Sugden C51 + P51 Combo - Low ground hum

[w7676south]

Hello all.

I have restored a P51 and C51 Sugden Combo, recapped to a point, but still have a ground hum, for the life of me I can't find it.

Have cleaned and sealed the transformer, helped a tiny bit.

Re capped some, helped a little, new input leads. Moved to different room.

This hum does not continue when an input is playing, only when there is not a track placing.

I love this amp, also looking for another P51 to use as a mono block. If any one knows of one for sale at a realistic price let me know.

Regards, Will.

[percival007]

If you disconnect all I/Ps to the Pre, is the Hum still there?

If you just have the Power Amp running, no, or shorted I/Ps, is the Hum there?

I am assuming this is through the Speakers, it's not a Mechanical Hum from the units?

[w7676south]

Thanks 007... hum is through speakers... and yes with no inputs still have it.

No hum if power amp is just running...

W

[JimFromSurrey]

i had a similar problem with a different pre/power combo, the only thing that would make it go away were some ixos 103 interconnects - every other pair of leads i tried (including some exotica) the hum was still there, but the 103's cured it... never did find out why.

there's some confusion on the web about these, the genuine ones are quite thick, pinkish-red in colour and have black, locking RCA's on the end

[Trevor]

Is this an earth loop i.e. you have both the pre amp earth and the power amp earthed you may have to disconnect onl one of the earths and then try it out
Trev

[dave cox]

OK, no hum with with just the power amp.

Next, try with the pre-amp / power amp connected (no inputs connected), with the pre-amp plugged in but turned OFF. If this gives hum then you have an earth loop (as Trevor suggests). This is where some portion of power supply return current (with ripple) is passing through the power amps input 'signal-earth' path. Even the tiniest amount of impedance in this path causes the ripple to be superimposed on the amplifiers input - so you get hum. Unplugging the pre-amp (breaking the earth loop) should make it go away ...

Take care if removing an earth connection, it may prevent the hum but it may introduce the slight risk of electrocution! Breaking the earth connection, at one end, in the connecting lead is safe enough though.

dc

[Trevor]

I the disconnected earth works that.s great But their is a potential safety issue re the lack of earth
You can make a special set of phono leads and only connect the screen at the power amp end
You must of course only connect disconnect while the units are switched off
You can of course couple the screens via high value resistors say 100 ohm or even a capacitor
Trev

[w7676south]

Thank you everyone will give it a try and report back... Best, W