Wednesday 15 December 2021

Synthesizer Build part-46: 808 KICK for EURORACK. (Juanito Moore circuit).

The kickdrum from the famous Roland 808 drummachine. With four controls and a print small enough for Eurorack (although a bit deep). Naturally you can just as well build it in a Kosmo size if that's the size you're using to build your modular synth. 

Now that I've started to play the modular synth I built more and more, I felt the need for some percussive action so I started out with this famous kick drum sound using a schematic from Juanito Moore who is famous for building his modular system without using any circuitboards at all just 'dead bug' soldering and he's really good at it too. A real inspiration for DIY synth builders like me. 

The layout I made worked flawlessly right from the get go. This is quite a straight forward build. It requires 4 panel potmeters of different values and you must keep to these values too. You need a 5K, 10K, 100K and 500K panel potmeter. (470K instead of 500K and 4K7 instead of 5K will be fine too.)  
You can modify the T-filter by changing the 15nF caps. Smaller values will give you higher tones but it will disrupt the balance of the filter and cause the Decay function to stop working correctly. Also there's no CV control for this module because it's not practical to implement. Here's what Juanito himself had to say about that:
"The decay not working right with different cap values is due to the properties of a bridged-T filter that oscillates with a ping of voltage. I gave up on voltage-controlling an 808 kick because the decay, dictated by the laws of physics, changes with pitch. Also, if you use a fancy voltage-controlled resistor (LM13700 datasheet) when you change the CV, the kick will trigger. A Vactrol was the best I managed to get."
So you can get away with putting a Vactrol over the 'Pitch' potmeter but that's about it. I personally didn't bother with CV control.

Here are the verified layouts I made for this module. I marked two screwholes on the layout but I didn't use them. I just hot-glued the print straight to the back of the potmeters once I had the panel ready and this works just fine. I glued the topside of the print with the eurorack power connector pointing downwards (see pictures below). Beware that this does make the overall depth of this panel 7.5cm which won't fit some eurorack cases!
Wiring Diagram:

Print only. Pay extra attention to the connection of the transistor in the upper left. The emitter leg skips one copper strip and is soldered directly to the ground strip of the power rails. Strips B,C and D are all ground and I connected them together on the print by putting extra solder under the power connector so it bridged the middle three ground pins, shorting them together. Make sure to use polystyrene, polyester or silver mica type capacitors for all but the de-coupling caps (if you choose to include de-coupling caps. They're not on this layout). It's important not to use ceramic caps in the filter section because of various reasons. If you want to include de-coupling caps then solder some small ceramic 100nF caps over pins 4 and 5 and pins 10 and 11 of the TL074:

Here's an overview of the cuts and the wirebridges seen from the component side. As always; mark the cuts on the component side with a sharpy marker pen, then stick a pin through the marked holes and mark them again on the copper side and then cut the copper at the marked holes. Do this and the wirebridges first and check the cuts and wirebridges by measuring with your multimeter for continuïty. Then solder in the rest of the components.
Cuts and Wirebridges seen from the component side:

Here's the schematic I used to make the layouts, drawn by Juanito Moore in his distinctive cool style. There are two versions of this schematic in circulation and one of them has the clipper section wrongly connected but this is the correct schematic:

Here's a Falstad simulation of this circuit which allows you to test different component values and see what the result is on the output. I put an audio output at the end that allows you to play the result and listen to it. It comes really close to the real thing.

The 'Clipper' switch increases the amplitude of the drum sound extra (when the switch is open) and provides a little bit of distortion which makes the sound more audible. The low frequency of this kick drum can be so low that you can hardly hear it but through a good PA system you will feel it in your stomach because of the extreme low frequencies. It shakes the windows in my attic and makes the dust fall from the beams LOL. It really is an exact replica of the original 808 kick drum sound. I have the Behringer version, the RD-8 drummachine in my little studio and the kick sounds just the same.
I soldered the 33µF electrolytic cap for the Decay straight to the potmeter to save space. There was one opamp left over as you can see in the schematic. I used that opamp to drive a little LED connected to the output so we have a visual reference of the output without pulling any current from the output to drive the LED. It's always handy to have a visual indicator to see if the circuit is triggered correctly. Plus a LED always looks cool in a module.  The 1K current limiting resistor for that LED is soldered directly to one of the legs and reinforced with some heat-shrink tubing. 

Here's the Bill of Materials. Make sure not to use ceramic caps, except for the 100nF de-coupling caps for the chip if you want to include those, but they are not included in the layout or this BOM. 100nF de-coupling caps can be soldered directly on the copper side from the plus pin of the TL074 to ground and from the minus pin of the TL074 to ground. Make sure the legs have some heat-shrink tubing on them so they don't cause short circuits:

Here are some pictures from the build proces and the finished panel:



Like I mentioned before, the depth of the module as you see it here is 75mm (7.5cm) so it might not fit in some Eurorack cases. Keep that in mind. The width of the module is 6hp. (3 cm). You could save some depth by rearranging the potmeters to be directly underneath eachother and then hot-glueing the print straight to the back of the panel instead of on top of the potmeters. 

Instead of making my own demo I thought I'd embed Juanito's own video here. This video is over two hours long because he shows the complete build process but this link will start the video at the end where he demonstrates the functions. It should start at 2:23:36 If not, then just jump to that time manually.

If you can't see the video on your mobile device then CLICK HERE to view on YouTube directly.

Here's a link to Juanito's YouTube channel. Subscribe to his channel while you're there :)

Okay that's if for this one, with grateful thanks to Juanito Moore for his reactions and for just being awesome :). 
If you have any questions or remarks please put them in the comments below of post them on the Eddy Bergman Facebook group where we have an awesome little community willing to help you with any problems you may encounter.

If you find these projects helpful and would like to support the website and its upkeep then you can buy me a Coffee. There's a button for that underneath the menu if you're on a PC or Mac. Or you can use this PayPal.Me link to donate directly. All donations go towards the website and projects. Thank you!

Tuesday 7 December 2021

Synthersizer Build part-45: STEINER-PARKER DIODE FILTER for EURORACK.

This is the same Yusynth Steiner Parker diode multimode filter I posted in project 26 but with a new layout for Eurorack.

I'm busy setting up a Eurorack system dedicated to live performances, so I want to remake some of the modules I built earlier to make them fit the Eurorack 3U size. So here's the first one I converted, one of my absolute favourite filters, the Steiner Parker multimode diode filter with Lowpass, Highpass, Bandpass and Allpass. This is a Sallen-Key type filter with positive feedback so that you don't loose volume when you increase the resonance like you do with the Moog ladderfilter for instance.
I won't go further into how it works etc. You can go to the previous Steiner Parker article for more details.
I started out matching the diodes I needed by measuring the voltage drop but they all came from the same batch and the measurements were so close that I stopped matching and just put them in (and the filter works absolutely fine). The transistors however must be closely matched otherwise the filter won't be in balance. You can set the right balance with the 1K potmeter but that's only a fine control so make sure the transistors are matched. You can match them by simply measuring the HFE and look for two with the same values.
When you start out building, make the cuts in the copper strips first and then put in the wirebridges. Then you can put in the rest of the components.

About component values:
For the level potmeters I used 10K linear ones because that's what I had. You can use any value from 10K up, it doesn't matter for level potmeters. Keep to the recommended value for the Cut-Off and Resonance though. I used a 100K for the Cut-Off frequency potmeter and I changed R26 to a 100K to make the voltage drop over the potmeter the right value. This works perfectly fine. You can of course use a 47K (50K) potmeter but then use a 47K resistor for R26. (R26 is the 100K resistor in strip A to the right).
For the 1,5nF filter capacitors I would recommend using good quality polystyrene, polyester or silver mica types. These form the heart of the filter so don't use ceramic caps for those.
Btw, I left out the two 10 Ohm resistors in the + and -12V strips because this filter was designed for 15V but running on 12V so I wanted to avoid any further voltage drops. I also left out the bypass capacitors but if you want to include those just put a 100nF capacitor from +12V to ground and one from -12V to ground right above the location of the chip. There's room enough left. (I did put them in later, just to be sure, but they are not on the layout or the bill of materials.)

Below are the layouts I made for this project. They are verified as always. I used these for my own build. I left out the second CV-IN and the second AUDIO-IN potmeters and jacks to keep the layout free from clutter. You just copy the first input if you want two of them (which I strongly advise you to do especially for the CV). The stripboard is 24 by 41 holes. The switch to choose between Lowpass, Bandpass, Highpass and Allpass is a normal 2 pole 4 way rotary switch. 
Instead of using a reverse logarithmic 50K potmeter for Resonance I used a 100K linear type with a 100K resistor soldered onto it to get the reverse logarithmic characteristic. (See layout below. Two 100K resistors in parallel make for one 50K resistor). This is the recommended alternative in the original Yusynth article and it works really well. Of course, if you happen to have a reverse logarithmic 50K potmeter then use that instead of the 100K pot + 100K resistor solution. Should you have problems with resonance coming in too soon, put a 10K resistor in series with pin 3 of the resonance trimmer potmeter to get the throw of the resonance panel potmeter more to the clockwise side. Thanks to Nick in the comments below for the heads up on that one!  Here's an image of the alternative wiring of the Resonance potmeter:

Wiring Diagram:

Stripboard only:

About trimmer T1: 
I changed trimmer T1 from a normal one to a multiturn trimmer which made it much easy to set. You need to set this trimmer so that the Cutoff frequency potmeter has the correct throw with full resonance at about 2/3 clockwise with the resonance potmeter set to almost self oscillation. I measured the resistance of T1 when I was done and it was about 640 Ohm.

This filter works best if it has a 1V/Oct CV permanently connected to it, although you can't play the self oscillation as you can with some other filters where you can use the filter as an oscillator. This filter's resonance is just too agressive for that.

Making the cuts accurately:
Here's a layout of all the cuts you must make and the wirebridges you need to solder in. This is viewed from the component side. Mark the cuts on the component side, with an Edding pen, and then stick a needle through the marked holes and mark them again on the copper side. Then you can cut them with a hand held 7mm dril bit. The cuts are all over the place so concentrate and be accurate otherwise the filter won't work. Don't forget the cut underneath the wirebridge at position S-19:

Bill of materials: Buy a batch of 100 BC547 transistors if you don't have any, so you have enough to choose from when looking for a matched pair. If you want to include de-coupling capacitors then order two extra 100nF caps because these are not included in the BOM. Order good quality polystyrene or silver mica or polyester types for the three 1,5nF filter caps.

Here's the schematic drawing by Yusynth:

Here are some pictures of the build proces and the finished product. Notice I had to put two capacitors in parallel to create a 680nF capacitor. I didn't have one in stock.

I soldered all the wires directly to the copper side of the print and mounted the print with the component side pointing backwards of course, otherwise you can't get at the trimmers. I put some Gaffa tape over the pins of the 4 way rotary switch to avoid accidental contact with the print or wiring.

A look at the finished panel. I managed to fit everything in nicely. I had this piece of powdercoated aluminium left over so that was perfect for this project. I made the 3mm mounting holes wider to give me some room to move the module sideways to fit the rest of the modules (which are yet to come ^___^)

Finally a word about hum. This filter is susceptible to hum especially when using a switching powersupply. I have no problems with mine in my DIY synth because it uses linear powersupplies but my Eurorack filter did suffer from this. I solved it by putting two 1000µF/25V electrolytic capacitors over the powerrails. This provides enough capacitance to suppress the hum. It worked fine for me. See picture below. The overall depth of the module is 45mm with these caps installed. Still below 5 CM.
There's a forum post about the hum problem here -- CLICK HERE --

Okay that's it for this one.  
If you have any questions or remarks please comment below or post them in the Facebook group for this website where we have a great little community willing to help anyone encountering problems with the projects.

If you find these projects helpful and would like to support the website and its upkeep then you can buy me a Coffee. There's a button for that underneath the menu if you're on a PC or Mac. Or you can use this PayPal.Me link to donate directly. All donations go towards the website and projects. Thank you!

Sunday 10 October 2021

Synthesizer Build part-44: AD/AR ENV. GENERATOR.

This is a simple AD/AR envelope generator by Ole Stavnshoej.  With the little alterations I made it's a very useful AD/AR for use with filters.

After re-doing the layout for the 7555 AD/AR I decided to try an other design and I found this one online. It's a simple design and easy enough to build. I intend this particularly for use on the CV inputs of filters and after I built it I made a few changes to make it better suited for this role.
First of all I made the electrolytic capacitor smaller so that the circuit would react faster and be more accurate. Later I added a SPDT switch (ON-Center OFF-ON) with some other electrolytic caps soldered to the switch so you can choose how fast you want the envelope generator to be. I also changed the 1M resistor coming of the 200K trimpotmeter to a 820K to make that trimmer more effective. Later I included the attenuverter potmeter and I changed the trigger input capacitor from 10nF to 3,3nF to make the circuit react better to fast playing. That change made all the difference. It now works much better and is very responsive. This is now my go to AD/AR for use with filters.

The trimmer sets the zero volt line or offset voltage but the strange thing with this circuit (if you use higher value caps for C4) is that if you turn the Release potmeter you also change the offset voltage. If you turn it all the way open the offset can be as high as +5V. So on long release times it never returns to zero. That's why this design is really only suitable for use to excite filters where you have very short envelope times. If you connected it to a VCA it would stay open all the time with long release times. The attenuverter mod however could help a bit in setting this straight but I haven't tested that.
However if you use a 1,5µF cap for C4 it doesn't give this problem. It starts with higher value caps that's why I put in the time range switch with different value caps. I talked about it with Ole and we both think it's due to residual voltage in the capacitor C4.

Instead of the TL084 you can also use a TL074 or an LM324 or really any quad opamp with the same pinout. Same goes for the TL082.

Here is the layout I made. It's verified as always. The inverted output is grayed out because I didn't use it and with the attenuverter mod you get both at the same time. But I leave it up to you whether you want to include it or not. I sent the normal output to a second opamp buffer, which I originally put on there for testing, and connected that to a second output. Always useful to have more than one output I think.
Oh and this circuit runs equally well on +/-12V as on +/-15V but the output envelope can reach almost the positive voltage rail so if you need lower amplitudes install the potmeter in the attenuverter mod. (Schematic + layout is further down the article)
Wiring Diagram:

Print only. (The green wirebridges indicate connections to ground):

Here's the schematic drawing. You will note that some values on the schematic are changed on the layout. I did this after testing so follow the values in the layout. Like I mentioned earlier I changed the 10nF trigger input capacitor to a 3,3nF one because the circuit was too slow if you play fast on the keyboard. 

Here is a Falstad simulation of the above schematic. I had to change the timing cap to a bi-polar one of lower value and the 1M offset zero resistor was changed to a 500K. Try changing values of components (right click on them) and see what happens. The circuit is triggered by a 1% duty cycle squarewave of 7V. If you want a Gate input then change the duty cycle to 50%

Bill of materials for the version without the attenuverter:

Here's the version with the attenuverter modification, which I of course only discovered after completing this build. But it's easy enough to implement so I changed my module later and included this option. It only requires a potmeter on pins 8 and 12 of the TL084. It did mean that I had to find a spot on the panel to put the potmeter. I ended up putting it between the output sockets. A bit awkward but at least it works very well.

Here's the layout, adapted to include the attenuverter potmeter. The only other change is that resistor R19, the 47K from pin 12 to ground, is removed.

Print only:

The way the attenuverter is wired up in the layout, you will get the uninverted output if you turn the potmeter fully clockwise and inverted output fully counterclockwise. (On my panel I had it the other way around. ^^) And wow does this make a difference! If I send the inverted signal from this AD/AR into the CV IN of the Steiner Parker filter you get almost a flute like, very clear sound if the filter is set a certain way. I loved it.

For extra clarity, here are the layouts showing just the cuts and the wirebridges, which you should do first before soldering on the components. These layouts are the same for the version with or without the attenuverter.

Cuts and wirebridges, component side:

Cuts only, COPPER SIDE!
Bill of materials for the version with the attenuverter. 

Here are some pictures of the print and panel. I didn't have the attenuverter connected yet in these pictures:

A look at the panel. You can see the Time Range switch I added to the left of the Trigger/ Gate switch.
I used a dual pole switch with a middle off position so I could have three ranges, short, medium and long. The Short setting (in the middle) uses just the 1,5µF cap on the print. The Medium setting adds to that a 2,2µF cap and the Long setting adds a 4,7µF cap to the one on the print. You can see the attenuverter at the bottom crammed in between the in and output sockets.

Here are some images from the oscilloscope:

Fast squarewave on input and then turning release up. 

Again opening Release on a fast pulse train with a little bit of attack. Note how the pulses go a bit below the zero volt line here. (Not a big deal for use with filters):

Fast Attack, tiny bit of Release:

Turning the attenuverter from negative to positive output. I'm running this module on +/-12V and you can see the maximum output is +/-11 Volt! Just beware that the output voltage can be quite high, especially if you decide to run this on +/-15V..

Just turning the Release knob without any input using a 4,7µF cap for C4. Note how the voltage on the output changes. This should be a flat zero Volt line. If you use the Time Range switch and set it to longer times and you turn up Release, it doesn't come down to zero volt anymore. Again, it doesn't do this with the 1,5µF cap only with higher values.

And finally a little demo video of how the attenuverter influences the filters while turning it from positive all the way to negative envelope output. Note the tuner on top of the case. It's connected to one of my Thomas Henry 555 VCO's and it's rock solid in tune! I really love those 555 VCO's. If I could I'd marry one :p

Doesn't this synthesizer sound great?? It's a real bass monster if you want it to be. I bet it can rival a MiniMoog in that respect.

Okay, that's an other article done. If you have any questions or remarks, please put them in the comments below or put your question to our awesome little community of DIY synth nerds on the EDDYBERGMAN Discussions Facebook Group.

Friday 17 September 2021

Synthesizer Extra's No.3: THE GRISTLEIZER.

An effects unit made famous by British Electronic 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. Alternatively, 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. I did later change the 1N4148 diodes for Germanium type 1N60 diodes but it didn't have any effect on the signal.
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. The offset control only has a visible effect if the waveshape that is selected is the triangle wave.

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 filter sweep sound combined with distortion. See the demo video below to get an impression of what it can do. 
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. 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.
EDIT: I have tried it with my Steinberger Spirit XT-2 bass guitar which has passive pickups and the signal gets through quite loud. In many ways this works mostly as a distortion and also as tremolo effect. If you turn the rate up you get a nice beating of 2 frequencies against eachother as the bass frequency comes near the LFO frequency. When they used the Gristleizer on Genesis' voice they must have has it in VCA mode. You can hear that in songs like 'Hamburger Lady'.
Overall I was not very impressed with the sound it produced when using it with my bass guitar. I think synthesizers sound best. 

I have not experimented with CV control for any of the parameters but I don't think putting Vactrols over the potmeters will work very well, but you'll have to experiment if you want CV control. I am however reliably informed that you can put an external signal (LFO) on the wiper connection, pin 2, of the Depth potmeter. You can use a toggle switch to switch between internal LFO or external CV or use an input socket with built in switch normalled to the internal LFO. Then you can connect and external ADSR and use it as a weird sort of filter. I have not tried this myself however.

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 to 1 Volt peak to peak.

Here's the new (verified) layout I made for it. The previous version that was up here until the 16th of November 2022 had a mistake in the layout. I had forgotten to ground pin 5 of the TL074. Now that that's all corrected the Gristleizer works like a charm. I included a bypass switch in the layout which I thought might be handy to have. (The weird thing in my unit is that if the Level potmeter is fully clockwise or fully counter clockwise, the bypass signal doesn't get through.) 
The Bias potmeter 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 if you want to build it like the original unit but I find putting those potmeters on the front panel much better.)
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.
You can connect together all the potmeter lugs that need negative voltage and all the ones that need positive voltage with hookup wire and then power them all at once with two wires coming off the power rails on the stripboard. That way you keep the wiring to a minimum.

Here's the 'stripboard 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. 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 stripboard and the wooden case which I also built myself from 3,5mm plywood. I forgot to take pictures of the stripboard 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. In certain settings it acts in the same way as the Bias control, adding a little more Bias as C6 is turned on. But it's not necessary to use a switch. Just solder in C6 as it normally should be. It's not a useful feature to have this capacitor switchable.

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

This is the front panel: 

The finished product:

Here's a new demo video I made of my Gristleizer directly as I tested the revised version. After correcting the mistake of forgetting to ground pin 5 or the TL074 everything works much better as you can see and hear. You might have to turn up the volume a little.
If for some reason the video doesn't appear then here's the direct link to YouTube:

The effects are I think very useful. You can get a tremolo effect going and turn it up into audio range and then mix it with a resonant filter so you get that filter sweep sound. Very cool I think. And I haven't even tried it with other sound sources like guitar or a (dynamic) microphone.

To close off this article I'll show you some screenshots from the oscilloscope.  These are the different LFO waveforms in each of the four switch settings:

The next one shows waveforms created in VCF Mode with an audio Sinewave as input:

This is in VCA mode. The LFO shuts or opens the VCA giving a Tremolo effect.

The above screenshots give an impression of the main effect the Gristleizer produces. There are of course numerous shapes in between with all the different potmeters you can set in so many ways. 

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. He had no idea his circuit was used and made famous by Throbbing Gristle until 2007 when he learned about it.

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

I made a Falstad simulation of the Gristleizer circuit which you can view by -- CLICKING HERE --

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.