Hello Forum,
I've just bought a relatively simple amp to learn some basics from.
It's a Class A with a torroidal transformer leading to two DC power supplies inside, a separate left and right channel board with each channels transistor tabs bolted to the left and right heatsinks of the chassis/case.
It produces a (probably 50Hz) hum ONLY when both sets of left right phono leads are plugged into the RCA input terminals on the amp (whether or not they are also plugged into anything else). If either lead is not plugged in, there is no hum.
Before I start grounding everything in sight and cause a short or other such problem, I wondered if someone could talk me through the best way to safely troubleshoot the issue please?
If I try the amp with one channel (or the other), it is working fine.
Many thanks everyone.
I've just bought a relatively simple amp to learn some basics from.
It's a Class A with a torroidal transformer leading to two DC power supplies inside, a separate left and right channel board with each channels transistor tabs bolted to the left and right heatsinks of the chassis/case.
It produces a (probably 50Hz) hum ONLY when both sets of left right phono leads are plugged into the RCA input terminals on the amp (whether or not they are also plugged into anything else). If either lead is not plugged in, there is no hum.
Before I start grounding everything in sight and cause a short or other such problem, I wondered if someone could talk me through the best way to safely troubleshoot the issue please?
If I try the amp with one channel (or the other), it is working fine.
Many thanks everyone.
Can you draw out your wiring arrangement and then post it up - best way to get a sensible answer.
Here is the link to Joffe's paper
http://www.updatemydynaco.com/documents/GroundingProblemsRev1p4.pdf
See Figure 4 - this may be your problem.
Here is the link to Joffe's paper
http://www.updatemydynaco.com/documents/GroundingProblemsRev1p4.pdf
See Figure 4 - this may be your problem.
sources ON or OFF will have little effect.
It's interference impinging on the interconnects that is generating an interference voltage on the signal return that the amplifiers sees as a signal input. The amp input does not need an audio signal for this condition to exist. It amplifies the small amount of hum interference and you hear/measure it at the amp output.
By turning off I was pointing to physically unplugging them. Earth-wire groundloop.
The same aplies. It's the loop in the signal returns that is usually the cause of Hum in a stereo amplifier.
The safety earthing can be a problem but do the D.Joffe solution first.
Then look for other causes.
Thank you very much indeed everyone.
AndrewT : I will read that paper later tonight - thanks very much
Bonsai : Thank you very much for the link - my first amp project (I didn't build this one myself) will likely be one of your designs by the way! I really like the look of the Ovation SX-Amplifier.
AndrewT : I will read that paper later tonight - thanks very much
Bonsai : Thank you very much for the link - my first amp project (I didn't build this one myself) will likely be one of your designs by the way! I really like the look of the Ovation SX-Amplifier.
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