Lunetta Synth - CMOS and Modular Discussion

DIY STUFF
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So I wanted to bring up Lunetta synths and have a few questions about it.

My biggest question is - how do you get it to communicate with other modular formats safely?
I have heard utilizing comparators for the inputs and transistor buffers on the outputs - but am wondering if anyone could further explain this.

Lets start a discussion about CMOS Modular!

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A lot of nonlinear circuits module are CMOS logic and analog mux chips with eurorack front and back ends.
For example the 4SEQ uses a common Lunetta chips 4052 dual 4:1 mux and somewhat less common 4029 up/down counter in a Eurorack module.
4SEQ — Nonlinearcircuits is a page link; the BOM, build guide, pwb and schematic are linked on that page.
Anything on an input is considered “active”, “high”, “logic 1”, etc. if it is above around 1 volt. So it works with unipolar envelopes and gates as well as with bipolar LFO signals etc.
Linear outputs are regular Eurorack style buffers with 1k series output resistors. Gate outputs are somewhat power hungry transistor emitter followers into a voltage divider for 0-5v gate outputs. When “high” they’ll draw 5-6mA plus whatever it takes to light the LED. But that’s not bad and the circuits are straightforward. There is a lot of Lunetta content in nonlinear circuits modules, in addition to the chaotic oscillators, rectifiers and such that makes these kinds of modules so much fun.

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My DIY modular is using some CMOS chips powered by 5v. I interface with eurorack with a module that adapts banana cables to 1/8in jacks. Is there a reason I shouldn’t be doing that that I’m unaware of?

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The issue isn’t connectors, but the signal levels – electronic circuits generally don’t like input voltages outside the supply rails. CD4000 inputs are speced for −0.5 V to VDD+0.5 V, so in your case going above 5.5 V may damage the chip.

Some opamps like the TL074s are even pickier; they can misbehave if you get closer than 4 V from the negative supply (see here).

The 4SEQ linked above solves that by powering the opamps with ±12 V and the CD4000s with +12 V only, and using diodes on the way in, to avoid exposing the CD4000s to negative voltages. The input stage uses opamps as comparators, with a threshold of 12×10k/(100k+10k) = 1.1 V. Here’s one of the inputs:

image

That’s for a non-inverting input; swap the + and − inputs if you need inverting (i.e. the CD4000 input is marked with an ▻ or written like T̅H̅I̅S̅). Also, that circuit will still mess up if the input goes a bit below −8 V, but it should handle inputs from −8.5 to at least 11 V just fine.

EDIT: Updated image to include the R89 pulldown, which is needed to avoid floating inputs.

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I would not want to hold my work up as a good example of CMOS design, but I haven’t had trouble with it:

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Thank you for the explanation. I have another question if you don’t mind. When it comes to the “electronic circuits generally don’t like input voltages outside the supply rails” part, how does that apply to things like plugging my MS-20 mini or Model D into the eurorack?

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I don’t have either but my understanding is the Model D is designed to be Eurorack compatible, so that should present no problems. The Korg site says regarding the MS-20 mini:

Signal In: Amplifier Out,
Band pass filter Out,
CV Out (0 - +8.4V),
ENV Out (0 - +5V),
Trig Out (+5V–>GND),

and any competently designed Eurorack modules ought to be able to handle those voltages without risk. However the MS-20 CV is V/Hz, not V/oct, so can’t be used to drive Eurorack oscillators — not in tune, anyway. And the trigger convention is different from Eurorack, where a trigger is a positive going pulse based at 0 V.

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Yeah I know about the v/hz v/oct thing. I was just wondering the difference between plugging into another synth that outputs 5v and my DIY modular which also runs off of 5v.