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 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 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 and 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. 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:

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. 

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!

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.

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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 circuit was based on a design by a then 15 year old Roy Gwinn. You can find this original article in PDF form in the 'Files' section of the EDDY BERGMAN Discussions FaceBook Group.
I first became aware of this sound effect unit when I watched the documentary 'Synth Britannia' on YouTube. (Click here to see the scene 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 straightaway.

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.

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 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.)
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. 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.

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 not dramatic.

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 an extra 'Drive' option. I also noted 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 go to my youtube channel if you can't see the video here.

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.

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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Monday, 19 July 2021

Synthesizer Build part-43: VACTROL VCF-1 by Skull & Circuits.

This is a bit of a weird one among filters and because it uses Vactrols it doesn't sound like any other filter. It has some percussive qualities too if you feed the CV IN with an LFO.

After not having built anything for two months I decided to get going again and this filter seemed like a nice one to try. It's a very simple design and thus easy to build. Because it's such a small print and because it works on a dual 12V powersupply I made the layout more Eurorack friendly.  I have not tested this circuit on a dual 15V powersupply but I think it will work just as well on 15V.  
This project is more of an experimental one than one where the end product behaves in a way we expect from VCFs. What I mean is that the audio from this filter sounds different to all other filters and the CV IN behaves differently too. When I put an AD (envelope generator) signal on the CV IN, the resonance only comes up as the AD signal fades out, the opposite of how other filters behave and when I try to use a negative AD signal by putting it through the Dual Voltage Processor's Attenuverter, it doesn't give the expected result. Not even when I apply an offset voltage to the AD signal. So this is an experimenters dream. It works, but not like you expect. Just know that before you proceed to build it. ^____^

This is a Skull and Circuits design and you can find the original information by clicking here.
This is a 12dB LowPass filter, so a two pole instead of the usual four pole filters  This means the frequency rolloff won't be as steep as other (4 pole) filters. 
The Vactrols are something you'll need to make yourself. Well, you can buy them ready made but I advise to make them yourself because new ones can be hard to find and expensive and the ones from China will be fakes, I guarantee it. In the information link above is a video that shows you how to make your Vactrols step by step. I used two 5mm red LEDs (you need two Vactrols) and two LDRs no. 37 from a set with 10 pieces of 5 different types of LDR from eBay. They come in 5 little plastic bags and they are all numbered. I used the ones from the bag numbered 37. I had used those before on the Lopass Gate and they react very fast to light changes. The way your filter will turn out will be different from my filter. There are so many variables here that filters may turn out to be alike but never quite the same. This is mostly due of course to the Vactrols and the way you set the three trimmers.  I tried two different types of LEDs in my Vactrols. First I tried bright white LEDs but with those I needed to turn most of the trimmers completely clockwise or counterclockwise to get the filter to behave correctly. The second time I used normal red LEDs (5mm) and they worked better. Now I could use the trimmers more accurately to get the right sound. So I advise to use red LEDs for you Vactrols. The LEDs in the filter itself, mounted on the front panel, do not shine very bright in my module. I used two 3mm yellow LEDs and they only come on when the trim-potmeters are set to extreme positions. I can get them to shine brightly when I turn the trimmers a certain way but then the filter doesn't work properly. Again this is one of those things that will be different with your filter because of component tolerances and differences in the Vactrols etc.
Just out of interest, here are some measurements I took of my DIY Vactrol (out of circuit) with red LED:
OFF resistance = >200MOhm.  ON resistance with a +/-5Vpp signal = about 10K, with a 0/10Vpp signal = about 5K. This is with a 1K resistor in series with the 5mm red LED of the Vactrol.
Of course these values differ when the Vactrol is in circuit because there you can set offset voltage etc. which alters the resistance.

NOTE: This module pairs extremely well with the Voltage Processor and the Lopass Gate module. If you create a beat with an LFO into the CV IN of the filter but first put the CV through the voltage processor you can accurately control the sound and if you then send the audio through the Lopass Gate you can turn your beat into a sort of galloping beat. Connect a signal from the same LFO used for the VCF to CV1 of the Lopass Gate and use a faster LFO signal on CV 2 of the Lopass Gate. Used in this way the Vactrol VCF can be used to make some cool Techno kickdrum sounds.

Here is the layout I made for this filter. As always the layout is verified. You can see it's small enough for a Eurorack module and I incorporated a Eurorack power connector to make life easier for those of you using that format. I myself didn't use the connector in my build but just soldered the power cable straight to the print like I always do.

Print Only. Note that the copper strip between pins 3 and 12 of the IC is NOT CUT! Both pins are connected to eachother underneath the chip:

Strangely enough I found that I needed to reverse the wiring of the Cutoff potmeter for it to work the right way around. I left it the way it is presented on the schematic, in the layout so you can decide for yourself whether you need to do that or not. 

Bill of materials:

Trimming this circuit is a matter of trial and error. You need to set the trimmers in such a way that you get a good deal of self oscillation when the Resonance potmeter is set almost completely open and you need to get the right range for the Cutoff potmeter. I can not give you a procedure to help you do this. You will have to figure it out yourself but it's pretty straight forward. Just put a squarewave or rampwave signal on the audio input and connect an LFO signal (+/-5Vpp) to the CV input and then connect the output to your VCA. Changing R7 from a 47K resistor to a 100K trimmer helped a lot in getting this filter trimmed. It was suggested in the text of the original Skull and Circuits article about this VCF and I also saw they implemented it on the PCB so that's why I put in a trimmer for R7 in the layout. It might be a good thing to use multiturn trimmers instead of the single turn ones I use in the layout. I used normal trimmers on my print but it can be fiddly to set these correctly. I might replace mine with multiturn ones too. You might think it would be a good idea to put an attenuation potmeter on the audio input but I tried that using the passive attenuator on my mixer but it didn't help at all. The filter only worked if I had the potmeter fully open so there's no need for an audio input level potmeter. As you can see in the schematic, the audio is attenuated by a factor of almost 18 times by the 1K and 56 Ohm resistor voltage divider right at the audio input. Then it is boosted up again in the output opamp.
The whole upper portion of the schematic dealing with the Cutoff and CV input goes straight to the LEDs of the Vactrols so you can see that he whole filter hangs on these Vactrols. The Offset and Range trimmers control the Vactrol LEDs and determin when the LEDs turn on and how sensitive they are.

Schematic drawing:

Here are some pictures from the build proces. In these pictures the third trimmer is not yet put in. I tested it first with R7 as a normal 47K resistor. I later changed it for a 100K trimmer.
The print:

Print mounted behind the panel not yet wired up:

Video of the first test. You can see here that connecting an AD (or Envelope) signal to CV IN results in the opposite reaction to what normally happens. Using a negative or inverted AD signal doesn't fix this issue. Not even with a positive offset voltage applied. That just proves that this is a very quircky filter and all the more interesting for it ^__^  I've heard better results than mine from other people who built this VCF, so it depends a lot on how your vactrols turn out.

Okay, that's it for now. I hope you found this article useful and if you did maybe you would like to contribute to the upkeep of this website and to keep these projects coming. If you do, you can buy me a coffee. There's a button for that underneath the main menu if you're on a PC or Mac. Otherwise you can use this PayPal donation link.  All donations will be used to buy components for future projects. Thank You!

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Tuesday, 13 April 2021

Synthesizer Build part-42: 8 RANDOM GATES by Yusynth.

 Creates 8 random gate outputs from one gate input signal which can be as high in frequency as an audio signal. Lots of creative possibilities with this module.

There is an other random gates project on my website already. That one is included in the Noise Module article and it creates random pulses on one output. With this module we have 8 different outputs which trigger in a completely random order. It needs a squarewave on the input that can come from an LFO, the gate out from a sequencer, the clock pulse from a sample and hold or even the pulse wave output from a VCO. The pulse width doesn't matter for the functioning of the module, it can handle any pulse wave. I tested connecting it to a VCO but the output is nothing very useful. It's just a lot of clicks and even putting it through a filter didn't make it sound any better. So as a sound source it isn't that good but that's not what it's designed for anyway. There are a lot of other ways you can use this module. To trigger drum modules for instance.

The module is fed with only positive voltage so no dual powersource needed. It works fine on both +15V or +12V. You can feed the gate-in with signals that have a negative cycle to them. It will simply ground the negative part of the cycle through diode D1. The output gate signals have an amplitude of 8 Volt when powered from a +15V powersupply.
This build consists mainly of wirebridges. My layout has 37 of them. All the output stages are made on separate pieces of stripboard with just 4 strips of 10 holes. They are soldered straight to the output sockets. I did this to save space otherwise I would have had to make a separate print with all the outputs on them. This way saves space and also hookup wire. The three 100nF capacitors you can see on the layout are meant to be de-coupling caps but where they are positioned is really too far away from the chips to be effective. So instead of putting them where the layout shows them, solder them straight over the plus and ground pins of the IC's (top right and bottom left of each chip).

Here's the layout I made. First the wiring diagram:

(Last revised: 14-April-2021: Added missing 1K resistor to output prints.)

In the box, on the wiring diagram above, you can see the schematic drawing of the output stripboards. I left out the 1K resistors in series with the output in my original design. I had simply forgotten it but I have now updated everything and the 1K resistor is now included. It helps to protect the transistor against short circuits, smooths the output voltage a bit and also determins the output impedance.
The 270 Ohm and the LED together with the 1K resistor to ground form a voltage divider that determins the voltage of the outputed Gate signal. That voltage is normally 8 Volt but if you want it to be higher you can make the 1K to ground a higher value like 1K5 or lower for a lower output Gate voltage.

Here's the main stripboard. It's only 24 by 48 holes but you could try to redesign it and make it even more compact so it would fit in a Eurorack system. For instance, if you connected the outputs straight to the correct pins of the chip instead of using the wirebridges you can save about 8 or 9 holes in width. Certainly enough room to make it fit a Eurorack system. And because it's a "Random" gates generator, the correct order doesn't really matter does it?

And here's a close-up of the little output stripboard that is soldered straight to the output socket: (If you print this one, choose the A6 format to save some printer ink.)
You need to make 8 of these output prints. Some cuts are a bit hard to see but the top two strips are cut at position 5 and there's an other cut at position C8.

(Last revised: 14-April-2021: Added missing 1K resistor to output prints.)

Here's the Bill of Materials:

Here's the schematic by Yusynth. You can find the original YuSynth article by clicking HERE.

As you can see it's actually quite a simple circuit. It mostly consists of connections between the three IC's. It is mentioned on the YuSynth website that this module needs a bit of time before it starts behaving correctly. When you first start it up it will probably not fire on all cilinders and display a repeating pattern with only about 4 or 5 LEDs lighting up and after at least ten cycles this will change into a random pattern using all the outputs. However, since I changed the new CD4070 I had in there for a used vintage CD4070 from the 1980's that I had lying around, the module works good right from the start. 
The module that I built was at first prone to hanging. It would suddenly stop being random and get stuck in a 4 or 5 LED pattern. Only by changing the input gate frequency or pulling the Gate-In cable in and out a few times would I get it working again. It turned out this was also due to the IC's I was using. I don't know if it was a fake chip or if it was damaged but I changed IC-3 for an old stock CD4070 that I once de-soldered out of an organ circuit board and the problem was solved immediately.
So make sure the chips you're using come from a reputable source!

There's a mode switch that lets you choose between two settings. In the ON position the output stays high until it detects the next pulse, so the pulses don't have any dead time between them. In the OFF position the output pulse stops on the negative slope of the input gate pulse, so the output pulses will have the same length as the input gate pulses.
There's also an option to advance the pulses manually with a momentary switch (normally off). This switch is connected to the internal switch of the Gate input socket so it will only work when there is no cable connected to the gate input socket.

Some screenshots from the oscilloscope. The first one shows how extremely fast the risetime of the output gate signals is. Just over 123 nanno seconds! That's 0 to 8 Volt in 0.000000123 seconds. This means theoretically that it could handle signals upto 40MHz! (Agreed, this knowledge is of no use in the synthesizer world but it fascinates me personally because I also have a background in radio technology and transmitters.)

Here's what the output sequence of one of the random gates looks like. A non-repeating sequence of pulses with an amplitude of 8V. 

Here are some pictures of the build proces:

Wirebridges. In this picture there's a little wirebridge missing connecting pins 7 and 8 of IC2 (CD4051).

Here's the finished print. Like I mentioned earlier, the de-coupling caps are much to far away from the chips to be effective so get some small ceramic 100nF caps and carefully solder them straight over the plus and minus connections of the chips on the copper side. I myself left it like this and it works just fine because I don't use a switchmode powersupply but a linear one, with a big transformer. 

Here's the main board with the 8 output prints. My output prints are missing the 1K resistor in series with the output sockets (I had forgotten those) but they are included in the layouts. That 1K resistor helps to make the output waves smoother. I could see that on the oscilloscope images. It also protects the transistors by limiting the current going through them should the output be shorted. (Although damage will be very unlikely even without the 1K resistors because the pulses are so short).

Finished panel backside wiring:

Frontal view of the mounted panel:

And here's a little test video showing the module firing randomly on all cilinders :)

If you want a fast pulse train with random gaps in it, then connect 4 outputs from this module to the 4 inputs of a mixer, like the mixer/passive attenuator module on this website. At the output of the mixer you will get a pulse train with random gaps in them. It's cool to use this on the cut-off of a filter to add some random spice to the sound.
If you then set the switch on the random gates module to 'Stay high until the next gate pulse' you have sort of a random voltage generator, although there will still be random 0V gaps in the output but that makes it unique :)
You could even make a little TL072 mixer print and include it in this module. Choose how many inputs you want (less than 8 of course) and connect those mixer-inputs to whichever outputs you choose and then make an extra output socket on the panel that carries the output from the mixer and label it "Pulse Train". It's just a thought but there are many ways to adapt this design to your own needs.

Okay, that's an other one done. If you have any questions or remarks please put them in the comments below or post on the special Facebook Group for this website where we have a great community of synth enthousiasts willing to help you.

If you successfully built this module and you're using it in a cool way that others might enjoy, please make a video, put it on YouTube and contact me with the link. I'll add it to the article with full credit given.

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