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Buzzy MC2300 In Bi-Amp Set-Up

retrosonic

retrosonic

AK Subscriber
Subscriber
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.
 
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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.
 
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.
 
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.
 
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.
 
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.
 
Johnny_Law said:
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.
Click to expand...
Well, right you are. Throttling back the gain controls to about 50-55% does diminish the level of audible hum to point that I’m hard pressed to hear any buzz or hum over the noisy bloody fans. Speaking of which, is it normal that the fans come on immediately upon power up? That seems excessive and—since I mothballed the MC2300 for many years until I recently had it serviced and put back into service—I don’t recall whether the fan came on right away before. I guess I’ll check for a thermostat.

Anyhoo, even though it makes general sense that input related hum/buzz would attenuate with a lowered gain setting, I’m still left with the nagging question of why there is such noise to begin with. Am I naive to hope that with proper cables and healthy equipment the signal path should largely be devoid of such racket?

Thanks again for your suggestion. I’m working with the relative levels still between the crossover output control and amp gain control to find a low(ish)-noise sweet spot where the high/low bi-amp levels also agree acceptably.

Carry on,

Michael
 
1Y and early 2Y units had the fans running always. Later units were thermostatically controlled. The schematic for the thermo circuit is in the service manual so you can convert it ...
 
damacman said:
1Y and early 2Y units had the fans running always. Later units were thermostatically controlled. The schematic for the thermo circuit is in the service manual so you can convert it ...
Click to expand...
Right, that is starting to sound familiar. This is indeed a 1Y and I vaguely remember this subject coming up years ago. I’ll probably leave it alone rather than dump more $$ into the 2300. The rehab was substantially expensive and the truth is this is the garage system and, well, garages can be noisy.
 
Yep, my dear wife banished both me and this system to the garage. Life is not always fair.
 

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SaturationPt said:
Well I hope hers is the car left out in the rain then!
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No, it didn’t quite play out that way. The JBL stacks are sitting on HD moving dollys so—you guessed it—they can be moved out of the way when it’s time for the car to be garaged. There’s still a modest shop space at the back of the garage, but talk about compressing a sound stage!
 
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