Phono stage earthing

[deek]

Just a quick question as I can’t seem to find the answer elsewhere.

There is a ground-loop hum when my TT phono cable is connected to my external phono stage (via 2 RCAs and the ground). The external phono stage is connected to the aux-in of my main amp - obviously.

There is NOT a hum when the TT phono cable is connected to my amp’s internal phono stage and ground (i.e. no external phono stage).

There is NOT a hum when the TT phono cable’s RCA sockets are connected to the external phono stage and the phono cable’s ground are connected to the main amp’s ground.

I can’t work this out, so at the moment I use the RCAs on the external phono stage and the ground on the main amp.

Phono stage is a DV P75 mkii. I did notice the power supply (wall-wart job) plug has no earth (just a plastic pin). Could this be the issue?

 

[deebster]

Same DV here. The hum I think is down to the DV's ground pin being just an unconnected pin. Either bypass it or make sure it has an ongoing link to something with a real earth, like the amp. I connect as per the 3rd way as it's easiest connection-wise.

 

[Brennus]

Same here, I recently bought a P75 mkii and had to connect the turntable earth to the preamp, not the phono stage to avoid hum.

I my system the 0V in the amplifier is connected to earth through the turntable. The earth connection on the p75 is only connected to the metal bits of its casework.

The casework on the amplifiers are earthed but not tied to 0 volts. This I believe is standard practice.

 

[cobbers]

You don't say what the tonearm and cartridge is - it could make a difference.

 

[Bairnstorm]

Try not connecting the earth and see if that works.

 

[deek]

I did try without connecting the earth and that didn't work.

TT is 1210GR with a 2M blue. I've got a 10x5 but not tried that yet, I doubt it will make a difference.

I think the issue might simply be as @deebster and @Brennus have found; the phono stage itself might just need earthed (sometimes?). Is this common?

At least I can get rid of the hum and it's good to know I'm not the only one who has this issue.

 

[Craig B]

Another option with DC powered phono stages is to connect the tonearm ground to the phono stage and then bridge the phono stage ground to the amplifier via a second ground lead.

As you've obviously found a solution, the above isn't necessary, other than perhaps to allow for better siting of components vis-à-vis more slack in the wires.

 

[sq225917]

Just ground it to the amp, there's no reason not to.

 

[Rockmeister]

^This...if the ground lead will reach the amp, just ground it there, or extend it to do so temp and se if it solves the problem.

 

[DavidS]

Yes my experience of this is to ground at the pre amp not the P75.

 

[sq225917]

Would be interesting to see if theres signal continuity from the case of the p75 to the rca out, I suspect not.

 

[Brennus]

sq225917, I have measured this and there is not.

 

[sq225917]

Then that explains the hum. If the case isnt grounded, and turntable gnd plug is on the chassis only then its pointlessly floating and requires an additional wire to pre amp, or just grounding the arm to the pre.

 

[deek]

Thanks everyone for your input.

I'm quite happy running the ground cable to my amp but I really wanted to understand why and now I do. Hopefully this will help someone else out too.