A solved ground loop problem , but who can explain why

Audio Components Amplifiers
1

Hi all

I have an old style simple 2 channel amp. 100w output, mains powered (is two wire mains connection amp is double insulated). no switch mode -big transformer

I have been getting a mains / ground loop hum from the amp.

for those who like a mystery

If the amp is connected to two speakers and the two phono inputs are left unconnected-there is no hum. (ie no input)

If the amp is connected to two speakers and left channel (one rca phono input) is given a signal –no hum

If the amp is connected to two speakers and right channel (one rca phono input) is given a signal –no hum

If the amp is connected to two speakers and left and right channel (both rca phono inputs) is given a signal –There is a Hum.

So to diagnose it further up the line as possible, and be precise the hum only appears when the two RCA input grounds on the back of the amp are shorted together. (so this happens when most plug in most equipment is used to input. Generally Left and Right grounds are common 3.5 jack to phono lead for instance)

To verify this just taking a link wire and connecting shield /ground of Right channel at the RCA output on the case and left hand shield / ground of output connector makes the hum appear.

The amp seems to work fine for sound output with either L/H ground being used or R/H but not both.

So I have internally disconnected the L/H ground from the input RCA socket within the case , so now both channels ground through the R/H socket. It seems to work fine like this.

Can anyone with more knowledge than myself offer a theory as to why this situation has come about , or why there is the hum when both channels are using separate grounds (as that was how the amps was made).

Is using just the R/H input ground going to be a problem ?

(in reality now as soon as anything is now plugged into the input R/H and L/H grounds are connected by the plug / lead or equipment and become common)

(but I guess the L/H channel in the amp will be floating until a connection is made)

Thanks in advance to anyone who is able to offer some understanding.

Regards Richard

2

Sounds like a problem with your preamp.

3

If you disconnect your turntable from the phono amp with everything connected normally, and the hum goes away, I would check the grounding configuration of your cartridge. You may need to isolate the headshell from the tonearm.