GuitarBuilder's Triple Nu Multi Pickup Guitar Build

Started by GuitarBuilder, July 17, 2019, 02:13:29 PM

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GuitarBuilder

After my first succesful build with Cycfi Research Nu Multi 6 active hexaphonic pickup, I wanted to explore the possibility of using more than one hexaphonic pickup on a guitar and focus primarily on single-string processing.  My previous effort (https://www.vguitarforums.com/smf/index.php?topic=21551.0) successfully used the Nu Multi 6 in the bridge position by mounting it on a humbucker backing plate.  By adding a GK 13-pin jack on the Cycfi Research Nexus Breakout Box to connect to Roland/BOSS synths, this guitar functions just like a GK guitar, but with one important exception: the Nu Multi pickup output is very high fidelity compared to the Roland GK pickup, due to a low impedance, FRFR output. The dry guitar sound is excellent compared to a GK-equipped guitar (of which I have many!).

This started me thinking: what would those Nu Multi pickups sound like in the middle and neck positions?  Granted, those are not ideal for driving synths due to excessive crosstalk, but how about for single-string audio processing?  There was only one way to find out - build one!

The challenges are many: I did not want to output 18 audio channels in addition to several key CV channels.  Custom circuitry would be required to properly switch three hexaphonic pickups.  How does one control the volume and tone of a hexaphonic output?  Finally , how do you stuff all that in the limited space of a guitar?

In addition, I was working on a concept for single-string processing using Eurorack modules, so this guitar design had to interface properly with that project.  For more details on the Eurorack Guitar Processor, see my build thread elsewhere in this forum:

https://www.vguitarforums.com/smf/index.php?topic=25929.0

I settled on the following features:

1. Three Cycfi Research Nu Multi 6 v2 active low-noise low-impedance pickups
2. Custom internal breakout board to interface 6 audio channels and 6 control voltages (CVs) with Eurorack Guitar Processor
3. Custom hexaphonic pickup switching system with hexaphonic summing mixer
4. Touch switch control of pickup switching system (why not go all the way?) similar to Strat 5-way switch using pickup selection CVs
5. Custom hexaphonic tone control using hex VCFs and CV pots for frequency and resonance control
6. Custom hexaphonic volume control using hex VCAs and a CV pot for loudness control
7. Standard Cycfi Research 19-pin LEMO-compatible jack and cable

A good candidate for such a project is the Stratocaster because of the easy customization of the pickguard and body cavities.  My starting point was a Warmoth Strat body with Universal Pickup Rout, to maximize cavity space:


It turns out I needed to rout out additional cavity space to accommodate the prototype boards for the custom circuitry!

I decided to prototype the custom circuitry with through-hole components to facilitate experimentation and troubleshooting.  I'm glad I did, because some boards took 5 revisions!  The downside is that they are very large compared to the (final) surface mount (SMD) versions.  Only the Internal Breakout Board was designed from the start with SMD.

The first step involved creating a custom pickguard.  Unlike my first Nu Multi guitar, where the pickup shared the inside HSS bridge position, I wanted to place the bridge Nu Multi pickup as close to the bridge as possible.  This required additional routing of the Universal Pickup Rout and a custom rout of the pickguard:


Note the position and angle of the bridge Nu Multi pickup


The middle and neck Nu Multi pickups are in standard Strat positions

With the pickups placed in the pickguard, the next step was to make room for the prototype custom boards.  An ideal area for this is under the pickguard towards the lower bout.  Below is a shot of the newly routed board cavity with a printout of the first board as a fitting guide:


Fitting the prototype switching board layout in the new board cavity.  On the left is the original Cycfi Research Internal Breakout Board, later replaced with a custom version

For the custom hexaphonic pickup switching system, I identified several options:

a) Using miniature audio relays.  Advantage: clean audio, no issues with distortion or amplitude limits.  Disadvantage: larger current draw
b) Using analog audio switches.  Advantage: smaller size, lower current draw.  Disadvantage: possible distortion, amplitude limits
c) Using analog pickup power switches.  Advantage: very simple and small.  Disadvantage: potential pops during switching

I decided to build all three and test them out.  The first candidate, a Nu Multi Relay Switching Board, was created using three DPDT miniature audio relays per channel (pickup) for a total of nine relays:


Each of the three 10-pin connectors accepts a standard Cycfi Research Nu Multi Pickup cable.  The 24-pin header on the left carries the selected pickup outputs (up to 18) to the Summing Amplifier Board that is piggybacked onto the Switching Board, as well as providing power to the Switching Board.

Here is the Nu Multi Relay Switching Board in the guitar board cavity:



Next up was the Summing Amplifier; its function is to take any combination of the three sets of hex pickup signals as selected by the Switching Board and mix them to the single hexaphonic output:


The Summing Amplifier communicates with the Switching Board via the 24-pin connector on the right.  Near the bottom are a 10-pin Cycfi Research compatible 6-channel audio output and a header for power and the three pickup selection CVs

Below are shots of the Summing Amplifier piggybacked onto the Switching Relay Board:


Front view showing three pickup connectors


Rear view showing 24-pin connector


Side view showing audio output and CV/power connections


The piggyback assembly in the guitar board cavity

Next up was a test of another method: switching the Nu Multi active pickups by enabling/disabling the power connection.  This circuit uses only 3 analog switches and is the simplest of all:


Top view of the Nu Multi Pickup Analog Power Switching Board; connector layout is identical to the Relay Switching Board


Front view of the Nu Multi Pickup Analog Power Switching Board

The third method for pickup switching uses analog switches to switch the 18 audio channels.  That prototype board is in fab and will be reported on in a future update.

Here is a mockup of the circuits so far along with the original Cycfi Research Internal Breakout Board.  Not the 19-pin jack has not yet been installed:


The pickguard was not cut yet!  I was also waiting for another pickup cable and CV pot


Two of the three CV pots are hooked up

So far we have a pickguard with the three Nu Multi pickups in position and a hex pickup switching assembly with 3 in, 1 out controlled by 3 pickup selection CVs.  Next up is the 19-pin jack installation and the custom Internal Breakout Board.  Mounting the jack on the lower part of the guitar (instead of the traditional Stratocaster position) makes it easier to route the cables for connection.  A standard elliptical jack plate with a 15mm hole was used to mount the jack:


The Cycfi Research 19-pin LEMO-compatible jack bring medical-grade jack reliability to guitars, far better than a plastic 13-pin GK jack!

A 3/4" bit was used to provide cable access:


Note that I cut the pickguard horizontally to allow for easy parts swapping without removing the strings while still experimenting.

The Cycfi Research Internal Breakout Board is a standard accessory to the Nu Multi pickups. All 19 connections are broken out into headers for easy access and signal distribution.  I started by using this board, but realized that I needed a bipolar power supply (such as the +/-12V in Eurorack for my Summing Amplifier, VCFs, and VCAs.  The Cycfi Research Nexus supplies +10V only and while it is possible to design the amplifier and filter circuits with a single-ended supply, I did not want to introduce any compromises.  Hence a custom Internal Breakout Board was created, along with an accompanying Eurorack Nexus Breakout Module:


The two 10-pin connectors along the top edge accept standard Cycfi Research pickup cables.  The headers in the center are for channels 7-15, either audio or CV and provide additional power for other circuitry.  The 19-pin jack connectors are near the bottom edge


Side view of the Custom Internal Breakout Board

So I have the ability to switch 3 pickups with 18 audio channels into 6 outputs using 3 pickup selection CVs.  How do I create those CVs?  An easy solution is to wire a standard 5-way switch to provide the correct combination of CVs.  It only takes +10V and three wires once you figure out which switch terminals to use!  That just wasn't high tech enough for me, particularly after seeing the touch switch implementation on the Relish guitars.  I couldn't find anything that was commercially available, so another custom design was necessary!  The requirements are as follows:

1) Capacitive touch pads (starting with 5, although 6 and 7 are easy)
2) LED indication of "switch" position below each touch pad
3) Digital logic to create three pickup selection CVs that latch
4) Able to fit in the same space as 5-way switch

This was the most challenging design to date!  I had to take a crash course in capacitive touch sensing and pad design; I also decided not to use a micro-controller for the logic (I wanted to minimize any digital clock noise).  I settled on an Atmel capacitive touch sensor chip coupled with discrete logic.  The prototype is huge, but now that it works I can reduce it to the final size:


All the elements of the prototype pickup selection touch switch.  Touch a pad and the LED below it lights while the logic circuit latches the appropriate pickup selection CVs


Close-up of the touch pads and LEDs.  This entire assembly will be shrunk down to 2mm thick using SMD components!


The Atmel capacitive touch sensor chip will be incorporated on the final logic SMD board


This logic board will be reduced to fit in the same space as a 5-position switch and connected to the touchpads/LEDs on the pickguard

That's it for now!  In the pipeline are the prototype Nu Multi Pickup Analog Switching Board and the prototype Hex VCF and VCA Board.  Once those have been assembled and tested, I can reduce the sizes by consolidation and using SMD components.  The pickup touch switch selector will likely remain a separate module, able to retrofit a standard 5-way switch.  I will investigate the possibility of combining the Nu Multi Pickup Switching, Summing Amplifier, Hex VCF, Hex VCA, and Internal Breakout Boards.

Stay tuned!  More to come shortly!
"There's no-one left alive, it must be a draw"  Peter Gabriel 1973


jim-analog



Greetings,

Peter, you're off the deep end! The entire project looks absolutely great.

  What are you planing to use for a VCA as mentioned in item #6 and for the VCF in #5? Keep up the great work and keep us updated.

BTW, the body has a really nice grain pattern; is that a different one from the first photo?

Regards, Jim


vanceg

Did I ever tell you; you are my hero?

Serioulsy - this is fantastic. 

GuitarBuilder

"There's no-one left alive, it must be a draw"  Peter Gabriel 1973

gumbo

...."Honey, I shrunk the TouchPads  !! " ........

;D
Read slower!!!   ....I'm typing as fast as I can...

GuitarBuilder

Now that all three prototype candidates for a triple hexaphonic switch have been completed, it's decision time!  Only one of them will go on to the next step of miniaturization and integration with the rest of the Triple Nu guitar circuitry.

Here they are:

Triple Nu Switching Circuit Candidates (all in prototype form): miniature relays, analog audio switches, analog power switch

My initial design for the summing amplifier that plugs onto these worked, but was susceptible to interference - it received AM radio stations really well!  The redesign with better filtering is in the works, after which I can give each of the three prototypes above a thorough evaluation.

Candidate 1 - Relay board.  This design uses 3 DPDT miniature audio relays for each hex pickup.  A control voltage of +12V for each bank switches the 6 channels on or off.  This is by far the cleanest audio path, but has obvious size disadvantages.  A major advantage is the ability to select relays of different voltages without affecting the audio signal integrity.

Candidate 2 - Analog Audio Switch Board.  This design uses 74HCT4066 Quad SPST Analog Switch ICs, 1.5 ICs for each channel.  The same +12V control voltage mentioned above switches the 6 channels for each pickup on or off.  These require a +/-12V power supply for lowest distortion operation.  It's easy to clip the signal in these if you're not careful.  The advantage is that they take up much less space, particularly in surface mount form.

Candidate 3 - Analog Power Switch Board.  This is the wildcard: if it works, it would be the most elegant and smallest solution.  The design takes advantage of the fact that the Nu Multi pickups are active.  In principle one can simply turn the power on or off to activate the 6 pickup channels; in practice care must be taken to prevent power-on surge pops, so the design uses a Zener diode and large capacitors to suppress those.  Again a single 74HCT4066 is used to control the supply voltage for each pickup.  We'll see how well this works!

These are all still prototypes - the winning design will be revised with surface mount devices (yes, even the relay board!) to allow it to reside inside the standard guitar cavity without routing.  The selected pickup signals feed into a summing amplifier, followed by a hex VCF for tone control and a hex VCA for volume control.

Stay tuned!
"There's no-one left alive, it must be a draw"  Peter Gabriel 1973

GuitarBuilder

The Nu Multi Switching Summing Amp redesign went very well - I added lots of filtering and converted the design to SMD.  This seems to have helped with the RFI susceptibility and now I'm getting a clean signal!  I also chose the Texas Instruments OPA1692 audio op amp for its excellent noise performance.  Here is the board plugged into the Nu Multi Relay Switching Board:


Nu Multi (Hex) Switching Summing Amp piggybacked onto Relay Switching Board

I call the Summing Amp the Judge because it allowed me to evaluate the three pickup switching candidates in a previous post - miniature relay, analog switch and power switch.  As you might guess the miniature relays work flawlessly.  The analog switch was fine as well!  The power switch still produced minor power on pops even with lots of filtering on the power line, so it is eliminated!  The choice between the miniature relays and analog switch is mainly based on PCB real estate; I tend to lean towards the relays at this point and have identified a SMD relay that is truly tiny!  More on that when I publish the final SMD design!

This photo shows the progress so far on my prototype custom electronics for the Triple Nu Guitar:


Triple Nu Guitar prototype electronics with traditional pickup switch

It all works!  I can successfully switch or combine the three hex pickups and feed the result via the LEMO 19-pin connector to the Cycfi Research Nexus breakout box, interface and DAW (or directly into 6 amps) or to the Eurorack Single String Processor.

The final challenge is to replace the traditional 5-way switch with an electronic touch version.  The prototype worked fine and I designed a final SMD version that is a drop-in replacement for the mechanical switch.  It is still in the testing and revision stage; I will report on that when it's done!

Meanwhile, the VCF + VCA boards will be arriving and tested soon.

Stay tuned!
"There's no-one left alive, it must be a draw"  Peter Gabriel 1973

GuitarBuilder

Quote from: jim-analog on August 07, 2019, 10:08:53 AM

Greetings,

Peter, you're off the deep end! The entire project looks absolutely great.

  What are you planing to use for a VCA as mentioned in item #6 and for the VCF in #5? Keep up the great work and keep us updated.

BTW, the body has a really nice grain pattern; is that a different one from the first photo?

Regards, Jim

Sorry for the late response, Jim!  The VCF is based on the SSI2144 and the VCA on the SSI2164, both from Sound Semiconductor.

Yes, the first body photo is a stock image from Warmoth (I forgot to photograph mine before routing!), while my prototype is a Warmoth Southern Swamp Ash body with Quilted Maple top.
"There's no-one left alive, it must be a draw"  Peter Gabriel 1973

GuitarBuilder

Update - All prototype boards for the Triple Nu Multi guitar have been completed and tested!

A large number of discoveries were made in the process, leading up to the final configuration:

* Subminiature audio relay switching for all 18 incoming string signals
* Low-noise high-quality audio mixer amplifier to generate the final 6 channel output
* 5-Position touch switch with LEDs for pickup selection
* CMOS logic circuitry to decode touch switch positions and drive relays (no microprocessors were hurt in this process!)
* Custom Nu Multi Breakout Board with multiple power supply voltages

Here are some images showing the prototype boards interconnected and in operation:


Triple Nu Multi Guitar Prototype Boards interconnected.  Relay/mixer boards are on the bottom right, touch switch logic on the bottom left.  The Nexus Internal Breakout board is on the guitar body.  Pickup 1 (Neck) is selected on touch switch.


Triple Nu Multi Guitar Prototype Boards interconnected.  Pickups 2 (Middle) and 3 (Bridge) are selected on touch switch.

Now on to the final product!

As you can see in the photos above, the prototypes take up a lot of space in the guitar.  While it was OK to rout out additional space in my test body, this would not be desirable for the final product.  The goal is to fit all electronics inside a standard Stratocaster cavity.  In order to assess feasibility, I created a mockup of the final boards.

Below is the first mockup of the final product; it consists of 2 interconnected boards with 3 CV pots and a touch switch/logic board assembly (not shown):


Mockup of final board assembly showing location of SMD parts.


Side view of final board assembly mockup with CV pots.  The pots are used to attach the boards to the pickguard.


Mockup of final board assembly showing location of SMD parts.

The only wiring will be the three pickup cables, two 19-pin connector cables, and touch switch logic ribbon cable.  All circuitry will be surface mount technology, including the relays.  This is the most efficient way to pack this much circuitry into the standard cavity.  Reliability should be high due to a minimum of connectors used.

What about the hex VCF and VCA?  That circuit was completed, but even in SMD form it took up too much space and could not fit into the final stacked boards.  This is actually not a huge issue, since the hex output of the Triple Nu Multi guitar is fed directly into the Eurorack Guitar Single-string Processor and Synth via a Nexus Breakout Module:


This is a 19-pin breakout module in Eurorack format, with multiple combinations of audio and control voltages possible.

This Eurorack modular synth contains 6 advanced VCFs and 6 VCAs that can be directly controlled by the three CV pots on the guitar (master volume, tone, and resonance control voltages are available on the Nexus Breakout Module).  You can follow progress on Eurorack Guitar Single-string Processor in this thread:

https://www.vguitarforums.com/smf/index.php?topic=25929.0

So off I go - I will keep you updated on progress with the final board, as well as future availability (yes - this will be a product you can buy!).

Note: even though this exercise involves three hexaphonic pickups, the design can accommodate any number of them! Two pickups (bridge and neck) could be a popular combination.  In addition, a combination of one hexaphonic pickup and several (1-3) standard magnetic pickups could be accomodated, as well as the inclusion of hex piezo pickups.  All of those could require some minor circuit modification, depending on the desired configuration.

Stay tuned!
"There's no-one left alive, it must be a draw"  Peter Gabriel 1973

Chumly

GuitarBuilder, I am extraordinarily impressed with everything from your luthier skills to your electronics assembly! I agree that avoiding pitch to MIDI and staying with a hexaphonic signal patch is the key.
I think it's much more interesting to live not knowing than to have answers which might be wrong. - Richard P. Feynman

GuitarBuilder

Quote from: Chumly on June 23, 2021, 07:08:03 PM
GuitarBuilder, I am extraordinarily impressed with everything from your luthier skills to your electronics assembly! I agree that avoiding pitch to MIDI and staying with a hexaphonic signal patch is the key.

Thank you, you're most kind!
"There's no-one left alive, it must be a draw"  Peter Gabriel 1973