With this thread I hope to provide regular updates on my latest idea: a guitar processor (mostly analog) that retains single-string audio processing all the way to the output, including the ability to drive synths. I'm a huge fan of control voltage (CV) systems (I built my first guitar synth in the late 70s using CV control from the guitar and foot pedals) for synth control, so the Eurorack format was a natural choice for me. I like the performance of my Fishman TriplePlay (FTP) system, but there is something about guitar to MIDI conversion that feels unnatural to me; my motivation is partly driven by the superb playability of my Roland GR-300 - I would like to see if I can come close to that with this new processor. Of course MIDI will be used for presets - there can be no knob twiddling while playing guitar.
The concept is as follows for a 6-string guitar (but can be expanded/reduced for any number of inputs):
Guitar with hex pickup > Eurorack hex breakout > Hex pitch shift > Hex audio controlled oscillator (ACO) > Hex audio mixer > Hex voltage controlled filter (VCF) > Hex voltage controlled amplifier (VCA) > Hex output mixer
The main goal is to retain analog audio processing as much as possible; this is obviously not the case with pitch shift. At the ACO stage the signals will be split for parallel processing (guitar + oscillator) with a voltage-controlled mixer ahead of the VCFs. Of course the requisite support modules (Hex VU meter, low frequency oscillators, envelope generators, CV-MIDI-CV, etc.) will all be there. Another advantage of the Eurorack approach is the ease of experimenting with different modules, either home-made or commercially available ones, with essentially unlimited parallel/matrix processing.
Modules that are currently not commercially available include the hex breakout, hex pitch shift, hex VU Meter, and hex ACO. For the hex breakout I have three designs: a) GK 13-pin input breakout with amplifier/attenuator, b) GK 13-pin output breakout with amplifier/attenuator, and c) Cycfi Nexus 19-pin input breakout with amplifier/attenuator. The GK 13-pin input and output breakout modules have just been completed; the Nexus 19-pin input breakout module is in the PCB fabrication stage. For convenience all modules have input/output pin headers on the back which are useful for permanent connections, such as audio paths.
GK 13-pin Input Breakout close-up:
Next to Antares ATG module and input Hex VU Meter:
GK 13-pin Output Breakout close-up:
Next to output Hex VU Meter:
The 16HP input module breaks a GK guitar output into 6 individual string channels and 1 traditional pickup channel. These are fed into 7 amplifier/attenuator circuits with up to 10x gain to match the typical Eurorack module signal levels. This module also supplies +/- 7V to the guitar, just like a Roland/BOSS synth would. All GK control voltages are broken out as well.
The 16HP output module does the reverse: 7 attenuator circuits bring the signal back down to GK levels and provide a 13-pin output to your favorite synth, as well as all GK control voltages.
In another post I showed my first attempt at an Antares Autotune for Guitar (ATG) Eurorack module:
https://www.vguitarforums.com/smf/index.php?topic=25404.0. This module uses the ATG Luthier Kit parts and replicates the entire guitar installation in a 10HP Eurorack module. Since Antares would not provide a hexaphonic output (not sure why, the DSP is processing 6 channels in parallel!), I decided to splurge and create six Eurorack ATG modules, each processing only one string! Brute force and expensive, but I happen to have a lot of ATG Luthier Kits! The hex pickup inputs are routed inside the case via a custom ATG connector breakout board, so no patch cords are needed on the front!
One unintended benefit of this approach is the ability to process each string with different ATG settings; this opens up a large number of new tonal possibilities beyond even the Axon or FTP fretboard split approach. Not shown is a master module that controls the parameters of all six ATGs at once, as well as MIDI control via iPad with MIDI Designer.
I love visual level indicators and a Hex VU Meter is a must for proper input and output level control. I couldn't find one of those either, so here is my design.
Hex VU Meter 10HP Module:
The ACO module will essentially be a Sonicsmith ConVertor+ (
https://sonicsmith.com/products/convertorplus/) in Eurorack format, based on the new ACO160 audio controlled oscillator available this summer (
https://secondsound.com/aco160/). Since that is a monophonic device, six modules will be necessary for this application. The outputs are triangle, square, and sine waves; CV out includes pitch (V/oct), envelope, and gate/trigger. There is the option to create MIDI note out via an SPI interface as well. although that is not my first choice at the moment. As mentioned earlier, the six ACO synth outputs will have their own parallel channels from the six single-string channels. The key mixing points are at the Hex VCF and/or Hex VCA inputs, voltage-controlled of course!
I plan on using commercially available VCF, VCA, Hex Mixer, LFO, AR, and ADSR modules to start; another key feature will be the ability to use effects loops on each single string processing channel and patch in cool guitar pedals either before the VCFs or at the VCAs. The output section will be hexaphonic - I feel this is an important feature: why process single-string audio only to mix it back to mono or stereo in the end? I'd like to experiment with six amplifiers and speakers in different spacial arrangements (perhaps I'm still in awe of my first Pink Floyd live quadraphonic concert!). The VCAs will come in very handy in moving the sounds around spatially!
Below is a shot of the Eurorack chassis I will be using for the initial phase of this effort:
This was taken at an earlier time when all I had was one Hex VU Meter and a bunch of blank panels! BTW - all panels were made using Front Panel Express (
https://shop.frontpanelexpress.com/index.php#Shop). The TipTop Audio Mantis case (
http://tiptopaudio.com/mantis) is quite nice, but not large enough to hold all modules in the final configuration. I will likely move to two Intellijel 7U Performance Cases (
https://intellijel.com/shop/cases/104hp-84hp-7u-performance-case/) in the end - these have cool 1U strips where I can place the effects loops, MIDI, and various pro audio connections.
Progress is slow but steady due to my guitar build projects (stay tuned for a triple Cycfi Research Nu Multi pickup Strat with electronic hex pickup switching and built-in hex VCF/VCAs!) and I will make a point of providing regular updates as I progress. Sometimes I wonder why I took on such a huge project!
One interesting intermediate product is the standalone fully Hexaphonic ATG; this is not extremely large like my crazy idea above - only 112HP total in a single 3U rack; here's a mock-up using MODULARGrid:
I plan on building this first using a Roland VG-99 stand and iPad; with MIDI Designer controlling all six ATGs it should be usable without any patch cables!
Finally, I will be looking for VG guitarists who would be willing to test and record demos of the Hex Autotune Processor and the Single-string Analog Guitar Processing monster in my studio when finished. PM me if you're interested!
