Grounding problem in the amp.

[legarem]

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

 

[Osvaldo de Banfield]

It will depend on how skilled are you, and the knowledge and instruments available.

 

[legarem]

I have an oscilloscope and have good skills.

 

[legarem]

So nobody can help me to diagnose this problem ?
Why this amp with no preamp connected to it don't hum ?
Why there is hum only when the TWO inputs are connected to the preamp ? if there is only one, there's no problem.
How to diagnose such a problem with a scope ? Thanks

 

[fab]

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

 

[djoffe]

Take a look here...

The PAT-4 Pages

Look down the page for "Library of Grounding Problems"...there's a pdf document there:

https://www.updatemydynaco.com/documents/GroundingProblemsRev1p4.pdf

I think what you're describing falls under the heading of the "Cross channel ground loop"...

Good luck...these are never easy!

 

[etalon90]

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?

 

[legarem]

I checked the resistance between the two RCA shells and there's 0 ohms even if What I consider the faulty wire is disconnected. Also they both have 0 ohms to the chassis. As this is a dual mono amp this is probably not normal,

 

[fab]

Yes it dos not meet dual mono criteria. You can try isolating both RCA ground from the chassis (thus from each other) and then test with preamp.
Fab

 

[Alessandro]

It's for sure a ground loop. As already mentioned is not easy to find. My only suggestion is to disconnect all and test one channel at time looking very well ground connections.

 

[scottjoplin]

Did you look at the paper djoffe posted?

 

[Bonsai]

Sounds like a cross channel ground loop. See link below to presentation.

Has this amp been modified or had a bad repair?

 

[legarem]

Problem solved

Someone worked in the amp before I got it and inversed RCA input grounds from one Channel to the other.

Thanks a lot for your help.

 

[Bonsai]

I had exactly that problem on an SME arm I bought a few months ago. Someone had worked on it and swapped the left and right grounds. Hummed like hell. Fixed it SMD it’s now absolutely quiet.

 

[Mark Tillotson]

Thinking about the topology of the ground loops with such a swap I think it would show opposite phase hum on each channel, so switching to mono ought to reduce the level - a useful diagnostic?

 

[Bonsai]

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.

 

[legarem]

In fact, I realized I still have a very little hum only on the right Channel (the one which is coupled with a cap to the chassis) I will install 10 ohms in series on two wires that goes from RCA shells to chassis.

 

[Bonsai]

No you should not do this. Any cross channel ground loop current will flow through the resistors and generate a noise voltage that will appear in series with the signal.

See page 37 and page 40 in the presentation.

 

[legarem]

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.

 

[legarem]

Here is how it is connected. This is solid one amp so mine (Solid 2) doesn't have pots at inputs.