LOTS of noise when gate is open

I have a SMR4 mk1 (0.5 PCB rev) that I am having problems trouble shooting, when the gate is open there is a fair amount of glitchy noise coming through. I’ve ruled out the control board (does it with any control board) and the opamps have been replaced as well. It doesnt come from the osc or filter since I can disconnect the 12/24 jumper and noise is no affected and I dont hear it when I probe those pins themselves.

It might be normal - depends on your definition of lots…

Here’s a naive way of quantifying noise: record a note of the init patch and wait for a few seconds after the end of the note, then stop the recording. Normalize the amplitude of the whole recording. The keep only the last 2 seconds. Check which gain you need to apply to bring the noise near clipping Audacity and Wavelab display the gain when applying normalization).

Is the noise still there if you connect all signals between the control/digital board except the OSC one?

Is the noise affected by cutoff/resonance?

Is the noise related to activity on the Shruthi (eg: does it change pitch with blinking LEDs)?

Is it related to the 50 / 60 / 100 / 120 Hz hum?

I have a ssm board on hand to compare and it is considerably worse, I use patch #6 to test. It does kind of sound like 60 cycle but with more hiss, clicks, and pops

>Is the noise still there if you connect all signals between the control/digital board except the OSC one?

Yes, when the gate is open

>Is the noise affected by cutoff/resonance?

Does not seem to but its hard to tell. The fundamental sound of the noise remains regardless (same as if I unplug the 2p/4p jumper)

>Is the noise related to activity on the Shruthi (eg: does it change pitch with blinking LEDs)?

No

>Is it related to the 50 / 60 / 100 / 120 Hz hum?

Again, hard to say. Its higher than 60 cycle for sure

Remove the digital board. Hook up the +5V pin to both VCA> and F>, ground Q> and OSC>. This should be enough to reproduce the problem with the digital board out of the equation.

Remove the pole selection jumper to rule out anything before and including the filter.

Is the noise already present at pin 8 of IC4? If so it’s the VCA.

Is one of your supply rails noisy? The SMR4 directly uses the supply rails to compensate for an offset at the VCA input - this is risky if the supply rails are not clean. I’m thinking this is what you are seeing - maybe one of the large capacitors near the power supply are kaput. You can try changing them.

If you are confident with your desoldering skill, try temporarily removing the “DC” trimmer. Should make the envelopes more clicky / poppy.

A couple more suggestions.

What sort of power supply are you using? it’s fairly well known that switching supplies can be noisy. The regulator and caps in the Shruthi-1 should help smooth that out, but it might be really really noisy.

Also, there may be a grounding problem on the audio side. Have to tried another audio lead?

Ok, shorted pins per your instructions and noise persists so not coming for digital side.

Looking at the power rails, I am getting a ton of ripple at the input of the Vreg. I did try another PSU and the hum (which indeed looks like 60 Hz) is better by about a factor of 3 but I still have clicks and pops and the hum is still there. This is odd since this is the same PSU I use with all my shruthi stuff and I dont see this behavior on a different filter board so I like the power supply idea. I’m reading 1.2K between power and ground, is that reasonable? A shorted 5v rail would cause what I am seeing, no?