Synthesizer Build part-31: NOISE MODULE with 5 TYPES OF NOISE + Random Gates.

A very easy to build noise module with 5 different sorts of noise, two of them being 'Grainy' noise with adjustable graininess.  Works on dual 12V so Eurorack friendly.

(The Random Gates section is located half way down the article)

This is a module I adapted from the MFOS Noise Cornucopia schematic by Ray Wilson. It's turning out to be quite a popular project because I'm getting lots of feedback from people who built it and are really happy with it. Especially the addition of the Grainy Noise.

So, I needed a good noise source in my synth and this one seemed perfect. The original schematic has a random gates section which I didn't need but which you can easily add on if you want it. But I left that out. (I made a separate layout for the Random Gates Generator section which you can find further down the article) I also changed the transistor used to generate the noise and I changed the way the transistor is integrated in the circuit. My way is simpler and generates 200mV worth of noise right at the emitter of the BC547. The transistor's Emitter-Base breakdown voltage is exceeded thus the transistor is operating in avalanche mode, creating nothing but pure noise.
This was a one day build for me. I spent the morning adapting the design and making a stripboard layout. Then I built it in the afternoon and by 8pm that same day I had a good functioning noise module built into my synthesizer. The layout I made worked right from the start. No troubleshooting needed.
In the layout below the transistor is shown as a schematic symbol, and not as it's normally shown in the TO-92 package, to make it clear that the collector is not connected. In fact, you need to cut off the collector leg completely to stop it working as an antenna. I've put the pin-out of the BC547 in the layout to make this extra clear. You might need to choose a BC547 that gives you the best noise results. I heared through feedback comments that there can be differences between transistors but you should get noise with any transistor. It's just that some transistors produce more noise then others. I myself put in the first transistor that I had, and didn't choose between them. It worked fine as you can se in the video. Should you experience hum or something, from the power supply, then you should resort back to the transistor arragement in the original design as shown in the original Noise Cornucopia Schematic. 

Here is the verified layout.

(Last revised 16-May-2020: Added grounding wires to output jacks and pinout to noise transistor.)

Stripboard only. 

For extra clarity: connect the 'Base [B]' of the transistor to copper strip 'I' and the 'Emitter [E]' to copper strip 'G'.

The cut on position D17 in the layout above, has been moved to position D21 to make it more visible.

Below is an overview of the cuts and the wirebridges alone. This is seen from the component side! As ever, mark the cuts on the component side with a black waterproof marker and then stick a pin through the marked holes and mark them again on the copper side. Now you can cut the copper at the marks with a sharp hand held 6 or 7mm drill bit.

Bill of Materials. Instead of the TL084 and TL082 you can also use the TL074 and TL072 opamps:

Here is the altered schematic, made from the original Noise Cornucopia design:

Btw, if the BC547 doesn't produce noise for you, try a 2N3904. Remember it has the opposite pinout of a BC transistor. E and C are changed around. Not all transistors are created equally and some produce more noise than others.

The noise output from the transistor goes through a highpass filter consisting of the 100nF capacitor and the 2 MegaOhm resistor. This creates a filter cutoff frequency of 0.8Hz letting through all the frequencies and rejecting any offset voltage. Should you experience an offset voltage after the filter then lower the resistor value from 2M to 1M. That will make the cutoff frequency 1.5Hz.

I changed the 500K trimmer, used to set the amplitude of the noise, for a 200K panel potmeter so you can use it as a level control on the front panel. In my panel I used a 500K panel potmeter but that really is too high a value. When I turn the potmeter 1/3rd open, the amplitude reaches it's maximum at 10V peak-to-peak and the rest is just maximum volume and starting to clip, so I think it's better to use a 200K potmeter. (However I haven't tested it with a 200K potmeter) .

If you only have a 100K potmeter you can try changing R5 from 10K to 4K7 to get the gain right, and then it should work with a 100K potmeter. I've had confirmation that this solution works just fine.

To be honest, you don't need a gain option in a noise module like this, so you can just as easily forget about the Gain potmeter and put in a trimmer, set it so the output of pin 7 gives +/-5Vpp noise level and leave it at that. That's also how it was intended in the first place. The gain option was just my own idea.

The opamps used here are not critical. The schematic says to use TL074 and TL072 but I used the TL084 and TL082. I think you could even use an LM324 instead of the TL074. The pinouts are all the same. 

This module is designed to work on a dual 12 Volt powersupply (so ideal for Eurorack systems) but it will work equally well on 15 V.

How Grainy Noise works:

The Grainy Noise consists of very short pulses with an amplitude of plus and minus 5V. The Opamps IC2 a and b are set up here as voltage comparators which are being fed on the non-inverting inputs with white noise and on the inverting input with a voltage that can be set with the Graininess Potmeter to between 0V to + or - 8.25V (roughly). Each comparitor has a diode on the output so handles only one part of the voltage phase (either positive or negative). So if the voltage on the negative inputs is very high, the noise will only occasionally go over it and we'll get only a few Grainy Noise pulses. When the opamps are not producing a pulse they are at rest in either full positive or full negative voltage on the output pins but those voltages are being blocked by the diodes. So only the Grainy Noise pulses are being fed to the output. As you lower the voltage, the threshold will become lower and the noise will tip over the boundary more often creating more and more Grainy Noise pulses.

The 'Grainy' noise is a real asset to have. It's very useful because of its harsh sound. It sounds a bit like the noise you get from old TV sets. If you look at the scope image in the video you can see that most pulses from the Grainy Noise go into negative voltage. The more you turn up the Graininess, the more pulses you get that go positive and that's what is used to create random gate pulses. In fact only the positive noise pulses are used in the Random Gates generator and the negative ones aren't used because that diode has been left out. See the original Noise Cornucopia Schematic for that. The Highpass grainy output is a bit low in amplitude. That's not just in my build but other example videos show the same thing. Maybe a different type of capacitor would make it better but to really change it you should put it through an extra opamp and give it some extra gain but I didn't bother with that. I don't think I will be using that output much anyway. If you change the resistor to ground you also change the highpass filter so I don't think that is advisable to make it louder.

Btw, the LowPass noise is not exactly the same as Pink Noise in my opinion. The LP noise has more rumble (bass) in it I think but you may have a different opinion on that. I leave that open :) I think to call it LoPass and HighPass etc is more intuïtive than to assign different colour-names to the noise. It's also a useful type of noise to have because a lot of people prefer mixing LoPass noise into the signal path instead of white noise because the LP noise sounds less muddy.

RANDOM GATES GENERATOR:

I've made a separate layout for the random gates section of the MFOS Noise Cornucopia design, for those interested in adding this on. The two 7 pin headers are there to provide a choise in randomness of the Gate signal. See the original Noise Cornucopia article for the schematic drawing. The signal has the most randomness if you place the jumper on the lower settings. The higher up you go the less random the Gates get. 

NB.: Place only one jumper on the pinheaders!! 

If you have a rotary switch with 7 positions you could use that, instead of the pinheaders, and make a feature of it by placing it on the front panel. That's up to you. Let me just say also that I have not built this random gates module myself but I've gone over the noise cornucopia circuit schematic with great precision and it's a very simple layout so it should work fine. If you have built this layout please send me some feedback about how it's working. I've had some feedback saying if it doesn't work like it should to put a 2,7MOhm resistor in parallel over the 10pF capacitor.

Personally I don't find this type of random gate circuit very useful. It's not synchronized in any way and you only get one output with a random pulse train on it. I would prefer the Yusynth 8 Random Gates project where each pulse has it's own output. What I would prefer even more is to pair a Sample and Hold circuit with a noise generator like the one above. But it's all up to you of course. 

(Check the comments below to read about Tim's findings when he breadboarded this circuit to test if he could build it with 7 outputs instead of one.)

Here's the layout I made for this section: 

Here's a demo video I made with sound samples of the different types of noise:

And finally some pictures of the finished product. As you can see the finished module is very small. In fact it is only 3 centimeters wide so it won't take up much space in your modular set-up:

To finish I want to direct your attention to a great video by Moritz Klein about building noise modules where he explains the theory behind it very well.  Click here to see the video on YouTube.

Okay, that's number 31 done. A very satisfying build because everything worked right from the get go. Any questions or remarks? Please put them in the comments below or post your questions on the EB Projects Discussion and Help Facebook Group.

Synthesizer Build part-16: SAMPLE and HOLD.

Creates random voltages from noise or turns an LFO signal into a stepped signal which you can use to control a filter. Lots of options.

NOTE: THERE IS NOW A NEW REVISED VERSION OF THIS PROJECT WHICH IS THE BETTER OPTION TO GO FOR IF YOU'RE PLANNING ON BUILDING THIS PROJECT.

CLICK HERE TO GO TO PROJECT 51, THE SAMPLE & HOLD VERSION 2.

This doesn't mean the design in this article doesn't work well. On the contrary. It works very well but the new one is  better designed and has a better output impedance so it's more stable when connected to other modules. So for a S&H you really should build project 51. It uses the same components as this version. I'm leaving this old version online as an archived article so people who built it can refer back to it.

Original text:

Every synth needs a sample and hold circuit in my opinion to have an extra source of control voltages. The S&H samples a voltage when triggered and holds that voltage until it is triggered again. If you feed it a white noise signal it will give you random voltages on the output which can create random tones if you input that signal into a VCO. If you feed it a signal from the LFO it will turn that signal into a stepped signal. The LF398 chip samples the input signal in 4 to 20 millionth of a second (!) and is used in many more applications that just synthesizers.
For this build I used the schematics from Rene Schmitz called 'Yet Another Sample and Hold'. (<-- click to have a look at the schematic)
I had ordered the LF398 chips a while ago and had a try earlier at building this circuit but I couldn't get it to work, but this time everything went fine and the circuit works very well. I added some extra's to this circuit in the form of a DC offset feature so I can control the voltage range of the output signals a bit better and I installed two input sockets between which you can choose with a SPDT switch. I also installed a switch that gives me the normal output voltage range (0 - 10Vpp) or half the normal output voltage range (0-5Vpp) which is better as input for the VCO's. The DC Offset in particular has proven to be a very useful addition. If you turn it into the negative the random notes get very deep and if you then put that through, say, the Steiner-Parker filter, you get the most amazing sounding low notes that sound really deep and sharp and in some cases can resemble the sound of drops of water if you put reverb on it. I can experiment for hours with this module.

This module will work fine on both +/-15V or +/-12V.

Here's the layout. All green wirebridges refer to connections to ground. All potmeters viewed from the front:

(Last revised: 19-Aug.-2021: Cosmetic changes to layout)

At the bottom right on the layout you can see the circuit for the DC Offset feature. I re-designed this from the previous version. This is a better way to add DC offset and it makes use of both opamps in the TL072 chip. (You can also use a TL082).

Here's a close-up of just the stripboard:

This S&H has an internal clock pulse generator based around one Schmitt Trigger NAND gate of the CD4093. You can also choose to trigger it externally by selecting the external input with switch S1.

Here's the schematic drawing of the extra features I added myself; the DC-Offset and the output range switch. A very observant reader noted that my output range switch does alter the low impedance that the normal opamp output would provide and this might be problematic in some cases. He suggests to put the range option in between the two opamps (see comments below). My reasoning is that the signal from this S&H usually goes back into an opamp like the CV input of a VCO or of a filter and most of the times these are opamp buffered and those inputs have an infinitely high input impedance so in those cases it really doesn't matter, but if the signal goes into an opamp inverter with resistors than that resistor balance can be upset. That's nothing serious but it would mean the amplitude of the S&H signal can be influenced in a way not anticipated. 

If that's all gibberish to you just ignore it and proceed building ^___^

[EDIT: This is one of the reason I re-designed this project in early 2023 and wrote a new article about it (project 51).  I'm keeping this article online because this is an archive of the modules I built and the progress I made over the years and as reference for people who already built this project in case they need to refer back to the schematics or layout.)

At first I used a potmeter with center detent for the OffSet control but I later decided to change it back to a normal one because it was difficult to set the offset accurately with the center detent spring pulling on the potmeter around the middle setting. 

The CV output goes through a resistor voltage devider that halfs the output voltage. This puts the different random tones closer together which sounds better. It's something I added after testing and seeing the output signals on the oscilloscope. Later on I added a switch that bridges that voltage devider and gives the original output voltages. I labeled it "Output x 1 and x 0,5". I did this because I wanted the full voltage available in case I want to use the output of the S&H to control the Cut-Off frequency of a filter (among other things). The resistor voltage devider however is something I strongly advise to include in your circuit if you're building one of these. The range switch is a good feature to have.

I didn't have any more space in the synthesizer I build to put this S&H in as a separate module so I cut a hole in the wood above the panels and mounted it there. This works very well and adds yet more buttons and switches and a flashing light. That always looks cool ^___^

Here's a picture of the finished panel and one that shows the placement within the synthesizer:

Here's a little video to demonstrate the sound you get when you put white noise on the input. This sound is going through the Dual Korg MS-20 filter described in the previous article.:

Here's a cool demonstration of the S&H with the Triple Wavefolder and the Steiner-Parker filter:

Okay that's another one done. Hope you enjoyed it and if you did please consider following this blog to get notified of new uploads and while you're here, leave me a comment please!

If you want to know more about sample and hold circuits I refer you to this Wikipedia page.

Here's a link to the LF398 sample and hold chip datasheet in PDF form:  (Click here)

The DIY Modular Sessions YouTube channel made some videos about building this sample and hold module which you can watch by clicking the links below:

PART-1 Preparing the stripboard

PART-2 Soldering the components in.

PART-3 Making Noise with the new S&H. Wow it's amazing!