Grounding problem in the amp.

I acquired a Counterpoint Solid 2 amp. The amp works correctly but there is always a residual hum caused by grounding problem inside the amp. If the preamp is not connected to the amp, there’s no hum. If only one of the two input is used (R or L chanel) , there’s no hum. When I connect both inputs, residual hum appears. I opened the amp and tried to find the problem. On both input jacks, there’s an additional wire going from outer shell to the power supply ground. As this is a dual mono amp, the two wires dont’t go at the same place. The left input jack, shell wire seems dead with no connectivity to ground. Wire seems open. I tried replacing the open wire with a wire with a alligator clips and grounding it elsewhere in many places in the amp. It helps but not completly. As the power supply is hard to have access, I must dismantling all the amp to have access to the place this wire is normally connected. Is there something I can do without having to do this job ? Is replacing the faulty wire at his normal place will correct this situation ? I’m really not tempted to do all this job if the problem will be still there. This is a real nightmare to work in this amp. Thanks
 

fab

Member
Paid Member
2004-03-24 1:16 am
Canada
fabaudio.online
Hi legarem
Looks like a ground loop to me. Real Dual mono amp should not have low impedance connection between left and right channels ground with no input connected. First You can measure with a multimeter that both channels ground are isolated or at least with a ground loop breaker if connected to chassis.
Fab
 
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hard to say.

Try to feed both input thru a "Y" splitter and check if hum is present.

Maybe your Preamp shield connection or RCA cable are defective.

Inside the amp, both "ground" may go to the signal ground but sometime, they do so thru a resistor (35 to 500ohm), if this resistor is blown, that could explain it

that counterpoint, do you have the schematic?
 

Bonsai

Member
Paid Member
2003-07-25 10:44 pm
Europe
www.hifisonix.com
Best diagnostic I’ve found for CCGL is to simply connect the L and R channel with a cable. I always do this as part of my debug procedure when building new amps.

At the back of the Ground Loop presentation I’ve put together a debug sequence that can help isolate the hum cause. It’s not exhaustive, but it does help to isolate whether it’s CCGL, Common Impedance or an external ground loop.
 
Interesting, Here is the schem of this amplifier. I thought placing the 10 ohm resitors on Wire 3 that goes to the chassis. On one channel, wire 3 seems to go directly on the chassis. On the other channel wire 3 seems to go to the chassis but there is a capacitor between RCA and chassis.
 

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