Hexaphonic Pickup Project

[rjb]

...Read the rest here: Singles or Doubles?. As always, I'd love to hear your thoughts.

No thoughts, just a dumb sidetrack question.

Comparing the Strat and LP plots, can you explain why the LP shows spikes at slightly unexpected locations?
Why is there a "double spike" at ~110 Hz and 120 Hz?
It looks like the expected spikes at 180 Hz and 240 Hz are shifted down to ~150 Hz and ~220Hz.
Any idea what's going on there?

[cycfi]

No thoughts, just a dumb sidetrack question.

Comparing the Strat and LP plots, can you explain why the LP shows spikes at slightly unexpected locations?
Why is there a "double spike" at ~110 Hz and 120 Hz?
It looks like the expected spikes at 180 Hz and 240 Hz are shifted down to ~150 Hz and ~220Hz.
Any idea what's going on there?

I have no idea. I didn't pay close attention to the details. I was only concerned with the overall noise levels. Could it be other noise getting picked up? I'm not sure. Apart from that, there are also other noise and lots of it. My best guess is some low frequency RFI. I do have other takes and here's one without the extra artifacts:



I'll probably replace the graph with this one. Thanks for noting!

[cycfi]

Let it breathe (part 3): Frequency Response

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

[Steve Conner]

Interesting plots. They are tilted 6db/octave compared to how pickup responses are usually plotted, though.

In the write-up you mentioned using an integrator on the output. I think it is usually done by using a drive coil with more turns, and constant voltage drive. This makes the drive coil current fall off at 6dB/octave. When you connect a low resistance coil to a headphone output without much drive capability, you get something more like constant current drive.

A simple experiment would be to try the existing drive coil with a speaker output from a hi-fi amp, instead of a headphone output.

Can we buy these hex pickups yet?

[cycfi]

Interesting plots. They are tilted 6db/octave compared to how pickup responses are usually plotted, though.

In the write-up you mentioned using an integrator on the output. I think it is usually done by using a drive coil with more turns, and constant voltage drive. This makes the drive coil current fall off at 6dB/octave. When you connect a low resistance coil to a headphone output without much drive capability, you get something more like constant current drive.

A simple experiment would be to try the existing drive coil with a speaker output from a hi-fi amp, instead of a headphone output.

It is indeed tilted

I like the approach done here: The Secrets of Electric Guitar Pickups. I'm not sure I like the idea of having a drive coil with more turns. Wouldn't its own frequency response skew the results? I'll probably build a proper test setup sometime, but for now, I got what I needed to know from the tests.

Can we buy these hex pickups yet?

:-) I'm currently talking with a couple of manufacturers. The difficult part was obtaining the right bobbins and the pickup enclosure (will write about that soon). The tooling and setup cost for those is a quite steep, but so far, things are looking good. Of course, all the designs will be open and shared.

[cycfi]

Cycfi Six Pack Hexaphonic Pickup v1.0

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).

[David Schwab]

The difficult part was obtaining the right bobbins and the pickup enclosure (will write about that soon).

Have you looked at the bobbins here:

Cosmo Corporation - Coil Bobbin Catalog Search By Bobbin Type

For covers, you can get EMG style strat covers.



Why reinvent the wheel? A nice part about the covers is they have an area on the bottom that can house a circuit board, preamp, etc.

Of course if you have the finances to have something injection molded, that's better. But that's expensive too.

[cycfi]

Have you looked at the bobbins here:

Cosmo Corporation - Coil Bobbin Catalog Search By Bobbin Type

I just looked. The minimum width is 11.51mm for their Pot Core Bobbins with Terminals. Other types are even wider. What I need for proper spacing should not exceed 10mm.

Anyway, if you read through the link, I already solved the sourcing (it was difficult! we searched through similar companies like that you linked to). We finally found one (in China) that satisfies our requirements.

For covers, you can get EMG style strat covers.

Why reinvent the wheel? A nice part about the covers is they have an area on the bottom that can house a circuit board, preamp, etc.

Of course if you have the finances to have something injection molded, that's better. But that's expensive too.

Using EMG covers looks like an interesting idea. Indeed the tooling cost for injection molding is steep! We found some nice offerings, but it's still too steep. I'll look into using EMG covers. I might have to compensate though because ours have a lower profile. Initially I wanted a very low profile PU, but later realized that I will not have enough adjustment space for Strat profiles with proper body to string distance. The only way to have a low profile is to have the PU top mounted. What's is nice about a body mounted PU is that there's no longer need to route a cavity.

Yes, I try not to reinvent the wheel :-)

Thanks, David!

[David Schwab]

I just looked. The minimum width is 11.51mm for their Pot Core Bobbins with Terminals. Other types are even wider. What I need for proper spacing should not exceed 10mm.

Here's a round bobbin with with 9.53mm dia.

http://www.cosmocorp.com/en/ppHandle...=length&DIM=MM

They have smaller too... 6.22mm, etc.

I did not read the whole thread, so if you found something, great.

[cycfi]

Here's a round bobbin with with 9.53mm dia.

http://www.cosmocorp.com/en/ppHandle...=length&DIM=MM

They have smaller too... 6.22mm, etc.

I did not read the whole thread, so if you found something, great.

Oh I see! You are looking at the bobbins only, while was I was looking for bobbins with terminals.
The terminals (SMD or through-hole) are important for manufacturing (either pick-and-place
or manual).

[Steve Conner]

This has probably been covered before, but why use differential amps? You use more components, pay a 3dB noise penalty, and the only advantage is rejection of noise picked up capacitively. A differential amp won't reject magnetically induced noise.

I'd suggest using single-ended amps, plus a Faraday shield for the pickup. It might be enough just to wire the coils so that the outer windings are grounded, then they will help to shield the inner ones.

[cycfi]

This has probably been covered before, but why use differential amps? You use more components, pay a 3dB noise penalty, and the only advantage is rejection of noise picked up capacitively. A differential amp won't reject magnetically induced noise.

I'd suggest using single-ended amps, plus a Faraday shield for the pickup. It might be enough just to wire the coils so that the outer windings are grounded, then they will help to shield the inner ones.

Yep, it's ben asked before. I say now that the single best reason is isolation from the noisy ground. Ground lifting and isolation is inherent in a balanced design. People go to great extents lifting and isolating the noisy ground. Even a wireless set up will reduce the noise a lot, as many have experienced. David noted here that EMG pickups have the two coils connected to the op amp in a differential manner. Why would they, if there's no merit? I've done single-ended FET before. It's never as good as floating diff amps. I'm an advocate of balanced design. And, in case you haven't checked recently, new generation Op Amps are magnificent! I'm using one with a 2.2 nV/√Hz noise spec. A 1K resistor alone has 4.1 nV/√Hz noise! And at $1 per Op Amp, it's well worth it. And they are getting better every year!

Having said that, Hmmm, I might look into a floating, single-ended, pseudo-differential design. Grrr, I hate negating myself :-) ! What I dislike most about OP Amps (OTOH) is the current consumption. For that matter, single stage discretes rule. I'll get back to you on that with some experimentation. Hah! then you can say "I told you so". I love this forum! It forces me to rethink :-)

Now about magnetic induced noise, I 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.

[Mike Sulzer]

Yep, it's ben asked before. I say now that the single best reason is isolation from the noisy ground. .

This is completely unnecessary. There are circumstances where this problem can occur, but not with such low impedance coils, and especially not with the electronics right there.

[cycfi]

This is completely unnecessary. There are circumstances where this problem can occur, but not with such low impedance coils, and especially not with the electronics right there.

OK, you are probably right. I'll go try and do some noise tests and compare.

[David Schwab]

Having said that, Hmmm, I might look into a floating, single-ended, pseudo-differential design.

You can do it like EMG does. Differential connection to a standard op amp, in this case an LM4250 (disregard what the schematic says).