Seventh Circle Audio

So, when I said my C84s were working, I hadn't yet tested it much on dynamic or ribbon mics, and so I hadn't noticed a hum problem that happens when the knob is above 7 or so. This problem is not in the other channels in my chassis. I checked that the power harness was wired correctly and I checked it in another room to eliminate the room environment as a factor.

When cleaning out the paint from the switch holes with a drill, I made the holes for these channels bigger than on the other channels. Could that have caused a grounding problem?

If the rotary switch bushing is not making good contact with the front panel, or if the front panel is not making good contact with the chassis, you will likely have a hum problem. Do you notice an increase in hum when you touch the gain switch knob?

I'll check on these things. Thanks.

Touching the switch makes no difference in the hum.

Also, I should probably mention that, as far as I can tell, the front panel is making good contact with both the bushing on the rotary switch (this is the threaded thing that you screw the front ring on, right?) and the front of the chassis.

Do you get the same symptom with different types of mics? Do you have a transformerless condenser mic you can test with? Are you in rooms with AC dimmers or fluorescent lights? Have you tested against the other channels in the same room, with the same mics and gain settings?

tpryan \$m[1]:Do you get the same symptom with different types of mics?

Yes.

tpryan \$m[1]:Do you have a transformerless condenser mic you can test with?

I tested it with an Oktava MC012. Does it have a transformer?

tpryan \$m[1]:Are you in rooms with AC dimmers or fluorescent lights?

The first room I tested in didn't, the second one did. The hum occurred in both rooms.

tpryan \$m[1]:Have you tested against the other channels in the same room, with the same mics and gain settings?

Yes. The other channels did not have hum. Just the C84s.

Are the little, non-screwing rings that come with the rotary switch important? I left them off because when I had them on, the front panel did not make good contact with the front of the case. In any case, I tried putting them back on for the C84 channels, and it made no difference.

What should I try next?

What are you monitoring with? Is the C84 connected to a balanced input?

I am monitoring with speakers or headphones, and the C84 is going into a balanced input. The hum is being recorded, so I don't think it's a matter of what I'm monitoring with.

My C84 also has very serious buzzing problem, whenever I plug in the mic (Oktava 12, SM58), the buzz is seriously loud. I checked contact of the metal baffle between the PS03 and micpres, there is no contact! I want to fix this but the screw locking the ground cable is screw up and I have to find a way to cut that one before I can find how to fix it. Any other suggestion on this?
Also the 4 LED on C84 does not lit up, is this normal?
thanx!

My C84 also has very serious buzzing problem, whenever I plug in the mic (Oktava 12, SM58), the buzz is seriously loud.

How about with no mic plugged in?

I checked contact of the metal baffle between the PS03 and micpres, there is no contact! I want to fix this but the screw locking the ground cable is screw up and I have to find a way to cut that one before I can find how to fix it. Any other suggestion on this?

Why do you need to remove the divider? Are the PS03 LEDs lighting up? Or do you mean that the mu metal isn't grounded? That shouldn't make a difference, since the steel that's it's wrapped over is grounded.

Also the 4 LED on C84 does not lit up, is this normal?

Yes.

tpryan \$m[1]:

How about with no mic plugged in?

without mic plugged in, there's only tiny white noise that maybe the self noise when turned all the way to 5 oclock.

Why do you need to remove the divider? Are the PS03 LEDs lighting up? Or do you mean that the mu metal isn't grounded? That shouldn't make a difference, since the steel that's it's wrapped over is grounded.

All the PS03 LEDs lit up. Are you saying that the metal divider is grounded while the metal wrap isn't? Also sometimes the buzz is louder than other time. Overall, it is quite "clean" and neutral, only when the volume turned past 2 oclock position, the buzz become unacceptable. But knowing it's there, I wouldn't try to use it for any recording. Please help!

I had a look at cominginsecond's unit and found a number of small problems, all of which could contribute to hum. On your unit, make sure that:

Crimp connections in the wiring harness are tight
Crimp terminals are properly inserted into their housings
Front panel holes are free of paint
Front panel screws are tight
Chassis ground wire is secure

You should measure a short circuit between any exposed metal on the chassis, the rotary switch bushings, pin 1 on all XLR connectors, and the AC safety ground.

However, even after correcting all of the above I was able to duplicate the buzzing symptom on both C84s, but only with certain microphones, and only if the unit was close to other equipment that radiated a magnetic field. These fields can induce a noise signal into common mode choke L3. The fix is to jumper it out of the circuit by shorting pin 1 to pin 2 and pin 3 to pin 4.

I moved my two rev 1.2 C84 modules from slots 7-8 to 1-2, noticed the buzz, jumpered out the choke, and the buzz is completely gone.

my question is this: I notice that the current revision (1.9.1) doesn't include the choke, and jumpering it out of the circuit in my 1.2 board doesn't seem to have any discernible effect on the sound quality. so why was it included in the previous design, and am I compromising performance when I bypass it?

really just curious...it sounds great to me.

why was it included in the previous design

For RF rejection. The common mode choke was more effective at RF, but caused more problems than it solved.

am I compromising performance when I bypass it?

Not unless you're picking up radio stations.