Sunday 26 March 2023

Synthesizer Build part-52: 4 CHANNEL FEEDBACK EQ/DISTORTION (Monotropa) Eurorack.

A 4 channel feedback equalizer / distortion module that will fit a Eurorack system. 

I came across this circuit in a post on the LookMumNoComputer forum. Bpbby posted a Falstad simulation of this circuit and it intrigued me because I never heard of it before. He found the circuit on this website:

It's a pretty cool circuit. Simple too. We have 4 filters, each covering a part of the audio range, and then there's a feedback loop that connects the output back to the input. The circuit is called the Monotropa, which is the name of a plant. Don't ask me why. I don't see any logic in that. ^___^
Here's the schematic of this circuit:

Here is the Falstad simulation of this circuit:

Here are the layouts I made for this project. They are verified as always. I built it for my Eurorack case but you can just as easy make this for a Kosmo sized synthesizer. I that case you could even build the 7 channel version because you'd have more space on the panel for the extra potmeters. Yes there is a 7 channel version of this circuit but you'd have to Google that. This article deals with the 4 channel version. This circuit is designed for +/-12V but I can't see why it wouldn't run equally well on a +/-15V powersupply.

Here's the wiring diagram. For the first time in the history of this website I show the potmeters from the back side! I should have done that all along because it's easier with wiring up the panel but there it is. I started out showing potmeters from the front in my layouts and for the sake of consistency I stuck with that, upto now. I had to connect some components straight to the potmeters and audio jacks to save space on the stripboard:

Below is the stripboard only view. The stripboard is small enough to mount parallel with the front panel behind the potmeters and sockets. You could drill a hole through the lower two strips which are not in use and use a standoff to mount it to the front panel. The wiring will also act as a stabilizing feature. I just used some plastic tube and hot-glued them to the back of the potmeters and to the copper side of the stripboard. That's secure enough. I soldered the powerconnector straight to the stripboard without using pinheaders and sockets. That way you only have 3 thin wires coming from the board with a Eurorack connector (female) on the other side to plug it in. If you want to use bypass/de-coupling caps there's room enough to solder those in over the powerrails and add some 10µF electrolytic caps if you want extra stabilization of the power supply voltage. These components are not in the layouts and are not listed in the Bill of Materials!

The Cuts and wirebridges as seen from the COMPONENT SIDE!!

Here's the Bill of Materials:

Here's a look at the finished product:

Here are some screenshots from the oscilloscope showing the influence of the feedback on the output signal: 

And finally a little DEMO video I made. I built my version with 100K potmeters because that's all I had and consequently it doesn't sound as good as it could be with 10K pots. I assure you though, it is worth building but keep to the component values in the layout and schematics. Some potmeters are more effective than others depending on the frequencies that are put through this circuit because this is of course an equalizer. So a Low Frequency potmeter isn't going to have much effect on a high frequency bit of audio that's put through it. In the video I have it connected to a 555 VCO that is fed by the Sample and Hold of the previous project.

There's a useful tip in the comments below suggesting to use this EQ with a squarewave and then play with the Pulse Width Modulation of the squarewave in combination with the feedback of the EQ. That should sound pretty awesome!

Okay that's it for now. Not much of a write up I admit but real life issues got in the way. I might revisit this article later and expand on it. I hope you understand and don't mind. For now I just wanted to give you all the necessary layouts etc. to build this Feedback Equalizer. I already heard from one person who built it and he's very happy with it. If you have any questions please put them in the comments below or on the special Facebook Group for this website.

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  1. Hi Eddy,
    I found the schematic online here:
    On my version (Bpbby from LMNC website), I kept the 10k pots for the EQ parts but I used 100K pot for the feedback and level ones as they are voltage divider. But it is a very strange module so it is hard to say if it sounds as it should.

    1. Yes I think for those two pots 100K is not a problem but because the other ones are filters I think they do need to be 10K ones.

    2. Btw, thanks for the link. I'll put it in the article.

    3. The reason I mentioned Feedback must be a 10K is that I experimented with that and I noticed some difference between 10K and 100K but in hindsight that was probably just due to the higher overall resistance of the 100K potmeter in the feedback loop. Like you say, it's a voltage divider so it shouldn't make much difference. Anyway I just recommend in the article that people stick to the bill of materials to avoid any disappointment :)

  2. Of course, no problem ! :D BTW, I can get some pretty nice sounds when using a square wave and PWM, the feedback signal makes it go completely crazy. Love it.


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