Learning a lot here. Thank you!
Your observations are very interesting.
When I read it, I remembered a schematic posted with a broken link in the Modwiggler thread that reflects a good part of the modifications you are suggesting.
I downloaded this some time ago.
This one:
Posted by Modwiggler user fg
In this comment:
https://www.modwiggler.com/forum/viewtopic.php?p=3412361#p3412361
EDIT: I pointed to the wrong user and link. Sorry. Fixed now.
Interesting… As mentioned in the following comments, despite the label that isn’t a low pass gate, because there’s no capacitor. The 5.6k resistor replaces it. It’s a vactrol VCA, attenuating but not in a frequency dependent way. But if you use a cap instead it’s an LPG.
Not sure why it uses a Zener.
Here’s another LPG, from https://sfcs.neocities.org/module/SFP20/:
The diode OR means it uses (more or less) whichever CV is larger (and also protects the LED).
I’m a little surprised by the capacitors — parallel caps add so the possible values are 30.2, 22, and 24.2 nF. A pretty limited range. Whereas if the 22 nF and 2.2 nF caps were swapped you’d get 10.4, 2.2, and 24.2 nF. But it’d need breadboarding to see what range really makes sense. And as usual the resistor value would be very dependent on the vactrol (in this case a homemade one).
This is so cool.
To have VCA and LPG in the same module I only need a switch that changes between resistor or capacitors.
It looks fun. A simple, compact and useful module to take a break from the hardest builds.
Thank you.
Here’s 3 of those passive LPGs at work
I’m no expert, and I just happened to glance at this, but it looks like you have an unused opamp. Is that correct? I think the - input needs to be connected to the output instead of the + input which should be grounded as it is. This would make another break under the ic necessary as well as another jump or two. I may be wrong, and it may have no effect on the circuit. I’d like to build this. I tried it on breadboard about a year ago (unsuccessfully, no doubt due to inexperience) when I saw the audiophool video. I want to try again. Moritz Klein has a similar circuit. I think someone else in this forum has a circuit like this too…
Right, unused op amps should have output connected to - input, and + input connected to ground (or VCC/2 in a single supply situation). TI has a 2-page document on this subject, if you can believe that: https://www.ti.com/lit/ab/sboa204a/sboa204a.pdf?ts=1707601183534
(It includes this picture:)
Nice techno sounds in a cute little build, good job
Yikes! Hopefully the poor opamp is ok! I will update the layout!
this may seem odd but concerning the SNARE/TOM from : “[Unauthorised Service]" stripboard layout posted, how would i wire the the 2X5 powerheader on the board plug into my rack?
perhaps a wee diagram would help thanks
you could use a larger stripboard to add what is missing, or an extension
yes thats the idea but what about -12v? does this circuit not require -12 connection? forgive my lack of understanding the basics of these conditions.
The -V pin on the TL072 (pin 4) is connected to ground, so presumably no, -12 V is not used. This is probably intended for or adapted from something battery powered.
I confirm, I built this module, it does not require -12.
thanks for sharing yall 
I’’m going to give your Stripboard a shot stmllr 
What values did you use for C1 and C2
And is C3 and C4 0.1uf? I like the toggle between action in this design.
Is c1/c2 10uf?
They’re all bypass capacitors, so conventionally, yes, C1 and C2 would be 10 µF and C3 and C4 would be 100 nF.
I suggest to redesign the stripboard regarding position of C3(100n) and C4(100n). They are meant to reduce noise from the TL074 opamps and should be placed as physically close to the power pins
of the chip as possible. Power pins are pin 4 (VCC-) and pin 11 (VCC+). That also means using four caps instead of two because there are two opamps.
Put one between pin11 and pin12 of both opamps.
For the lower opamp, put one between pin4 and pin5.
The upper opamp is a bit trickier, because there is no grounded strip next to pin4. You could put it between pin4 and pin10 across the opamp.
Find an explantion at https://www.analog.com/media/en/training-seminars/tutorials/MT-101.pdf
3,5 years later, I’d go for a different approach using a non-inverting opamp design for simplicity reasons and to safe components. For example Sams 1161 dual buffered multiple
There are advantages and disadvantages. With an inverting design the op amp input pins are always at 0 V, where the op amp performs best; with non inverting you can get phase inversion if the input goes below -8 V (on a ±12 V system), or possibly chip damage with input outside the rail voltages — both admittedly rare with most synth setups. On the other hand a design with non inverting voltage followers, a 1M resistor to ground, and output resistors inside the op amp loop uses fewer components and doesn’t damage 1 V/oct signals — the latter can be done with an inverting design but only with more components including precision/matched resistors or trim pots to adjust the gain. I have both in my synth.
