Author Topic: Passac Sentient Six guitar MIDI controller - connect via Gvox hex pickup?  (Read 7419 times)

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I have a conundrum...  :(

I recently picked up a 1989 Passac Sentient Six guitar MIDI controller very cheaply off eBay, however I bought it without the proprietary hex pickup system. This special, hex pickup system plugged into a 1/4 inch jack on the front of the controller. I've since bought one of those Gvox hex/MIDI units off eBay with the intention of adapting it for use with the controller, and am hoping someone here may have some experience with this unit. Anyway, the Passac seems to be a rare beast, and information has been rather hard to find. From the few anecdotal pieces floating around on the web, I've basically come down to the following: The pickup system used bridge-mounted piezo transducers, connected to circuitry on the trem block which multiplexed the signal to the sleeve of a 1/4 inch stereo jack. That's about as much as I know...

So now comes the tricky bit: How does one go about adapting the Gvox to work with the Passac controller?

For those who are unfamiliar with it, the output from the Gvox hex is via a small 8-pin DIN type plug -I'm assuming one pin for each string, one common, and one disconnected pin. The pickup plugs into the 9 Volt powered, Gvox processor unit, which has six small trimmers to adjust the sensitivity for each string. From there, the output is a 9-pin, serial "D" plug, designed to attach to the serial port of a PC. I'm curious, is the signal coming out of the Gvox processor MIDI, or some other signal which requires further "interpretation" by the PC? If the Gvox does indeed convert to MIDI from the get go, then I'm somewhat doubtful as to whether it will be of much use. Somehow I need to digitalise the six individual string signals from the hex, and multiplex them into one signal... but what sort of signal does the controller unit want to see? There are also six trimmers on the back of the controller for "channel level" of each string, but again, I'm doubtful whether this will be of any use here. Does anyone have any ideas or info on this unit that could help? T'would be much appreciated indeed!  :)

Anyway, here are a couple of pics of the unit in question:


Also, here are the few bits and pieces of info I was able to scrape together from the web:
This machine had a number of incredibly cool features, but I'm not sure if they were ever actually implemented.  The pickup was a piezo unit built into a Kahler wang bar with surface-mount multiplexing circuitry to run all six string signals on one wire.
To the best of my knowledge these can't be used with a hex pickup as they expect some kind of signal with the strings already multiplexed before the signal enters the unit, and it enters on what looks like a standard guitar cable (predates Roland 13-pin, which is why the 6 info from the strings have to be multiplexed, because they're using just 2 wires, not 13). I don't know what kind of signal the Sentient Six expects to be fed but I'd love to find out.
I've been a longtime computer geek (still am) and some of the rhythm track (bass and drums) was sequenced with Performer on a little Mac SE with no hard drive- it had two floppies, and 2 megabytes of RAM!! There is also a very transparent keyboard pad that I programmed on a Korg M1. I believe I played that too, using my MIDI guitar system (Passac- which used a special bridge/pickup system on the strat) on a separate pass. I used the MIDI thing live to help cover some of the keyboard parts too.... pretty advanced stuff for '88.

The LED thing was the power lite for the MIDI thing (Passac) I used live. Very slick system at the time- You had to use their bridge (which I didn't care for) and the circuit board for it was mounted on the trem block. The bridge saddles had piezo pickups in 'em and the hex signal was multiplexed on the ring of a stereo guitar cable. I also had one of the tone controls on the strat as a MIDI continuous controller- It could be modulation, filter or whatever for the sampler. I used the midi as an effect to bring pads and such under the guitar sound- never really up front.

The guitar looked pretty stock, but I had some stealthy stuff going on. Made people guess....
I'm not 100% sure and I hope someone will be knowledgeable enough to correct whatever I got wrong.

I think I first saw one at 1988 Winter NAMM. They were made by Passac in NSW Australia. I think one of the big problems, and the reason they are less well known and understood, and that there are Sentient Six boxes around but not guitars with pickups to plug into them is that they never sold pickups to install in guitars or published info on how that was done. I think people bought a system, shipped their guitar to Passac's US technician to have a pickup installed and tweaked, and received back the S6 rack unit along with their own guitar of choice, now equipped to plug right into it. The pickups mounted internally and were not labeled, so guitars that had this mod done that got separated from the Sentient Six rack units are floating around looking just like other guitars and no one knows how to contrive a pickup that will plug into an S6 rack box.

The pickups were at least sometimes mounted in Kahler tremolo bridges on electrics.

I'd love to have more info. These S6 boxes are around but no info on how to set up a guitar to plug into one.
The passac sentient 6 is quite amazing and can be bought cheap sometimes on ebay. It is no longer made, and there weren't enough made for word to get around. It is a 2U rack mountable unit and is pretty heavy, but it tracks really well and has some very cool features, such as midi echoes that are on different channels so you can play canons.


Hi, dunno if you need the wiring of the gvox pickup, but here's the little bit I figured out from mine:

Also it is a mini humbucker for each string, probably fairly different output than a piezo.


Thanks very much Ben, that string-to-wire colour info is good to have! I'm not sure if the magnetic pickup versus piezo will be a big issue though, as long as there is a separate signal for each string. The biggest problem, I think, is figuring out what kind of signal the controller wants to see, and how to achieve this signal.

I just opened up the Gvox processor box, and had a good look around (sorry, I don't have a camera, otherwise I'd put up some pics). Inside, there's one PCB, and components are surface mount. As you will no doubt be able to tell, this stuff isn't exactly my speciality, but anyway...

The first half of the board looks to be an analog input section. The six signals from the pickup travel through a fairly extensive resistor-capacitor network, along with one trim pot per string (signal strength adjustment), and one 14 pin IC per string. The ICs are called P7F4, which according to online data sheets is a single-phase, full-wave, bridge rectifier. I guess all this is flatten the six individual signals and 'prepare' them for the digital section (?).

The second half of the board looks like microcontroller set-up (has CPU/memory/clock crystal/etc.). The two 'main' ICs are unidentifiable: the smaller one (32 pin) has a sticker over it, and the larger one (68 pin) has been sanded (:icon_eek:!) on top to remove the print. I'm assuming these are the brains/processor of the unit.

Additionally, the digital portion of the PCB contains the following ICs (descriptions are from data sheets I found online):
HC373 (20 pin): 3-state, octal, D-type latch
CXK58257AM-10L (28 pin): 32768-word X 8-bit, high-speed, CMOS static RAM
HC04 (14 pin): Hex inverter (contains six independent inverters).

What's also interesting is that it looks like provisions have been made for a battery to be attached to the PCB, however, this provision hasn't been implemented.

In terms of output, there are only three wires (brown, black, red) going to the output serial plug (9-pin, "D" shape), designed to be hooked up to a PC.
Black is connected to digital ground.
Brown is connected to pin 10 of the HC04 chip.
Red is connected to pin 9 of the HC04 chip.

As mentioned above, the HC04 is a "hex inverter" chip, containing six independent inverters. From the data sheet, it would seem that pin 9 is the inverter number 4 input, and pin 10 is the inverter number 5 output. Considering the serial plug is meant to connect to a PC, I'm guessing the red wire  is therefore for the PC to send information to the Gvox, and the brown wire for the Gvox to send information to the PC.

(HC04 data sheet:

Given this, and the here-say info from my first post -that the hex signal was multiplexed to the ring of a stereo quarter inch jack- would it be safe to assume that I should try wiring the brown wire to the ring, and the black wire to the sleeve? The controller rack has a plain audio out at the back too; I'm wondering if an analog signal from the hex may have been fed to the tip of the input plug. But in that case, what should be used for the ground signal (sleeve), analog or digital ground?


Ok, tried wiring up the Gvox unit to a 1/4 inch stereo jack as per my last post, but no luck. Surprisingly though, when I did this, the power LED in the Gvox was able to light up, even without being plugged into its wall wart supply. The Passac controller is obviously supplying its own power via the input jack. No doubt this was originally intended for powering its own proprietary hex/multiplexer system. I'll open the Passac unit in the next couple of days or so and see what I can find...


Re: Passac Sentient Six guitar MIDI controller - connect via Gvox hex pickup?
« Reply #4 on: February 26, 2013, 03:55:20 PM »
I recently came into the split pickup/Khaler bridge.

It's a fixer upper, but I'm going to list on ebay with a low starting price.
which part of sin theta plus index times sin theta times ratio do you need me to clarify to you?


Re: Passac Sentient Six guitar MIDI controller - connect via Gvox hex pickup?
« Reply #5 on: December 16, 2015, 02:21:42 PM »
I just noticed this thread and thought I'd add a bit of info about the Passac

My buddy and I actually bought all of the remaining stock of the (240V) Sentient Six units when the company went into liquidation
The problem wasn't that they didn't make enough units, they actually made too many and got into cash flow problems
There were a few palletloads of 110V units that didn't get sold at auction because they'd already been paid for and were destined for the US

The piezo pickup system was even more complex than previously mentioned
The Sentient 6 had the ability to sense 'pick direction', which enabled the unit to send on different midi channels determined by up or down pick strokes (pretty amazing at the time)
For this to work, the Kahler bridge saddles had to be fine tuned by hollowing out cavities in the metal to match the resonance of the bridge piece with the piezo insert

So we had a bunch of units, some Kahler bridges, and a whole bunch of (unlabelled) bridge saddles that we had to match to the circuit by trial and error
Not being technicians it was a foolhardy exercise, but we did get a bunch of units happening and sold at the time
There would've been at least sixty rack units left when we ran out of bridges, and there's not much you can do with them
Anyway, they created a useful dividing wall at the factory heh

Such a shame that Passac failed, that unit was so ahead of it's time. Pick direction, built in sequencer, and the ability to change patches by hitting a footswitch and a fret. Features that you still don't see 27 years later