Buzzy MC2300 In Bi-Amp Set-Up

[retrosonic]

Greetings All. I’m trying to get to the bottom of a vexing problem I’m having with my recently refurbished MC2300 (caps, transistors, etc—major redo) in a bi-amped configuration. I’m using the 2300 to power a pair JBL B380 passive subs. The preamp is a C220 and I have a pair of JBL BX63A dividing/summing electronic crossovers (one for each channel) in line before the 2300.

The problem is I have a constant low level buzz on both outputs of the 2300 (heard through the B380s). I’ve tried a series of “process of elimination” steps but still I can’t figure out from where the buzz emanates. Here’s what I’ve tried so far (with no improvement):

1) I’ve swapped outputs from the C220 preamp (Main, Out 1, and Out 3).
2) I’ve swapped all cables (pre to Xover, Xover to amp, both channels).
3) I swapped the BX63As with a Dahlquist DQ-LP1 (Since I two of them, I actually tried both).

If I remove the line in cables from the 2300, the buzz goes away. Also of note, there is no buzz on upper end of the system (pair of MC60s into JBL 4333s).

Should I try adding a ground wire between to pre—>Xovers—>amp? Any other ideas/thoughts?

Any help would be appreciated.

Thanks.

 

[Johnny_Law]

Does the buzzing go away if you run your gain knobs at a lower setting?

 

[retrosonic]

I’ve turned gain knobs all the way down and, yes, the buzz goes away. Also, removing the input (line in) cables removes the noise as well.

 

[Johnny_Law]

What about putting them at 50%, with the line in cables connected (and also connected to the preamp)?

 

[retrosonic]

In other words, bypass the Xovers, set line in @ 50%, and see what happens?

 

[Johnny_Law]

No, keep everything as you have it, just turn the bias down.

I run my bias at 50% on my MC2300 and MC2105 because if I run it at 100% I get a buzz. I'm wondering if that's your issue.

 

[retrosonic]

Roger that. As it turns out, I throttle the output from the BX63As back nearly 50% already to balance levels between high/low. I’m away from home until the end of next week, so I won’t be able to test this until then. Thank you for your suggestion. I’ll let you know how it goes.

 

[damacman]

FYI - the connection for the neutral AC line terminal block is directly below one of the bolts that secures the power transformer. I had a 2300 where that bolt came loose, and was in contact with the neutral.

Just remove the bottom panel and it's all looking right at you.

 

[retrosonic]

Hmm, interesting thought. I might just do that as well. I’m wondering though, if that were the source of the problem, why the buzzing goes away when the input cables are removed. What your thoughts about that? Thanks, by the way, for taking the time to offer a suggestion.