JohnH

Thanks to you too. Ive got this built into my LP now, on a 0.7" x 0.7" scrap of Veroboard, tucked into a space behind the output jack. Its trouble free, it just sits there and does its job. John

oatmeal. thanks for your interest. The tone/crunch pot was a 100k dual gang with a centre detent. It would in fact be easier to use a seperate controls for tone and crunch, but I was trying to reduce the number of knobs for this built-in version. The resistors are all standard 1/4w 5%, but any type will do, there are no signifiacnt power dissipation requirements. For caps, I used polyester, but only because that was what was available. The FETS were 2N5457. I could do a parts list, but all the values are on the schematic diagram, The circuit as posted is built into an old Hondo Strat, which I have reqired to give series/parallel and phase options The circuit is a propotype, and I am currently working on a version with even lower current consumption (about 50 microamps instead of 250), with a better bass response, by reducing one of the source resistor bypass capacitors. I posted it for comment, in case anyone else might try it. Ill add my next version when I have it, and do a parts list for that. John

I have been playing with this circuit over the last couple of days, to pick a version to go into a new rewiring build. I wanted to explore the extremes of the various parameters in order to understand them. I'm using a 2N5484 JFET, and Im expecting it will perform similarly to the 2N 5457. I am biasing it with a 2.2M gate to ground and a 3.3M gate to positive. This sets the source voltage at about 4.7 V. I tried a wide range of values for that source resistor (discussed earlier as 220k or 22k). When feeding via a 20' cable into an amp, I found that even a 1M resistor sounded fine. I then tried it with a greater load on the output, representing perhaps some not very well designed effects box. I loaded the output with a 27k resistor to ground for this experiment. The result was, with a high source resistor, you get an increasing amount of low order harmonic distortion. Actually it did not sound too bad, but not what I was looking for. With the source resistor at about 150k or less, this was inaudible to my ears. So I have settled on a 100k source resistor, confident that this is a robust value that will deal cleanly with any practical load on the output. Current consumption is a nice small 50 microamps. The sound shows a good improvement in treble clarity compared to just passive, and no noticeable output level reduction. This buffer will also be the key to get my independednt volume controls in series and parallel to work. Thanks again for this thread. John

This is looking good with that mid-voltage bias. Im guessing that with that arrangement, it is less sensitive to FET properties. two more thoughts: My understanding is that with JFETS, the gate gets biased to a lower voltage than the source. To set the source voltage to mid range 4.5V, ie half the supply voltage, to get maximum headroom, should R7 be less than R5? (This could then bias the gate to say 1.5v to 3v depending on the FET). In fact, why not make them both part of a trim pot, then you can wire it up and set the bias exactly for that particular JFET I am always looking for a built-in design where the battery lasts for ages. The 10k values for R5 and R7, although not taking much current, are still more than doubling the current consumption. I suspect that those r5 and r7 resistors could be changed to something much higher, say replace them with a trimpot of 1M wired from + to -, with the wiper feeding the 10M resistor. John

Thanks Lovecraft - ill be interested in any feedback. Anyone else - any off the cuff suggestions from electronics experts to fine tune it?

Well this has become a very useful thread. I was interested in how the frequency response turned out to be much more robust than I thought. I also got the same results in a pSpice simulation. I used a 2N3819, which has a higher VGS than the 2N5457, and sure enough, the source voltage rose to about 2.9V. i wonder if this would give more headroom? Lovecraft - what is your view on the role of the 1pf and 4.7mfd capacitors in your posting?

Behringer have a large range of very cheap pedals in plastic boxes. I have not got any, but reviews are good, or bad when they break. Theres a discussion here: http://guitarnuts2.proboards45.com/index.c...read=1134460609 John

Paul, I thought Id write this out for everyone to see. Applogies to any Grandmothers who all ready know how to suck an egg! Ideal amp stages have high input impedance to take very little signal, but low output impedance, so they can drive whatever is downstream. In th eFET circuit above, 22k source resistor will result in lower output impedance than the 220k, though of course, if it sounds great, thats fine! In general however, the issue of treble rolloff due to cable capacitance is also affected by this. The rolloff starts to become significant at a frequency where the cap impedance matches the output impedance. Cap impedance decreases with increase in frequency, so with a low output impedance, the roll-off frequency is higher. To work out this frequency, with say a 22k output impedance: cap impedance = output impedance 1/(2Pi x F x C) = R F = 1 / (2Pi x R x C) C = 800pF = 800 x 10^-12 R = 22,000 F = 9042 hZ - ie quite high, which is usually what you want The equivalent with R =220k is 904hZ, which is significantly into the guitar frequencies. It might of course, sound great if a bit of treble roll-off is wanted. John

Lovecraft - the JFETS are 2N5457. I got the idea by mixing up a Tillman preamp with various circuits at http://www.runoffgroove.com/, then adding the idea of diodes where you can control the crunch using a pot. The best sounds, other than simple clean boost, are in position 2, setting an input level that just maxes out the first JFET stage short of clipping, then lets the second, which has no negative feedback, make a soft clipped overdrive. To that recipe I add a touch of diode compression with the tone/crunch control,but not too much. This adds a sort of breathy sound in the background - kind of smoky. psw - The circuit board is mainly the gain stage, and the other parts are mounted on the pot and the switch, which is a 4P3T rotary. These and the wiring are bigger than the main circuit!. This is a first prototype, I got the main circuit going first, then experimented with the eq, level settings and diode arrangements for the three switch settings. Next time, more will go on the board. Further development will be to tune position 3 - it needs some better shaping to make it less raw. Also, I think the low bass may be better if it is curtailed slightly. at the moment it has a very low cutoff frequency and 'burbles' a bit on heavy bass input psw - That circuit you posted I believe is also sold by gfs see : http://store.guitarfetish.com/activepreamps.html Its neat - but its more fun to brew your own. John

thanks psw. but I havnt got a board layout drawn up. I just tend to start at the left and solder components onto the board until I get to the right. Here is a picture of the board before adding the wires to connect the switches and other connections. Most of the eq parts and diodes were mounted on the switches and pot http://people.smartchat.net.au/~l_jhewitt/.../BlueDriver.jpg BTW. For a simple one FET buffer , the circuit around the second transistor is what I reckon is best, basically as Tillman with increased resistor values. John

OK - take a look at my on board booster post John

Heres is a prototype design for an on-board booster and overdrive circuit. I have it built into a Strat and it is alot of fun to play with. It can range from a clean boost, through some smooth crunch to a farly high distortion. Take a look at the circuit, and the words that describe it. I'd be grateful for any comments and suggestions. Feel free to point out any electronic sins that I may have committed! http://people.smartchat.net.au/~l_jhewitt/...ueDriverMk2.gif http://people.smartchat.net.au/~l_jhewitt/...verMk2words.pdf John

Yeah, a battery should last a very long time. Another nice little feature. I haven't verified what possible issues there might be with effects inputs downstream. I imagine it shouldn't really be an issue - as I see it, the buffer should only serve to make things sound better... ← What I was concerned about was roll off due to cable capacitance. I find the usual types of buffers such as Tillman, and the versions Ive built to be very good at fixing this by lowering the output impedance of the guitar. The concern I have is that this would raise the output impedance. With your circuit with the high 220k resistor, I am not sure exactly what the output impedance would be, but it would be at least closely realted to that 220k value. Guitar cables of say 20' can have a capacitance of 600-800pF (based on 30-40pF per foot), so if 220k was the output impedance, then treble roll-off could start at a frequency as low as 900Hz. My understanding is that a circuit like that would work just as well with say 22k there, in which case the roll off would start at 9000Hz, which is above guitar frequency range. Current drain would still be less than 0.25mA, so still very good battery life. As you can tell, Im interested in this theory but I am not an electonics engineer, so I am grateful and interested in any corrections to the above.

One thing that interests me about that circuit is the high value of the 220k source resistor. That should make it super low current, and hence a battery should last for ages. But does it cause any issues driving past cable capacitance or into amp and eeffects inputs downstream? The Tillman uses very much lower resistors (and is a different configuration). I have had success with the Tillman design, multiplying all resistors by x3 to extend battery life, and it is still a very effective buffer. John

Cool - that is the JFET type that I use too. I use 6.8k source resistor and 22k drain resistor. With no cap, I get a small signal boost, and very clean results. To balance two such circuits for your two pups, how about putting a 1M trim pot, wired like a volume control, on the input of one or both? John

I have also experimented with that circuit, and what Saber says is exactly right. I'd Keep the 10mfd cap and the 2.2k resistor, and try a 10k trimpot in series with the cap (using just the centre and one outer lug, as a variable resistor). That way, you will get a boost across the whole spectrum, but an amout which increases the lower the value of the trimpot setting. Set the trimpot for the boost you need, without clipping, or instead just try different fixed resistors until you get the right value. Another issue is that these circuits are sensitive to the type of JFET, and even JFETs of the sae type have significant variation. What type are you using? I adjust the source resistor to set about 6V at the drain, which is halfway from full supply 9v (JFET fully turned off) and the minimum voltage (wth JFET full on) set by the resistors, which is about 3V. One last idea to throw in - I build these with all resistors x3 in value. It works the same but draws less current, and still has enough of a low output impedance for my purposes.

OK - so how about this: establish a + and a - 'bus' as a pair of wires along the length of the fretboard. Between each of these bus wires, connect 12 sets of LED, LED, resistor. One each for the edge and centre of the fretboard at each fret marker. It would allow another issue to be addressed. I have had LEDs of the same spec, but with very different brightneses at a given supply voltage. The pairs and their resistors could be tested and matched before installation. Individual resistor values can be adjusted if required. In fact make sure the whole setup works before installing it. EDIT another thought. make the resistors a bit low, so you get a somewhat brighter glow that you really want. Then put a switched wire wound pot in series with the whole lot, as an overall brightness control for different lighting conditions and to allow adjustment for battery strength, or to click the leds fully off! John

Lovecraft makes some good points about forward voltage drop and the number of batteries. How about basing the idea on some AA size NiHm rechargeable batteries?. These have 1.2V percell, but unlike alkalines, then hold a much more stable voltage as they discharge, so you can set them up with a suitabel series/parallel arrangemet and then get a consistent performance, and be able to top them up when you want. They are easy to buy these days. I think with 4 of them (4.8V), you could probably get 2 leds in series with a resistor, and have 12 groups in parallel. Lets say you go for 5mA of current in each, total current drain is 60mA. These AA batteries can hold 2000mAhours, so you could get 30 hours theoretically between charges. You can get by with one resistor, put: (12 in parallel) in series with (another 12 in parallel) in series with one resistor. Does that sound like a design? John

I am curious as to how to power all these LEDs. Typical current through each one might be say, 10-20mA. Lets say you have them as 12 groups of 2 in series across 9V with suitable resistors, that will require 120 to 240mA. Thats a lot of current. Small 9V batteries give about 600 mAhours, but that is for depletion to half their voltage, by which time the leds will be too dim. Im worried that the batteries wont last very long, maybe a couple of hours effective, unless you have more heavier batteries. - or rechargeables? John

Syndromet - Thanks for the encouragement!. It is spooky, but the guitar I built this into is also an '80s Hondo - a 760 Delux Strat copy, which is a nice solid basic guitar. Is yours the same? Mine has a slightly different shaped pick guard to a standard Strat, which is not trimmed around the bridge. that influenced the placement of the controls. It also has a nice open routing, so plenty of space. However, my friend in Texas has built this design into two more standard Strats. The switches are really all just normal on/on. when I speak of on/off, I mean that that is the job they do in the circuit. Flick one up, the pup is on, down is off. That is constant in series or Parrallel. If you want to keep splittable 'buckers, you can add a coil cut switch between the pickup and the rest of the circuit, eg using a diagram from the Seymour duncan site. It is more switches to place and control however. The blender was by far the hardest part work out. To do it as I did, you have to open up the back of a dual gang pot, and scrape away the carbon track in a zone about 3mm wide, at the centre of the track (just to one of the two tracks). It is not hard, but if you dont like that idea, use a 2 x 500k pot instead of a x 100k. Sound range will be the same , but the blending will not vary so smoothly. As I put in the post, the blender is not essential - it can be omitted and still have all the combination options. John

Just to be clear; when talking about 1V 'peak to peak', does that mean a signal that goes from +1V to -1V, or +0.5V to -0.5V? John

If you are interested in series/paralel options with toggles. I have a design with a 4PDT switch for series parrallel, and dpdt's for on/off and phase. Its over on Guitarnuts2. You can read about it here: http://guitarnuts2.proboards45.com/index.c...read=1130243150 John

All true. Theres another effect as well. I made a rig to measure current and voltage across a Schotky at very small signals. Knowing those, you can work out an effective resistance for the diode ()R = voltage/current), which changes with voltage. It turns out that as the voltage reduces to zero and becomes negative, the Schotkys resistance is continuosly changing. From +20mV to -20mV, resistance changes from about 70k to 40k, and continues to change with larger signals. Those resistances are right in the money zone for changing the tone of the pickup. Hence the schotky is changing the sound in a non-linear way all the way down through the decay phase, and way below their forward voltage. By contrast, as silicon diodes approach zero voltage, their reistance is several mega Ohms, which has no effect on the signal, and they only clip as their forward voltage is approached. John

I used a 2n5819, wired directly in place of a tone cap. It sounds better with a little resistance from the pot is series. One diode sounds much better than two. it doesnt matter which way round you put it.

those are electrolytic capacitors, with a value 4.7 microfarads (if all are as the top one). They are polarised, and the wire nearest to the white stripe goes towards negative. There is usually a '-' symbol there, but I cant see it in the picture. The circuit you posted has two electrolytic caps, C3 and C4 which are 1 microfarad. tHe 4.7's will probably work OK in their place, in this application, as long as you put the negative in the right direction, indicated by the curved side of the cap symbol on the diagram. John