Ground loop between pre-amp and power amp?

[Simon_LDT]

Picked up a couple of 10ohm metal oxide resistors this morning. So what do I need to do to test if this'll work or not? I assume I can use speaker wire to connect to each end of the resistor and then place one end on a chassis earth point? Whereabouts do I place the other end?

Should hopefully have some time tomorrow to have a tinker.

[Firebottle]

See post #96.

[Simon_LDT]

Thanks Alan, that makes sense!

So I am safe to do this while the amp is on? (without touching the actual bare wire myself of course)

[Firebottle]

Yes you are perfectly safe, touching the wires as well, as the case is now fully earthed

[Simon_LDT]

Just had a go at adding the 10 ohm resistor but nothing happened. I scraped a little bit of the paint under a screw to get a decent connection to chassis, added the resistor and then touched the other end to the shield on an rca socket (tried a few different ones also). No change in level of hum. I'm still getting louder hum as each rca is plugged in (silent with only 1 in). Seems I might have to live with this. Fortunately, with Alan adding the earth the hum is not as bad as before but it hasn't reduced by much at all.

Would using an rca (pre) - xlr (power) cable help reduce the hum at all? Would there be any point in wasting money on a cable like that?

[Reffc]

Just had a go at adding the 10 ohm resistor but nothing happened. I scraped a little bit of the paint under a screw to get a decent connection to chassis, added the resistor and then touched the other end to the shield on an rca socket (tried a few different ones also). No change in level of hum. I'm still getting louder hum as each rca is plugged in (silent with only 1 in). Seems I might have to live with this. Fortunately, with Alan adding the earth the hum is not as bad as before but it hasn't reduced by much at all.

Would using an rca (pre) - xlr (power) cable help reduce the hum at all? Would there be any point in wasting money on a cable like that?

It wont work unless the chassis earth bond between the RCA ground return signals and the chassis is first broken. ALL ground return contacts between RCA sockets and chassis have to be effectively lifted otherwise the lowest impedance pathway remains the mains earth, hence the mains hum from the ground loop created.

Of course if the RCA ground return circuit pathway for the amp is insulated from the chassis, and the PCBs dont have a signal earth connection to the chassis, then an earth lift is of limited (if any) value.

[pedalhead]

Bit of the thread resurrection here. I've got a similar issue, suspected ground loop between pre and power amps. I read this whole thread & now you've left me on a cliffhanger! Simon, did you ever manage to fix the issue?

[Simon_LDT]

Bit of the thread resurrection here. I've got a similar issue, suspected ground loop between pre and power amps. I read this whole thread & now you've left me on a cliffhanger! Simon, did you ever manage to fix the issue?

Unfortunately not, I've just learnt to live with it. Luckily it's not audible from listening area but is definitely still there. Think it's just some funky design of the poweramp because my fronts which are powered via my 2 channel set-up amp don't have this problem and it's fed from the same preamp.

[StanleyB]

Ever since introducing separate Pre and Power amps into my set-up I've had humming coming from my speakers, which I am 99.9% sure is a ground loop. I'm currently running an Emotiva UMC-200 and Nakamichi AVP1, before that I was using my old Sony STR-DA2400ES as a pre into the Nak and also had the humming. Before I ever introduced the Nak I was using the Sony AVR solely and had no humming at all.

When the power amp is connected to the speakers but not the pre, there is no humming so that eliminates the power amp. Soon as the pre is connected up using unbalanced RCA cables the hum appears - before ever connecting any other leads in the pre itself. I can only assume (after thorough research) that because both amps have 3 pin power cables that both are trying to ground and it's throwing something off. Both are plugged into the same power socket via an extension.

What are my options to remove this ground loop (and safely)?

I've read that making up a piece of speaker cable to go between them and connecting it to a screw in each chassis is an option but is that even safe, and will it even work?

In some cases amp designers have a different earth for the case compared to the earth for the signal. Naim is an example that comes to mind. In that case you can try a wire between the case of the pre-amp and the power amp in order to neutralize that problem. It is one of the reason why there is an earth tag on the back of my DACs. Naim owners tend to need that earth tag to stop their amp from humming when connected to my DAC, which has several different earth lines.

[pedalhead]

Thanks for the update, Simon. Sorry to hear you've not been able to fix it. I've gotten around my similar issue by using a Jensen Iso-Max in between pre and power amps. It's not really addressing the fundamental issue, but at least it fixes the symptom!