and says, that's enough. What we need is something that is sensing what the string is actually doing, not what it has done to it. It is listening to the effect it is having on the signal, not the effect on the actual string's vibration...that is what we are looking for in this application. Hence...remember that this is not about making a compressor, it's about driving strings quickly to a desired level using feedback... we should use the threshold to tell the system when to stop pushing, not how hard to push So...we want a compressor that is going to adjust the level of the drive signal in response to what the string is actually doing and so compensating for things like the distance from the driver caused by fretting notes at different points along a string, thickness and ease of drive of a string, tightness of a string, picking strength, etc... We don't want it to make all drive even (as a compressor seeks to do) as some strings (for various reasons) require an extra push! Is that the idea? Maybe someone would like to clarify the intention some more, or put it more succinctly... I think you're getting it... As Curtis pointed out, there are two ways to set up the control circuit in a compressor - one is to get the compressor to try to make the signal louder or quieter depending on its own output, the other is to use its input signal as the deciding factor... (I was re-inventing the wheel describing the second version as an approach for the sustainer). In the sustainer system, the input to the circuit comes from the ...driver->string->pickup, so if the input is used to decide whether to add more or less drive, it will automatically adjust for any variations caused by string, fret, pickup etc. If a normal 'stompbox' guitar compressor that uses its own output signal as a control is used, the level is evened out _before_ the variations caused by strings and frets are added to the mixture... so they remain :-| My quote about "tell(ing) the system when to stop pushing, not how hard to push" is a different issue... With a compressor, when you reduce the threshold, the output level drops to that threshold. The problem here is that this would lower the level of drive signal. My thought on this (simplified and in an ideal world) is that if we let the threshold decide when to apply drive and when not to, and control the ammount of drive seperately, then you would have much more control over the response of the system. (still don't have it working yet though At this stage I have a circuit that kind of works (hehe sounds familiar) one problem is that if the threshold is set high, then it will be less responsive to low level signals (if the threshold is set fairly low, then low level signals get plenty of amplification) - maybe some overdrive can help here... In some ways, this is similar to your idea of using a 'Switch' that senses the level and switches the drive on or off, but it has a smooth transition between on and off so it can find a natural point of equilibrium where just enough power is being applied to drive the string(s) at the threshold level. I've been trying some of the ideas that curtis suggested about using bits of different circuits, but I fear I've reached the limits of my electronics knowledge and I'm having no luck. Maybe there will be a brainwave soon @curtis, you're right about the transistor being used to 'suck' current away from the LM13700 gain control line - how does that work ? The 2k resistor in the bias network is the one I changed, it was originally 27k, but i found that the circuit reponded more quickly to amplitude changes with the 2k in there... I guess I'll need to find another way... cheers Col Link to comment Share on other sites More sharing options...
psw Posted October 20, 2006 Author Report Share Posted October 20, 2006 So...we want a compressor that is going to adjust the level of the drive signal in response to what the string is actually doing and so compensating for things like the distance from the driver caused by fretting notes at different points along a string, thickness and ease of drive of a string, tightness of a string, picking strength, etc... We don't want it to make all drive even (as a compressor seeks to do) as some strings (for various reasons) require an extra push! Ok...so following on from this, could we use a compressor circuit with a dual op amp. One set up as a buffer or adjustable preamp, the other as a standard compressor that varies the adjustment, not by the effect on it's own adjusted output, but the unaltered signal of the first? Does this make any sense, or am I off the track with this? Relays aren't much more expensive than switches, but it's neat to have as few switches sticking out of pickuard as possible (at least to me) What do you think? Well, they were discussed briefly but as primal says, they take power to operate that we badly need to run the device. A better way that would achieve what you want is some kind of digital switching (think flipflop logic devices) that switch in response to tiny control voltages. Of course, then your circuitry gets more complex bigger and expensive, even as you save on the cost of switching. If you have ever seen the tiny momentary switches available (and very cheaply too) you would see the potential here. While push button switches on guitars have never been popular, on the sustainer I think they do have a real application. Such control would allow the activation of the sustainer mode easily with the simple push of a button (perhaps on the side of the driver itself) to activate the sustainer and the harmonic modes quickly and easily, and on the fly for single sustained notes or occasional effects or passages. Being able to control the device in such a way may help the device musically and change a "sustainer guitar" into a "guitar with a sustainer option". Using the device only as required would also mean lower battery consumption if it were only used intermitantly. It could promote less gratuitious use of the device and so more musicality... Relay's won't cut it, in part because of the power but also because of size and the fact that they too use electromagnetic coils to operate, allowing the possibility of switching noise leaking into the pickups and the guitars signal. Another approach that we did explore extensively was the idea of a "sustain box" that mounted like a tailpiece behind the bridge that contained all the circuitry, battery and controls... Tim/onelastgoodbye made and realized this superb design. However, I was unable to create a device sufficiently independant of the guitar's electronics to make this really practical, though it still could be done with some ingenuity I guess. This idea avoids the problem of finding space in the guitar, battery replacement, tweaking and switch hole drilling...but the number of wires required to operate pickup bypassing and such makes it a little impractical. What may be possible is that if an effective and compact mid-driver could be made, bypassing would not be an issue and it would be more a case of getting a signal from the guitar and a signal to the driver with minimum interferance. Switching would be easier and more compact too if we only had to concern ourselves with turning the thing on and reversing the phase. So...again, I am aiming for a mid-driver and am seeking to incorporate within it such functions as LED indicators and power amplification to minimise the risk of noise coming out of such a box (or internal circuitry in the control cavity)...and maybe even on-driver switching in the future. The possibilities are of no modifications to suitable two pickup guitars (or the loss of the mid-pickup) other than to obtain a signal and to send a signal to the driver. There is no way of avoiding modifications though, this will change a guitar, and it could be questioned that given that, one shouldn't settle for half measures and simply drill that hole Col has a switch pot though that may do the job with only the loss of a tone control...often expendable... An interesting question to explore, none the less... pete Link to comment Share on other sites More sharing options...
psw Posted October 20, 2006 Author Report Share Posted October 20, 2006 A compressor will amplify quieter sounding notes up to a given level and perhaps squash notes above a level (limiting) so as all notes come out the same, or similar level...sounds like what we need, and there are plenty of compressor circuits about...however... What a compressor does with a sustainer is lift the signal up to a certain level and no more...it senses it's own output and says, that's enough. What we need is something that is sensing what the string is actually doing, not what it has done to it. It is listening to the effect it is having on the signal, not the effect on the actual string's vibration...that is what we are looking for in this application. Is that the idea? Maybe someone would like to clarify the intention some more, or put it more succinctly... I think you're getting it... As Curtis pointed out, there are two ways to set up the control circuit in a compressor - one is to get the compressor to try to make the signal louder or quieter depending on its own output, the other is to use its input signal as the deciding factor... (I was re-inventing the wheel describing the second version as an approach for the sustainer). In the sustainer system, the input to the circuit comes from the ...driver->string->pickup, so if the input is used to decide whether to add more or less drive, it will automatically adjust for any variations caused by string, fret, pickup etc. If a normal 'stompbox' guitar compressor that uses its own output signal as a control is used, the level is evened out _before_ the variations caused by strings and frets are added to the mixture... so they remain In some ways, this is similar to your idea of using a 'Switch' that senses the level and switches the drive on or off, but it has a smooth transition between on and off so it can find a natural point of equilibrium where just enough power is being applied to drive the string(s) at the threshold level. Yes..ok, less of a switch but a variable resistor (gain contol) electronically triggered by the input of the signal...hmmm so...here's a simple compressor (the second circuit)...as an example and for reference... Would it be possible to take the adjustable feedback loop from another buffer of the signal by using a dual op amp? Would this create something of the effect we are seeking in a fairly simple way? If so, could an adjustment to this other control signal (buffers) gain provide control of the threshold effectively? If not, carry on...I am still vaguely getting it... pete Link to comment Share on other sites More sharing options...
curtisa Posted October 20, 2006 Report Share Posted October 20, 2006 Is that the idea? Maybe someone would like to clarify the intention some more, or put it more succinctly... I think you're getting it... As Curtis pointed out, there are two ways to set up the control circuit in a compressor - one is to get the compressor to try to make the signal louder or quieter depending on its own output, the other is to use its input signal as the deciding factor... (I was re-inventing the wheel describing the second version as an approach for the sustainer). In the sustainer system, the input to the circuit comes from the ...driver->string->pickup, so if the input is used to decide whether to add more or less drive, it will automatically adjust for any variations caused by string, fret, pickup etc. If a normal 'stompbox' guitar compressor that uses its own output signal as a control is used, the level is evened out _before_ the variations caused by strings and frets are added to the mixture... so they remain :-| I think there's still some confusion as to how exactly a compressor works and the difference between a comp and a limiter. A compressor does not amplify anything. Signals below the threshold do not get extra gain. All a compressor can do is turn the volume down once the input signal gets too loud. By how much the compressor turns the volume down by determines whether it behaves as a compressor or limiter. Here's a really poxy analogy Imagine the input signal as a row of trees and the compressor as a dude with a chainsaw. The chainsaw dude wants to take the tops off the trees to make them shorter so he lines all the trees up and lops the top off them - some of them are still a little taller than others, but they've all been trimmed a bit. Some of the shorter trees didn't need any trimming and he's left them alone. So we're left with a row of trees that are all a wee bit shorter, roughly the same height, but there's still some height variance in them. That's all a compressor is doing. However, all the trees are now shorter and we now want them brought back to the same height they used to be because we can see too much of the next door neighbours property (ya, it's a **** analogy). We then put the trees on an elevated platform and jack them all back up to the height they used to be at. That's the function of the makeup gain control, and it happens after the compressor. It can give the impression that there's some special amplification happening below the threshold, but it's entirely separate and unrelated to the act of compressing. Limiting is just taking the row of trees and lopping the tops off all of them so that they're all exactly equal with little to no deviation in height. My quote about "tell(ing) the system when to stop pushing, not how hard to push" is a different issue... With a compressor, when you reduce the threshold, the output level drops to that threshold. The problem here is that this would lower the level of drive signal. My thought on this (simplified and in an ideal world) is that if we let the threshold decide when to apply drive and when not to, and control the ammount of drive seperately, then you would have much more control over the response of the system. (still don't have it working yet though At this stage I have a circuit that kind of works (hehe sounds familiar) one problem is that if the threshold is set high, then it will be less responsive to low level signals (if the threshold is set fairly low, then low level signals get plenty of amplification) - maybe some overdrive can help here... Here's my thoughts - the feed forward compressor that Col's putting forward is the way to go. The trick is going to be making the compressor more or less responsive to particular frequencies and amplitudes (string size does play a part, but in the absence of being able to make a driver per string we have to ignore it and focus on frequency and amplitude - the sustainer is a monophonic device being asked to work on 6 different signal sources at once). I think we need to get a reliable simple feed forward compressor going, and then we need to tweak the behaviour of it somehow. My thinking is that we want a low threshold, but high(ish) gain before the compressor. The sidechain signal needs to be treated in some way so that it generates a smaller control voltage for higher frequencies so that the compressor "ignores" the higher strings and focuses more on turning down the drive to the thicker strings (it's the skinny bits of steel that need more drive afterall). In order to do this I reckon we need some frequency shaping network in the sidechain that removes a lot of high frequency content before it gets a chance to affect the compressor. We may also need some additional post-compressor frequency carving going on so that the lower strings are naturally driven less anyway. Here's the (image-free, sadly) signal flow I'm proposing: Guitar pickup -> Buffer (with extra gain perhaps) - > VCA -> High pass filter -> Fixed output volume (or trimmer - set and forget) -> driver amp (LM386 or other?) -> Driver. At the point where we tap off the sidechain signal, between the buffer and VCA above: Buffer -> Low pass filter -> Precision rectifier and threshold control (similar, but simpler than the THATcorp appnote) -> DC smooting -> VCA gain control pin. You could probably realise the above with a quad-opamp chip, VCA element (be it FET, LM13700 or otherwise), an LM386, and a handfull of resistors, caps and diodes. Should be buildable on a 2" square PCB (or a couple of stacked boards if space is at a premium). @curtis, you're right about the transistor being used to 'suck' current away from the LM13700 gain control line - how does that work ? In your diagram, the gain control pin is normally tied to the 9V supply via that series resistor, which sets a fixed gain as determined by the current flow into the input pin. You've placed the transistor in parallel with the gain control pin. So with the transistor switched off the LM13700 gets all the gain current, and as the transistor switches on more it steers more current away from the LM13700 and into the collector of the transistor. You should only need to feed the DC control voltage directly into the LM13700 gain pin via a resistor to get it to behave properly - no need for fancy transistor switching I wouldn't think. The 2k resistor in the bias network is the one I changed, it was originally 27k, but i found that the circuit reponded more quickly to amplitude changes with the 2k in there... I guess I'll need to find another way... That's probably more to do with the capacitor you have in there (1uF). You've formed a lowpass filter between the 1uF cap and the 2K base resistor. By changing the base resistor you've changed the cutoff freq of the filter, making it more or less responsive to high freq's, thus making the circuit quicker or slower to respond to changes at it's input. Also changing the base resistor changes how much current gain is reflected through to the transistor's collector - very roughly, current in the collector is base current Ib x hfe (the transistor current gain). Making the resistor smaller makes Ib bigger, which makes collector current bigger, but only to a point at which you reach saturation. Delving into the dim, dark recesses of my memory, having a bigger IB will probably give you a faster switching speed too. Cheers, Curtis. Link to comment Share on other sites More sharing options...
psw Posted October 20, 2006 Author Report Share Posted October 20, 2006 However, all the trees are now shorter and we now want them brought back to the same height they used to be because we can see too much of the next door neighbours property (ya, it's a **** analogy). I don't know, I quite liked it...is this thread starting to get more entertaining? A compressor does not amplify anything. Signals below the threshold do not get extra gain... That's the function of the makeup gain control, and it happens after the compressor. It can give the impression that there's some special amplification happening below the threshold, but it's entirely separate and unrelated to the act of compressing. Well...yep, I get that, but I kind of took the compression and susequent need for a make up gain as part of the process... Here's my thoughts - the feed forward compressor that Col's putting forward is the way to go. The trick is going to be making the compressor more or less responsive to particular frequencies and amplitudes (string size does play a part, but in the absence of being able to make a driver per string we have to ignore it and focus on frequency and amplitude - the sustainer is a monophonic device being asked to work on 6 different signal sources at once). Ah...now I am with you, but have always sought a different approach that may be simpler and more effective than an electronics solution...DRIVER DESIGN Take for instance the thin core driver design that I use. It is amplified by a treble biased amplifier (100uF output cap) and a somewhat overdriven clipped and powerful preamp with filtering above and below the audio spectrum. OK...simple 3mm steel core. But, a wider core (see the sustainiac device) would more effectively drive the lower strings that physically vibrate slower and with longer distances between nodes. So, we could afford to make it wider, it is operating below efficiency). The high strings are responding because the core is thin enough not to span between the nodes (thereby pushing and pulling the up and down string and cancelling the vibrations out)...or so my theory goes, but still enough mass to allow electromagnetic forces to be effectively transfered to the string (less core mass would result in less power)... So, the intention of the thin core is both to allow the vibration of the higher strings, and provide some balance by less effiecency in the lower strings. The bilateral driver has a similar appeal to me...possibly stereo driving with channels with treble bias and increased power for the high strings...perhaps, different windings (more, less, thinner, thicker) to bring out different responses...core width, magnet strength, adjustable screw poles...that kind of thing... Then there is the Hex designs I worked on...six individual electromagnetic elements driven by a monophonic source but each tailored to the strings needs...of course, perhaps this was going too far... Ok...so the driver design can influence the performance from string to string...at least potentially...with out resorting to compensation circuitry that appears to be getting increasingly complicated...(nothing wrong with that mind...) So...can you extend you analogy or mix some metaphors to explain why we need to take the reference for the compressor from the input, not the output, as is commonly the case? Could we not simply amplify the signal, uncompressed and take that as the reference for the compressing action of a conventional simple compressor as I have described, to feed the drive amp? Maybe that control signal amp could be filtered to amplify less of the high frequencies and so not trigger the compression (reduction of gain) so much in that range. Given the control reference signal is not heard but is only for that purpose, feeding low frequencies through a negative loop should cancel/compress those frequencies without effecting the drive siganl itself adversely. Anyway...that is my thoughts, but perhaps I am barking up the wrong tree...so to speak!!! Thanks curtis... pete All that said, I just picked up the guitar then, and it is struggling to sustain notes below c on the high e and is noticably more powerful above the 12th fret on all strings. I can't bring the driver up or the strings will hit it in the highest positions...bring the action down fixes it, but then the strings crash the frets a little. In addition to driver design, the mid driver would have less of this fretting deviation, and a bridge positioned driver (once a dream) none at all. Perhaps, the multi driver guitar is a solution...you want more consistant drive on all frets, use the neck pickup and drive from a driver built on the bridge pickup...I sense a switching nightmare there however...hmmm Link to comment Share on other sites More sharing options...
col Posted October 20, 2006 Report Share Posted October 20, 2006 Well...yep, I get that, but I kind of took the compression and susequent need for a make up gain as part of the process... Yeah, I've been talking mostly about a compressor as a 'black box' device... technically, I suppose the majority of them use a limiter followed by a gain stage, but whats really important is the overall effect - output/input... (although I've been a little guitly of mixing and matching 'black box' definition with internal details, so apologies for that) Col Link to comment Share on other sites More sharing options...
col Posted October 20, 2006 Report Share Posted October 20, 2006 In your diagram, the gain control pin is normally tied to the 9V supply via that series resistor, which sets a fixed gain as determined by the current flow into the input pin. You've placed the transistor in parallel with the gain control pin. So with the transistor switched off the LM13700 gets all the gain current, and as the transistor switches on more it steers more current away from the LM13700 and into the collector of the transistor. You should only need to feed the DC control voltage directly into the LM13700 gain pin via a resistor to get it to behave properly - no need for fancy transistor switching I wouldn't think. The transistor is there to invert the dc signal from the rectifier. Using variations on the 'super diode' precision rectifier, I have managed to get a reasonably steady dc signal that drops with a higher input and rises with a lower one... The problem is that it is relative to 4.5V, so sits roughly in the range 3v - 4.2v depending on the input level. The circuit I ripped from the book AC couples the signal prior to the passive rectifier, according to the description in the book, it then uses the transistor as a dc inverter... side stepping the issues I'm having with a virtual ground relative control signal... Aside from possible issues that you have highlighted, the transistor circuit provides a control signal relative to 0V which _works_ with the LM13700. How can I get around this and still use a rectifier similar to the THATcorp one? That's probably more to do with the capacitor you have in there (1uF). You've formed a lowpass filter between the 1uF cap and the 2K base resistor. By changing the base resistor you've changed the cutoff freq of the filter, making it more or less responsive to high freq's, thus making the circuit quicker or slower to respond to changes at it's input. Also changing the base resistor changes how much current gain is reflected through to the transistor's collector - very roughly, current in the collector is base current Ib x hfe (the transistor current gain). Making the resistor smaller makes Ib bigger, which makes collector current bigger, but only to a point at which you reach saturation. Delving into the dim, dark recesses of my memory, having a bigger IB will probably give you a faster switching speed too. heh, the first thing I did was tweak all the caps, then when it still wasn't enough, I fiddled that resistor... fwiw, that circuit works reasonably well in simulation - the unexpected current drain seems mostly to be coming from the dual op-amps, both are drawing abour 10mA which is more than I expected. cheers Col Link to comment Share on other sites More sharing options...
col Posted October 20, 2006 Report Share Posted October 20, 2006 (edited) ... The circuit I ripped from the book AC couples the signal prior to the passive rectifier, according to the description in the book, it then uses the transistor as a dc inverter... side stepping the issues I'm having with a virtual ground relative control signal... Aside from possible issues that you have highlighted, the transistor circuit provides a control signal relative to 0V which _works_ with the LM13700. How can I get around this and still use a rectifier similar to the THATcorp one? (I know - replying to ones own posts is an early sign of madness) I've managed to scrap the transistor - I just turned around the diodes to invert the rectified signal filtered it with a cap to ground and tweaked the line to the LM13700 with a resistor... (it even seems to work without a buffer - just take a line from the input amp, stick it through an AC coupling cap, 2 diodes, a filter cap and a resistor, then into the LM13700... So what about that 'precision rectifier' ? Would using a op-amp feedback style rectifier really be a big benefit for us? If so, how do we get that to work here? My existing setup involves generating a -ve dc signal so that a stronger input to the sidechain can reduce the current into the VCA control pin... How can we do that with the fancy rectifier? Col Edited October 20, 2006 by col Link to comment Share on other sites More sharing options...
curtisa Posted October 21, 2006 Report Share Posted October 21, 2006 I don't know, I quite liked it...is this thread starting to get more entertaining? Call it a moment of madness... So what about that 'precision rectifier' ? Would using a op-amp feedback style rectifier really be a big benefit for us? If so, how do we get that to work here? The reason why we want to use the precision rectifier is because the diodes have a forward voltage which must be overcome before they start conducting. With a 1N4148 the forward voltage drop is in the vicinity of 0.6 - 1V depending on the current flowing at the time. The problem with that is that the normal signal voltage generated by the pickup is anything from a few mV to a volt or so - some of those signals are obviously too small to overcome the fwd voltage drop of the diode, so for small input signals you generate no control voltage, causing unpredictable compression action. Even if you add gain to the circuit before the diodes you still are left with at least some signals that will have trouble overcoming the fwd V drop, and you're also starting to run out of headroom in the rectifier circuit, limiting the upper range of input signals you can rectify. With the precision rectifier, and the diodes sitting inside the feedback loop of the opamp, the fwd V drop of the diode is negated by the action of negative feedback, and you can accurately rectify input signals much lower than the normal fwd V drop of the diode - practically to 0V input. My existing setup involves generating a -ve dc signal so that a stronger input to the sidechain can reduce the current into the VCA control pin... How can we do that with the fancy rectifier So, am I right in assuming that the LM13700 VCA reduces gain as the gain control input current falls (I haven't tried fiddling too much with the circuit yet)? So we want to generate a control voltage that gets progressivly bigger as the input signal exceeds the threshold further, yeah? The easiset way I can see to do it is to have a static DC voltage applied to the gain control pin, and subtract the control voltage from it - that's just an opamp stage with a reference voltage applied to the non-inverting input (call it "VCA normal gain" control - could even be a variable resistor) and the control voltage applied to the inverting input. The output of that opamp becomes the control voltage for the VCA. I know - replying to ones own posts is an early sign of madness Hehe...some people would argue that trying to build a sustainer and writing 132 pages of concepts and ideas is madness BTW Col, what software are you using for your circuit simulations? Cheers, Curtis Link to comment Share on other sites More sharing options...
curtisa Posted October 21, 2006 Report Share Posted October 21, 2006 Ok, so I've just been fiddling with my (still handheld at this stage) pickup/driver sustainer, and I was running it through a limiter that was built from a kit published in an Australian electronics magazine (Silicon Chip if anyone is interested). The kit is sadly out of production now, but it featured the SSM2018 VCA chip at it's heart and also a very similar precision rectifier circuit as the THATcorp appnote. And it's a feed-forward design. AND it makes a helluva difference! I can get plenty of sustain on every string in just about every position. What's more is I can get the same performance at a greater distance from the strings than just using the plain old LM386 circuit, which means we don't necessarily need to have super-low action anymore. For some reason I'm getting better low string performance too, not sure why. Beforehand I could only go down to the note "E" on the 5th string/7th fret before it started sustaining harmonically instead of fundamentally. Now I can get fundamental sustain almost all the way down to low "G", 6th string, 3rd fret. Could be that I'm now no longer pushing the LM386 into clipping and accentuating the upper harmonics of the note? The limiter I'm using isn't really adaptable into a built-into-a-guitar arrangement though - power supply requires +/-12VDC, and the PCB board it's made up on is about 3" square, but as proof of concept it works extremely well. My next trick will be trying to tailor the response of the sidechain to emphasise the high-note drive. Good progress, we're on to something here! So...can you extend you analogy or mix some metaphors to explain why we need to take the reference for the compressor from the input, not the output, as is commonly the case? Dunno if I can do it using the trees/chainsaw story With feedback compression (eg, Col's original schematic, the Aussie comp etc) the output of the VCA is the signal that tells the compressor to start turning down the volume, so effectively the compressor doesn't care what's happening at its input, it just "blindly" goes about making it's output quieter everytime the threshold is exceeded. With feed-forward compression the input signal is what is used to tell the VCA to turn it's output down. Might seem like the end result is the same - signal exceeds a threshold, output turns down - but the difference in our situation is that there's another feedback loop outside the compressor: the driver, the string and the pickup. The compressor needs to know if the signal it's pumping to the driver is sufficient to maintain the sustaining action, and the way it does that is to "listen" to it's input. Feedback compression can't do that. Ok, thought of an analogy (possibly quite crap too!): Imagine a guy filling a bucket of water from a tap. He needs to turn the tap off when the bucket is completely full to the top, so he puts his hands around the sides of the bucket and waits until he can feel water trickling over the sides of the bucket to signal that he needs to turn the tap off. He's being reactive - that's feedback compression. Imagine the same guy filling up the bucket of water, but this time he's watching the water rise slowly as it fills. When he sees the water lapping at the rim of the bucket he knows it's time to turn the tap off. He can anticipate what the water will do as it nears the top. as he can see what it's doing. He's being proactive - that's feed-forward compression. ...And they lived happily ever after Cheers, Curtis. Link to comment Share on other sites More sharing options...
psw Posted October 21, 2006 Author Report Share Posted October 21, 2006 Ok, so I've just been fiddling with my (still handheld at this stage) pickup/driver sustainer, and I was running it through a limiter that was built from a kit published in an Australian electronics magazine (Silicon Chip if anyone is interested). The kit is sadly out of production now, but it featured the SSM2018 VCA chip at it's heart and also a very similar precision rectifier circuit as the THATcorp appnote. Hey...that sounds like the one I built...is it a 4 knob compressor/limiter? DSE and others used to sell it as a kit, right? I built mine and included a LM386 module so I could optionally use the driver direct from it...same idea, proof of concept... I am not sure if I ever tried it on conventional drivers, the above photo shows it being used on one element of a hex driver. It could be that it is simply a better preamp! Maybe I got to get the thing back and do a bit more testing...still a little big to get into the guitar...hahaha It's feed forward though, anything in the circuit schematics of use, or does it rely on this chip? Dunno if I can do it using the trees/chainsaw story Oh...I was at least thinking along the lines of watching the grass grow and waiting the time to mow!...or at perhaps a hole in the bucket, so as not to rely on the quantity of the water going into the bucket, but when it is actually full Hehe...some people would argue that trying to build a sustainer and writing 132 pages of concepts and ideas is madness laugh.gif Oh no, it's worse than that, there are so many things I have nievely tried that have never been documented...kind of a little sad really...lets just call it a distraction from other sh!t and leave it at that... Still...as real life is on hold this year...I was thinking today that I might put up a few of those crazy experiments that seemed like a good idea at the time...might be a little entertaining...coming soon I think... How about setting a magnetic signal down the string by passing them through a coil...or using a hall effect sensor as pickup... Meanwhile...holding the pickup above the strings is ok, but it is hard to guage the EMI problem as it is spraying EMI towards the guitar whereas when in the guitar it is level with the pickup... With this preamp, are you able to get the device closer to the pickup do you think? Has anyone got any improvements to the driver design? I think there may still be some work to be done there, if not the design, the construction at least! Perhaps there is a way to make it more efficient. I have another variation on the split blade bilateral design I posted earlier, half bladed, half adjustable poles!...and I think a stacked coil may also hold some posibilities, especially as a stand alone unit with some kind of internal magnets, more thought required there still... So...back to work again, very early start... see ya... pete Link to comment Share on other sites More sharing options...
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