Grounding issue with PAiA and MFOS equipment

danielwarner · Joined: Dec 18, 2006 Posts: 65 Location: Sacramento

I don't know whether to put this topic in the PAiA subforum or MFOS, really. Anyway, here goes:

I've got a PAiA Frac Rack with a couple PAiA 9700 modules, the MIDI to CV converter and the VCO module. They work correctly.

I just built a MFOS dual log/lin VCA module, fabricated my own frac rack panel for it and hooked it up to the power from the PAiA modules.

Well, the VCAs work, but there's a big, disappointing, frustrating, audible HUM.

The PAiA modules have four different power connections, V+, V-, G (power ground), and SG (signal ground) and the MFOS dual VCA only has V+, V- and G (common ground). Right now I have the MFOS module's ground hooked up to the PAiA unit's G (power ground) but not the SG (signal ground), what should I be doing differently to get rid of this hum?

valis

Hmm.

I've built a few of those MFOS VCAs and haven't had a problem. I use mostly 4700/2720 modules but I do have the 9700 MIDI2CV. On my MIDID2CV the SG and Ground have continuity.

Is the hum always there or only when it is passing a signal? Does it get louder as the gain is increased? Does it get louder when you touch a knob or the panel?

I'd have to reason that there is a grounding issue with the MFOS VCA. I'd double check the wiring on it and make sure the jacks are properly grounded.

Good luck!

danielwarner · Joined: Dec 18, 2006 Posts: 65 Location: Sacramento

The VCA hums whether there's a signal present or not, for instance if I just patch a cord directly from a VCA output to my amplifier there's a hum.

I've checked the grounding on all the jacks, and there's continuity between them.

The hum is constant when I touch the chassis, turn a knob, or switch a switch. It's unaffected.

I've checked the transistors with my multimeter's diode tester and they're fine. I've replaced the LM13700 and the TL084 to no avail.

I'm getting power from the PAiA 9720 VCO, and it's measuring +/-17VDC which seems high.

I'm stumped.

Uncle Krunkus · Posted: Wed Nov 14, 2012 4:16 am Post subject:

Is it mains hum frequency? (60hz in the US) and harmonics.
Or is it another frequency?

danielwarner · Joined: Dec 18, 2006 Posts: 65 Location: Sacramento

So, I also had a MFOS bipolar power supply in the works, so I finished that and am now powering the MFOS dual VCA from that.

The hum is gone! I wonder if the +/-17VDC was too much for the VCAs, which are recommended for +/-12VDC to +/-15VDC (on the MFOS website it says adjustments are probably necessary for 15V operation).

However it sounds like the trigger signal from the MIDI->CV converter is bleeding through the envelope generator from the 9720 VCO, whenever I press a note I hear a big THUMP.

Etamin · Joined: Jan 05, 2011 Posts: 26 Location: NJ, USA

Hi Daniel

This may be a stupid question, but what are you using to open the VCA? Did you patch the AR envelope to it from the Paia VCO? I just patched that and I do not hear any thump. With the attack pot all the way down, the attack is sharp, but definitely not a thump.

Peter
Edit: OK, I just re-read your post, and see that you are using the AR env in the Paia VCO. Have you tried contacting Scott at Paia? He is very helpful, and your best bet to resolve this.