Low level buzz on C84

[Ken Schumacher]

I just finished my second C84. When I listen to both of them I hear a slight buzz on the first one that isn't there on the second. Any suggestions on where to start looking to find how to fix it? I thought I put everything in exacty per the instructions. Help!

Ken Schumacher
Live Vibes Recording
Grass Valley, California

[Ken Schumacher]

I just read another thread on C84 buzz and wanted to clarify my testing. I'm using two Crown CM700 condenser mics, XLR out to TRS into a Mackie 1604VLZ. Cables are same length. Swapping the channels on the Mackie makes the buzz move, so it isn't the Mackie or cables out. Swapping the mics into the pres and the buzz stays on Ch 1, so the problem is not the mics or cables in. Mics are on a stand only a couple inches apart so its not the room environment. Both C85 mic pres are adjusted identically (18 volts plus minus and zero volts per the instructions).

The buzz is not loud, but is noticeable when I A/B the two pres. The buzzing one is located in slot 1, next to the power supply. I haven't tried swapping the boards in the slots or moving them into slots on the right. Eventually I hope to fill up the rack, so I'd prefer to have my rist ones be in Ch 1 and 2 slots. So, once again... Help!

Ken

[Ken Schumacher]

Final round of testing shows the buzz is caused by proximity to the power supply. Swapped the C84 modules and the buzz stayed on channel 1, slot one, so it wasn't the mic pre boards.

In another thread Tim wrote:

"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."

I tested all those points to AC ground and got 0.3 ohm (which is what my meter reads when I short the contacts).

I finally mounted the C84 boards in slots 7 and 8 and the buzz is gone! However, I would rather have the chassis filled from left to right. So...
What can be done to make slot 1 usable? Help!

[tpryan]

If you've done everything in this thread http://www.seventhcirclestudios.com/SCA ... 23756e2f9a
try this:

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.

[Ken Schumacher]

I'll try bypassing the choke next time I open up the chassis and report what I find out. Thanks for replying to my questions, Tim.

Ken