Friday, 17 September 2021

Synthesizer Extra's No.3: THE GRISTLEIZER.

An effects unit made famous by British Industrial Band 'Throbbing Gristle' from the late 70's. The band used it on everything, from guitars to synthesizers and even microphones and it will take all the different input levels without problem.

I built this as a stand-alone effects unit, as it originally was. There are now numerous variations of the Gristleizer (pronounce as: Grissel-eye-zer) available. Even as Eurorack modules with lots of extra functions but I wanted to build the original one as used by Throbbing Gristle back in the day. Band member Chris Carter built the unit from an article in Practical Electronics Magazine (July 1975 issue)
(The above link will lead you to a downloadable PDF of the entire magazine. The article starts on page 29.)
The circuit was based on a design by a then 15 year old Roy Gwinn. Next to the link above you can get just the relevant article in PDF form in the 'Files' section of the EDDY BERGMAN Discussions FaceBook Group.
I first became aware of this sound effects unit when I watched the documentary 'Synth Britannia' on YouTube. (Click here to see the part about Throbbing Gristle).

Here's a picture of the original unit built by Chris Carter. This was Cosey Fanni Tutti's original unit used until 2009 when it stopped working:

Here's a look at the inside (low resolution):

Below is a link to the website of Chris Carter which has tons of links on it to all sorts of Gristleizer related webpages. It starts with a lot of videos but if you scroll down you'll find links to the various webpages.

Chris Carter's webpage about the Gristleizer:  -CLICK HERE-

The Gristleizer on Boing  - CLICK HERE-

The schematic below is a modern update of the original one from the article in Practical Electronics, and this is the one I used to build mine. The two diodes in this circuits are 1N34A Germanium diodes but I used 1N4148 and this works just as well. 
This circuit runs on a dual 9V powersupply but I used a dual 12V powersupply built into the case. It's not an issue because the opamps can easily take it. In the picture above you can see the original used two 9V batteries to create a dual 9 Volt powersource. The circuit itself is really rather simple. The only thing that can be confusing is the wiring of the 4 way switch. Luckily I got that right at the first try.

In the version I built, all the potmeters are mounted on the front panel, even the ones initially intended to be trimmer pots that you only set once, like Shape and Offset, I thought it might be handy but these potmeters don't do anything at all whether the unit is in VCA or VCF mode. These controls are only meant to set the triangle waveform anyway. If you want to save some space on your panel you can leave out the Shape and Offset potmeters and put some trimmer potmeters on the stripboard instead. I made an alternative layout with trimmers that you can find at the bottom of all the other layout images. So please read through the entire article before you start building.

It's difficult to describe the effect this unit has on any sound you put into it. It has a built-in LFO with a choise of 4 waveforms, that gives a tremelo effect to the sound but that can be cranked up all the way into the lower audio frequencies which gives a cool sort of distortion. 
I did some internal measurements and I measured an LFO frequency from one cycle every 70 seconds to 115 cycles per second (115Hz).
It also has a resonant filter that is mixed into the sound. but you can't set any Resonance like a normal VCF. Watch the demo video below to see what it does. It's quite unique. I can see why it fitted the music of Throbbing Gristle so well.
One cool thing about this circuit is that it can handle a really wide range of input signal amplitudes. You can feed it line level signals or signals at the synthesizer level of -5/+5Vpp or 0-10Vpp. All you need to do is set the 'Bias' potmeter to the appropriate level. Bias has influence on the level but also the sound itself. It is the most important potmeter in the circuit. It needs to be set accurately to hear the tremelo effect. It's a touchy potmeter and it's only really useful in the last bit of the throw of the potmeter whatever the level of the input signal is. It's fortunate that there is an output level potmeter because the output can be very loud because this unit can amplify the audio, but if you turn the Bias down the audio level can go down too and with the output level potmeter you can crank it up again. Because the Bias pot is so twitchy, with small movements having a big effect, I don't think it is wise to put a Vactrol across it to create a CV input for the Bias control. I don't think that'll work.

When I first tested my unit with line-level input signals I could hear a ticking noise mixed in with the audio. I first thought this was a power supply issue but then I found out that I had forgotten to put in capacitor C6. This is the 100nF one on the left of the print from the cathode of the diode to ground. I soldered it in and, because I had an ON/OFF switch on the front panel that I hadn't wired up yet, I used the switch to turn on or off capacitor C6. You can just hear the effect it has. It smooths out the audio a bit. I can see why the cap is there. If I turn the capacitor C6 off, the sound is slightly rougher.
Btw, you can connect a dynamic microphone straight to the input of the Gristleizer and experiment with the effect is has on the human voice. Should you experience noise or weird sounds in the line-level audio then turn down the voltage from the powersupply a bit (if you can). I just did a test with dual 9V and dual 12V and a lower voltage is definitely better for line-level signals. I made my powersupply with LM317 and 337 regulators so I can easily adjust the voltage. Dual 12V is better for synthesizer level signals.
Btw, when I say 'line-level' I mean signals that come straight out of a guitar or microphone. Signals in the range of 100 milliVolts peak to peak.

Here's the (verified) layout I made for it. Luckily when I first tested the unit, everything worked rightaway which was fortunate because I couldn't test it until I finished building it all up. I included a bypass switch in the layout which I thought might be handy to have. The Bias potmeters is wired the other way around from what you would expect. I thought this was the best way but it doesn't really matter which way you wire it up. The layout below has the supply voltage at +/-9V but I run mine on +/-12V which works fine too. (There's an alternative layout with trimmers for the Shape and Offset potmeters further down the article and that is the version you should build.)
Wiring Diagram:

Be very accurate with soldering up the 2 pole 4 way rotary switch. Solder the resistors and jump wires straight to the switch first, before you wire it up to the stripboard.

Here's the print only view. There are no bypass capacitors in this design but in my own build I did put a 100nF cap over the plus and minus rails near the TL074, but it's not necessary, especially not if you feed it with batteries:

Cuts and wirebridges, component side view:

Cuts only, copper side view:

Here's the Bill of Materials:

Here is an alternative layout which lets out the Shape and Offset panel potmeters and replaces them with trimmers on the stripboard. This is the version as used by Throbbing Gristle and this is the version I would personally advise you to build! 
Beware that the stripboard is a little bit longer (43 holes in total) and that it has two extra cuts in the ground strip next to the trimmer potmeters, so the wipers don't connect to eachother or to ground.

Once again, be very, VERY accurate when wiring up the switch. You must get that right before doing anything else otherwise it won't work right.

Here are some pictures of the print and the wooden case which I also built myself from 3,5mm plywood. I forgot to take pictures of the print alone during building. In the pictures capacitor C6 is not yet put in. I had forgotten it first and after I put it in I could notice that it smooths out the rough edges of the sound, so to speak.

The picture below shows the powersupply print glued to the inside top of the case:

This is the front panel. You can see an ON/OFF switch to the upper right, which I intended for the powersupply but I didn't fancy putting 240V on a switch like that so I used it instead to switch on or off capacitor C6, just as an experiment. I can use that as a sort of extra distortion or drive switch but the effect is really not dramatic. So there's no need to replicate that C6 switch. You'll only be disappointed.

The finished product:

Here's the latest version of the front panel of my unit. I put red knobs on the potmeters I don't use and the ON/OFF switch now switches capacitor C6 on or off as mentioned before. I also marked some of the measurement values I got for the Shape and Offset pots, on the front panel.

Here's a demo video I made of my Gristleizer. It's a bit 'how yer doin' but it'll have to do for now. I'll make a better one later. You'll probably have to turn up the volume a bit.
For some reason my embedded videos from YouTube don't show up on mobile devices anymore. I don't know what that is all about but follow this link to my youtube channel if you can't see the video here.

As you can hear in the demo it's not a spectacularly amazing mindblowing effects box but rather something very practical for live gigs. Something to run a microphone through to make the voice sound weird or something to distort a synth or guitar. It is perfect for the sort of music that Throbbing Gristle were known for but don't expect miracles from it. 

To close off this article I'll show you some screenshots from the oscilloscope. The first one is a look at the squarewave with resonance behind it, the main sound effect this unit provides. The other show the signal on pin A3 of the 4-way rotary switch when it's set to Triangle and turning the 'Shape' potmeter.

Shape potmeter fully Clockwise, probed pin A3 of the 4-way switch:

Shape potmeter fully counterclockwise, probed pin A3 of the 4-way switch:

I used the 'Offset' potmeter to put the signal neatly on the 0V line. So if you build this unit with trimmer potmeters you need to probe the triangle pin on the A side of the 4-way switch and turn the trimmers until you get a signal that looks like a triangle and turn the offset so the signal is sitting on the 0V line or 0V goes through the middle. I'm not sure which is better but you can't hear any difference in the sound.

I want to leave you with a video of Roy Gwinn, the inventor of the Gristleizer circuit, which I recently found on YouTube. Hear the man himself talk about the circuit and how it came to be. The video was uploaded in 2017 and it mainly deals with the Eurorack Gristleizer that had just come out but it's very interesting to watch.

If you can't see the video then click this link: -- Roy Gwinn Video --

Okay, that's it for now. If you have any questions please leave them in the comments below or post on the EddyBergman DIY Projects Facebook group.

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