About VCA-1

[Arda Eden]

Hi all,

As many of you, I stepped into the fantastic world of modular synthesizers and the first module I built was the TH's VCO-1. It works perfectly and I already built the second one.



Next, I needed a VCA, and again TH's VCA-1 (https://www.birthofasynth.com/Thomas_Henry/Pages/VCA-1.html) worked, but in a strange way: The gate opens at negative voltages and closes on positive. When I press a key, gate is closed and no sound. On a depress, gate opens up. This is supposed to be the opposite way, isn't it?

I checked my circuit and it is exaclty the same with that TH designed.

What may have gone wrong?

Any ideas?

Thanks already.

[Arda Eden]

I figured out that:

1- TL071 is getting very hot and frying some time later. Even if I remove the TL071 from its socket, circuit keeps working in its abnormal behaviour. 071 has no function in my circuit.

2- Checked everything. No shorts or anything wrong.

Couldn't understand why TL071 is getting that hot and not working.

[drapdap]

The ota you are using, 3080 or 13700?
I’ve built several vca-1’s with the original layout ages ago, and i had this behaviour with fake 3080’s!

I’d try swapping that after gettting the right psu voltages on ic sockets.
Also the 071, it could be fried now.

[Arda Eden]

[marwatt]