humming line-out

[Alex R]

I'm fitting a line-out to a Vibrochamp, and it hums a lot. Attached is a schematic drawn by our cat, who is learning to use Photoshop.

I'm doing an L-pad from the speaker. I need a dropping factor of about 5 or 10. I've tried various values for the resistors, with R1 at 4K7, 10K, 22K, and R2 commensurately at 470, 1K, 1K5, 2K2, 4K7.

It hums.

The hum is visible via scope on the jack tip contact, but disappears from both jack tip and contact when the jack (to the PA) is removed from my lineout socket. It does not disappear when I lift the ground on the Champ. It instantly disappears when I turn off the mains to the Champ, even before the output fades away.

This amp is a USA model and the voltage is dropped from the UK mains via an autotransformer bolted in the case. The cat says his whiskers twitch when he looks at the transformer. Could the mains difference be the cause of the hum? Am I going to have to use a line transformer? Will even that solve it?

[Twist]

On your autotransformer do you have the low side of the primary tied to the low
side of the secondary? I'm thinking it's a coupling issue with the autotranny.
The ground on your amp may be floating to the ground on the other amp.

[Alex R]

Yeah I wonder... just noticed that it's one of those s/f power trannies with parallel 120v windings - double wires - should be able to hook them up in series for 240v, cut out the autotransformer and see what happens... tomorrow, after the party.

Happy New Year all!

[Enzo]

Has nothing to do with the Lpad, everything to do with your mains situation. Classic gound loop.

[dai h.]

you can try a small resistor on the ground of the line out (around 10 ohms, 15) or you can also break ground connections with transformers.

[Alex R]

- yes, there's a ground right across the mains stepping transformer, I can sort of see the issue there, especially if there's some phase reversal going on across it too - but I did try lifting the ground on the Champ completely to break the loop, with no effect at all on the hum. I'm thinking it's along the lines of the Champ's 0v trying to float to a different place from the PA's, but I am a bit vague about that still, I admit...

Off to try hooking it up as straight 240v. After that if it's still humming I'll see what dai's little resistor can do.

[Alex R]

OK well the 240v hookup worked and the hum stopped, so out comes the autotransformer. Thanks very much for the advice everyone. If anyone feels like having a look at the earlier posts and educating me a bit more about just where that hum came from & the situation with floating grounds I would be as grateful as ever. Oh and why do those small resistors lifting (for example) board earth from chassis earth work? (as in dai's recommendation.)

It will be pleasant to point out to the Champ's owner that the previous job was unnecessarily complicated, and for the benefit of UK techs who might not know this I will point out that many 120v silverface Fenders actually have parallel 120v primaries and can be easily converted to 240v by simply rewiring them in series. They are easily recognisable by their doubled primary leads. Actually I have only noticed this so far on Princetons and Champs.