hum with 2 connected amps

[paulster]

Another method would be to use an internal ground return lift (not a mains ground lift) in one of the amps, between the ground return path and the chassis using 2 x 6A diodes back to back in parallel with a 47R flame proof resistor and in parallel with something like a 1kV .01uF cap, whilst keeping all input and out sockets isolated from the chassis (like you see in many commercial amps these days, including the Fender RI standalone reverb unit)

+1.

It's a good, quiet, and safe solution.

[Structo]

How much does it help to have the input and output jacks isolated?

Say you take two identical amps, one you isolate the ins and outs and the other one has the jacks grounded to the chassis at the mounting point.

What would we expect to hear?
Less noise in the isolated amp or what?
Doesn't an input need a ground reference?

Because I see some guys like Aiken and Hoffman preach that kind of stuff.
Just always wondered if it made any difference.

[ChrisM]

My post got deleted?

[Structo]

Where?

[scotto]

thanks all. Grounding the jack didn't help. The isolation transformer sounds like a good solution. Would you recommend one ? I googled and there's a ton out there. I assume it goes between the amps in this configuration? BTW, Nate at Bumbox said he'd happily try to sort it out for free if i send it back, so he's at no fault here. Just want to make that clear. He's a pleasure to deal with and I'm using this thing in a way it may not have been intended, which sounds awesome. It will take a nice overdrive channel into EJ big violin territory when used in front of the other amp. Just need to kill the hummmmmmmmmm.

[C Moore]

I always thought "ground lift" was in reference to signal/input lift, not mains lift. You lift the signal ground on the second amp.
Am I wrong about this? I do not understand how this would present any "electrocution" potential.

[paulster]

How much does it help to have the input and output jacks isolated?

The method we were talking about separates the audio (including B+) and mains grounds, so the chassis no longer acts as the audio ground. This is why you'd need insulated jacks.

You still need to provide a reference between audio and mains grounds in case of a fault though, so the two back-to-back diodes and the flameproof resistor will provide a way to dump current safely until the fuse blows in a failure mode.

I always thought "ground lift" was in reference to signal/input lift, not mains lift. You lift the signal ground on the second amp.
Am I wrong about this? I do not understand how this would present any "electrocution" potential.

Ground lift, properly executed, is a separation of the audio and mains grounds.

Unfortunately it has been distorted in the guitar playing community by many who think these 3-into-2 prong adaptors are actually 'ground lift' adaptors.

Whilst it's true that they do lift the ground if you plug a 3-prong cord into them and then into the wall, it's actually exactly how they're not supposed to be used.

The idea is that on an old 2 prong outlet you'd remove the centre fixing screw, put one of these adaptors in and then screw the outlet plate back in place, through the ring terminal on the adaptor, which connects to the third prong to the metal (grounded) box in the wall. Instant grounded outlet!

With the potential for misuse it's no wonder that Canada outlawed them!

[Structo]

I don't think many two wire house wiring has the boxes grounded.

If the house used metal conduit, the box may be grounded.
In my area, I have never seen that in a residential house.

Just the old two wire going from box to box.