Cycfi

Well, no.

I imagine you will still have crosstalk relative to the original design, just to a lesser degree. It depends on how well the original design performs. Another factor to consider is how close the pickup can get. The closer the better, however if it's at the neck position this won't be possible. Also, if placed near the neck you will need to worry about string bends and the magnetic field. It's a lot of trade offs and balancing once things are refined, but the general applied concept could improve things.

Yes, I'm aware of that.

Basically, a larger magnetic field will have a bigger, possibly wider, sensing area but will be more susceptible to noise(magnetic) and crosstalk. A smaller field needs to be positioned more carefully. A stronger magnetic field relates to output, but does correlate since there are other factors to consider, largely inductance but there are a few more. Since the plan is to still use the neo magnets, the field strength may be stronger than the project above. Again, a lot of it is a balancing act of size and intended use.

Sounds good. I'll give it a try when I get a chance. FEMM is indeed interesting. BTW, are those vertical ferrite bars between the coils?

There's no space there anymore, but there might be in front and behind the coils. It's also interesting how some space between the magnet and the ferrite affects the whole.

It's fairly rough, but as you can see there may be something to gain. A tighter field will result in less noise in, less string pull, less crosstalk, and in this case more output since the coil's core is now increasing the inductance. Less noise and more power in the pickup stage means less power can be used in the amplification stage and maintain a low noise level.

The idea above is first use a ferrite core in the inductors and then shape the field using neo magnets. There is plenty of experimentation to be done to see output and noise levels with different configurations. A neo magnet has a lot more pull to itself and thus the relative usable field isn't projected as far, well sort of. This is good and bad. Another good thing about experimenting with this is the only modification needed to the original design in order to experiment is a change to stock coils rather than the neo coils. Another way to help shape the field is through the use of fins, or even small poles or rods, to give the magnetic field an easier path back to the other magnetic pole. This can also be good or bad. Good if balanced with your needs, bad if it removes most of the usable magnetic field.

Thanks for the explanation. Isn't that the same idea presented here: http://music-electronics-forum.com/t26380/ ? That project was plagued with weak signal and too much crosstalk. Maybe his inductors are too tall and his magnet too weak?

It's very rough, but...

That is indeed very interesting. Could you explain a bit more?

I like it a lot! Very clean.

I'm still not convinced on the neo magnets though, and you could still improve the magnetic design aspect a bit. If you can squeeze a bit more output from these coils then that would be less noise out since a big source of noise is from amplification. Also, narrowing or manipulating the magnetic field will improve issues with noise and crosstalk.

I'd definitely love to incorporate newer/better ideas, but I also want to get this out the door sooner rather than later. So, for this project, I'd say I've satisfied all my initial requirements. I don't consider noise an issue anymore as I've far exceeded the desired SNR (>80db). I also no longer consider crosstalk a problem as I am confident that this can be mitigated through cancellation in the DSP.

Tell you what: how about collaborating on a new PU project with your ideas in mind?

Do you have room under the pickup to place magnets? Or room for a small and compact enclosure system around the coil sets?

Yes, we can probably squeeze in some more room. What do you have in mind?

...and they're definitely the guys to be doing it! I was very much in agreement with RedHouse's statement on how we have gone through decades of iterations on losing the noise and perhaps trading off some of the "positive faults" inherent in old noisy gear. There are only 24hrs in a day and between building we should at the very least be playing too. I certainly spend far too much time hunting for better tones in my gear instead of improving my chops :-)

I agree 100%. We should play the instrument more Thanks for reminding!

That isn't to say that this will not be a fantastic project. Maybe it'll make some of that old noisy vintage gear sound a little less so!

Thanks. Well, I haven't deviated from my original goal: to create new sounds. I'm tired of the old vintage sounds, to be honest.

Speechless.

Well, since you are the Central Scrutinizer, you can probably start scrutinizing :-) It's interesting how the design got scrutinized in the music-electronics-forum the past few days starting from this post. In the end, I learned a lot! And got some exciting ideas.

Here’s the V1.1 prototype based on the V1.0 design and specification. The version is bumped to 1.1 since there are slight modifications from the original 1.0 design. Follow this link for more…

Fully Assembled Hex Pickup

Neo-2 PCB with components

Neo-2 Fully Assembled

FEMM 4.2 is a free magnetic modeling program. While limited, it can be very useful in visualizing magnetic fields and relative strength. I would suggest researching noiseless stacked coils for some examples of use of neo magnets. Lace pickups also have many interesting concepts, though many remain patented.

As for the bifilar coils, I'd be more interested in output vs size (sensing area related) Of course, these coils would have little effect on noise ratios from a design standpoint, and would need to be wired as you are doing with sets.

Thanks! I just googled a bit after relying. I'll definitely look into that. For now, I'm quite satisfied with the coils and it serves its purpose well. There are still bigger fish to fry and I need to move on.

Nice! Looking good so far! For a low power version, perhaps there are other opamps to consider? You could probably run on less than 1mA... for all 6 coils.

Yes. Scroll down a little bit: OPA376 is a lower power option at 760μA. Even lower power? Then how about OPA314 at 150μA. I have a list of Op Amps here: http://www.cycfi.com/projects/six-pack/op-amp-shootout/

And... in yet another development, I might be considering an alternative floating, single-ended, pseudo-differential design. For low-power, single stage discretes rule.

I suppose your using neo magnets? If you haven't already, I'd suggest playing around with FEMM a bit to see if you can improve magnetic containment a bit. To be honest, I'm not sure neo mags are the best way to achieve what your trying to do. They can be useful, look at ways they have been used in pickup design historically. I suppose in a way this goes hand in hand with bifilar winds too.

Please tell me about FEMMs and why they would be better.

Speaking of, did anything come of the bifilar windings? Any reason for or against it? I still have a project(on hold for the moment) based on bifilar designs.

It turns out that the current design is already optimal WRT noise and I was not able to squeeze more from using bifilars. A better approach, and one that I am taking now, is to take advantage of the multiple coils and have the coils alternate from north-south pole orientation, clockwise winding for the 1st, 3rd and 5th coils and south-north pole orientation, counterclockwise winding for the 2nd, 4th and 6th coils. That way, the sum of the pickup outputs will cancel some more of the noise the same way humbuckers do. It is possible to isolate the noise from the signal as well as do more advanced cross-talk cancellation in DSP.

Keep up the good work!

Thank you! I appreciate all the inputs I get.

Hi!

After countless hours experimenting and iterating over the basic design, I think I’ve reached a point where I have something that I am actually finally satisfied with. Here now, let me present version 1.0 of the Six Pack Hexaphonic Pickup. The design is modular. Visible in the xray view below, the Hexaphonic Pickup actually contains three smaller dual active pickups sitting on a main board which contains no more than biasing electronics and a micro-miniature header connector.

Read more about it following this link.

Features:

1. Six low impedance coils (2000 turns, 450Ω D.C. Impedance, flat frequency response, 20Hz-20kHz).
2. High performance, ultra low noise OPA209 (OPA2209, dual) differential Op Amps.
3. Single supply (4.5V to 36V).
4. Stainless steel height adjustment screws and springs.
5. Gold plated, 1 mm pitch micro-miniature positive lock connector (Hirose).
6. Premium 0.1% thin film low noise matched resistors.
7. Fender Stratocaster Profile (11.8 mm height excluding connector).

Found this picture recently regarding another Paul Vo invention (the guy behind the Moog guitar). If you scroll to the bottom of this page you can see a blurry image of his VO96 acoustic guitar hex sustainer being tested. You can just see that each pickup appears to comprise two coils per string (one driver, one pickup?), each string pair being slightly offset from the next so that they all align properly with the strings. I guess this acounts for the Moog guitars' trapezoid-shaped pickup covers.

Woa! Nice find! My best guess is that both the pickup and the drivers are phase-inversed (humbucking style) to cancel out the actual EM being generated.

Hi!

This is part 3 of the “Let it breathe” series. For proper context, you might want to check out Part1 and Part2. This third and last part of the series concludes with actual frequency response measurements for various coil configurations of the Hexaphonic Pickup Project and with the Fender Stratocaster middle pickup as benchmark to compare against. In addition to standard windings with 500, 1000, 1500, 2000, 2500 and 3000 turns AWG 42, we also tested bifilar windings (see Alternative Ideas) with 500 and 1000 turns AWG 42. Like before, the low-power LME49726 OP Amp is used in the differential preamp stage with a gain of 5.

See this link for more: Let it breathe (part 3): Frequency Response

On a related note, I tried a secondhand Moog guitar yesterday just for the hell of it. I suspect that this particular example was badly set up, or had something wrong with it as it didn't work as well as shown in some of the Youtube videos I'd seen of it, but it was a lot of fun to play with nonetheless. Sustain appeared to be weaker, and the "mute" function only just worked. I guess the drivers weren't imparting enough "oomph" on the strings - too far from the strings? Strings were old? Not enough amplification? I notice on the Moog website that they recommend strings with a high steel content, even going as far as saying that the guitar works best with their own "specially formulated" strings.

The thing that struck me was how well it could sustain cleanly with any pickup, something that the DIY sustainer and the Ebow always struggled with. With the DIY sustainer and Ebow there is always a degree of background distortion/fuzz superimposed on the raw guitar output. From memory the general concensus during the development of the DIY sustainer was that it was largely due to the driver circuit being horribly overdriven, and the proximity of the driver to the guitar pickup inductively coupled the noisy driver signal back into the pickup. The Moog guitar had none of these artifacts, which made it so much more appealing to me as an example of an instrument fitted with some kind of infinite sustain system. I love using an Ebow, but I hate that I can't use it for clean sounds.

There wasn't an awful lot I could gather about the technology behind the pickups/drivers during my brief encounter with the instrument (the sales assistant admitted he didn't know anything much about it either, other than the pickups were based around rare earth magnets), and there doesn't seem to be much revealing information floating around on the net.

The eventual deal-breakers for me were the facts that it didn't work quite as well as the videos I'd seen of it, and the that it was fitted with a bunch of on-board filters for synth-type sounds that always seem to be applied to the guitar signal even when wound all the way back, something that I could definitely live without - if I wanted my guitar to sound like a Moog synth from the 70's I would use external processing. Because of the fancy filter/tone controls I never really felt like there was any setting that let me listen the natural sound of the guitar. I guess Moog have included it to add extra credibilty to the brand name. The implication that I need to buy Moog-branded strings to get the best performance out of it was also not encouraging. However if Moog (or some other manufacturer for that matter) could offer the hexaphonic sustain system minus the filter processing as a kit I'd buy it in a heartbeat.

I've always liked the idea of a bifilar-wound pickup or one with several "taps" throughout the wind which can be summed later in the signal chain. I presume this wouldn't be a million miles away from how a stacked pickup works, but in a more compact form? Would there be appreciable crosstalk or other interaction between the two coils due to their extreme proximity in a near-identical wind?

I honestly don't know. I haven't tried this before and to be honest, electromagnetism is by and large still a mystery to me. I am learning each day. People who did it sing praises. For example:

"Well, I got around to hooking it tonight (using a two stage differential amp made up out of 3 x TL072s) - even with very old (dead) strings on my test guitar it's sounds very 'full bandwith'. And as Rick has just pointed out ...there's something about the low end through a distorted amp sim - it struck me as being less muddy vs a high impedance pickup.

I'm quite impressed. But what has really knocked me out is the lack of noise (that'll be the common mode rejection going on -my first real experience with it ...& I'm sold!)."

Excellent proof of noise cancellation. The exact same method is used in the classic 70s Aria Pro II SB-1000s and they were near silent. I have upgraded many many of them the last couple of years with pre-amps using metal film resistors and modern ICs like OPA2277s, OPA2107s, etc. and this makes the differential amplifier deadly silent. Almost to the point of it being a joke!

Is the rumble in the low frequencies just an artifact or actual noise? Truly fascinating results. You've killed the noise already....now let's see what games can be played with this thing!

The noise level is so low I can't hear anything even with my headphones set to max. I reckon I can hear it if I boost the signal with ample gain, but I haven't done that. I'll try when I get the chance.

So... we are currently using single-coils in floating differential mode. The coils are connected to the + and - inputs of the differential amplifier without any direct path to ground. This configuration is inherently noise canceling. Should we be using double coils in humbucking configuration instead to get even better noise cancellation?

Here, we talk about the limitations of passive humbuckers and propose possible alternatives such as the Bifilar Coil winding. Read the rest here: Singles or Doubles?

Wow that's really interesting.

This sample sounds much more mellow than the previous hex on strat one. Much more humbucker-like to me. More bass and less sparkle.

It was really interesting to open the previous samples in another browser window and listen to them back to back.

This one is by far my favorite so far (ignoring the Bosendorfer of course, haha.) I really like the tone.

It would be interesting to hear this new coil design with the previous amplification circuit just to hear what a difference the coil makes without changing the circuit. Is your intention to leave the previous circuit behind because the new one is better?

I'm certainly liking the sound and I'm very excited to hear your subsequent tests with the 2000 turn coils.

I am currently working on the tests for 500 turns, 1000 turns, 2000 turns and 3000 turns so we can see the freq. response side by side. I also rigged an (core-less) 8 Ohm driver-coil placed a few millimeters on top of the pickup-coil which is then driven by a 20Hz to 20kHz sine sweep while monitoring the preamp's output response. This will give us an accurate profile of the actual frequency response curve of the coil + preamp.

I like the sound of this one too. I can probably re-test the older circuit, but I doubt if it contributed to the 'color'. The changes in the electronics were meant to give improved CMRR (by using matched/precision resistors and better Op Amp) as well as lower the noise floor (lower noise Op Amp and lower valued resistors). I believe the pickup design with a more focused magnetic field and lower string pull (thus closer proximity to the strings) contributed a lot more to the color than anything else. If there's such a thing as proximity effect on pickups, I guess this is it (it would be easy and interesting to test that conjecture).

Thanks for your input! I really appreciate it. In the end, we can probably have a set of samples to choose from and have a poll which one you guys think is the best.

Let it breathe (part 2): More Frequency Analysis

Last time we did some (very rough) preliminary tests comparing an early version of the hex pickup with a Fender Stratocaster and a Gibson Les Paul. The initial test was done using a 2000 turns AWG 42 coil and the cheapo TL072 Op Amp in differential configuration. The goal basically was to see the frequency spectrum of lower impedance coils side by side with that of the single-coil Strat and Les Paul humbucker pickups. Here we will continue with the tests, this time focusing only on the Hex pickup installed on a Strat while continually improving on various aspects of the pickup design.

Tell me what you think.

It's fairly hard to compare such different designs. Impedance and inductance and a number of other factors influence the sound.

In practice, testing will prove more vital since I honestly don't know where that cutoff point will become noticeable. There is also still the question of whether high frequencies are desired and to what point. The simple answer, preference.

Oh it is noticeable. The pickup itself is a second order low pass filter. HIgh impedance designs have the cutoff point in the audible range. It's the thing we associate with the color or flavor of the pickup (e.g. dark, bright, etc.). It's easy to trim high frequencies with a second order low pass filter to shape the sound later, but not the other way around. IOTW, you can remove what's there but you cannot add what's not there to start with (i.e. higher frequency harmonics). That way, the player has the flexibility of tone shaping rather than having the pickup itself hard-wire the tone for you.

Why not use/make step up transformers? Or for that matter, wind larger pickups, higher output, thus less amping needed. I don't think it will alter the result too much, maybe to a slight degree but nothing too dramatic.

However, keep the drivers the way they are.

For the pickups, once the impedance (not resistance) is greater than about 25k, you will need a small jfet preamp to lower the impedance when inputting to an lm386(50k input?). However, with a larger input you will need less amplification, eliminating oscillation from high amplification and reducing the power needed in addition to the possibility of reducing noise in the system which in turn will improve your results.

Just some thoughts to leave you with, the inductance of a coil is related to the square of it's turns. Inductance relates to output. A coil less than hmm... say, 1k ohms? is probably wasting output with very little benefit. There are quite a few other variables that effect such things such as design or wire gauge or metal mass nearby, I'm not quite sure where a DC resistance measurement would be exactly... but that's my guess for your design given the facts I know. The best test will be to wind something up an try it!

The higher the inductance, the lower the cutoff frequency and the darker the sound will be. There *is* merit with low impedance pickups below the 1k range. No wonder Les Paul himself favors a low impedance pickup of 10 Ohms! (see Les Paul’s Favorite Les Paul – The Recording Model). He sends the balanced output direct to the mixer. I've had very good results with small, low impedance pickups in the 100 ~ 300 ohm range. Here are some preliminary spectrum analysis of low vs. high impedance PUs on the Strat and the Les Paul and a Bösendorfer grand.

hey, thanks a lot!!!

one thing i've been wondering about for the new version

before the LM1876 driving each "channel" (string) i need something to preamplify the signal from my handmade pickups.

what I have been using are some LM386's, but they havent been the best preamplifiers... they tend to go crazy and oscillate and arent effective enough until they reeeeaaally distort the signal. thats with the gain set at 200.

in the meantime i use my zoom h4 to preamplify the signals while i prototype.

are there any other simple circuits out there i should try? maybe like a TL072 headphone amp?

I won't use an lm386 as a preamp. It's not designed to be used that way. You are right, it will oscillate like crazy if the gain is set high. And you need a high gain since you have very low impedance (220 ohm) pickups. I'd use an Op Amp wired balanced like an XLR mic preamplifier. You can see lots of schematics of these if you do a search for "balanced preamplifier schematic". As for the Op Amp, yes, the TL072 is better suited. I have here a list of some more Op Amps: http://www.cycfi.com/2013/04/choosing-good-audio-op-amp/

Wow! That's pretty cool!

A class D amp is a more efficient way of amplifying vs a class AB amp like the LM386. My main issue with finding a suitable class D amp is that most are smd and designed for cell phones, etc. rather than driving a string at full power. I still have a class D test chip laying around... hm. Somewhere.

Anyway, for your application a class D amp or class AB isn't an issue, go with what works best. I assume your going to plug into the wall no matter what, you know with the whole 16 power amps and all, so why worry about efficiency? It's mostly relevant when relating to battery operated devices.

Those are very good points actually. I thought at first that this would be a portable instrment, but it seems it's not. So DarkAvenger is right, it does not matter much.

Wonderful idea! I too am going into "acoustic synthesis" starting with the hexaphonic pickup project thread in this forum. I'm very interested with your results, especially the drivers. I'd prefer class D amplifiers for efficiency, BTW.

10Ohm? Crazy. Nothing short of an EMP would spike noise onto that!

How low is the impedance of the Les Paul Recording anyway? I read here (from the comments): Les Paul’s Favorite Les Paul – The Recording Model that it's a stacked humbucker design with 5 Ohms per coil, for a total of 10 Ohms! Can anyone here verify that?

I did actually. I'm a fount of obscure information :-)

Oh of course you are :-)

Les was a much underestimated inventor. Most people think he stopped inventing after the solidbody guitar named after him. Certainly not!

Indeed. Anyway, obviously I am a fan of low impedance PUs. BTW, there's this guy at the music-electronics-forum who tinkered with extremely low impedance pickups with "proprietary" active circuitry. He proclaims that "nothing else sounds quite like a 10 ohm humbucker". 10 ohms! Now that got me thinking...

Two instruments spring to mind when it comes to being active and using very low turn counts/DCR. The Gibson RD Artist incorporated a large and perhaps overly complicated Moog circuit which was quite gainy on top of the low output pickups. The other is the Aria Pro II SB-Integra which was a basic PJ bass with a gainy pre-amp (~x10 if I recall). Both fine sounding instruments with much clarity. There will be many others, however these two are making me think more and more about the idea of low turn count pickups.

I doubt you will need to consider winding hotter however definitely do it. For science.

Not sure if you know this, but Les Paul actually favored low impedance pickups that are DI'd directly to the mixing board. He favored the wider frequency response and the possibility of sending the signal through long cable lengths (in a differential manner of course): The orphaned (and very rare) Les Paul Personal, Professional and Recording models use passive low impedance pickups. They didn't catch on at the time because the outputs were very low and couldn't drive guitar amplifiers.