Floating PS through 10Ω & .1uF to ground

[Guitarist]

I've been seeing/hearing this on a few quiet amps. Even soldered permanently. We're used to seeing heaters afloat, but B+ or the whole amp too? Would this work on a preamp, to alleviate hum even though star grounding was used? (not me of course But the Marshall 7100 goes there.
I had this circuit on a Rockmaster and didn't notice a difference.

[Steve Conner]

This is a commonly used trick, both in MI and stereo equipment, to break ground loops that are set up between the amp and other pieces of equipment you plug it into. The classic example being when you try to use two amps and switch between them with an A/B/Y box.

I never liked it, I always felt safer knowing that my axe is solidly connected to the green wire of the power cord. If I get a ground loop in my gear, I'll use an audio transformer or a balanced connection to break it.

[tubeswell]

I think its okay so long as it is only used to isolate/elevate the ground return path in the circuit from the chassis. The circuits I've seen have a pair of back-to-back 6A (from memory) diodes in parallel with the resistor and the cap too (for extra insurance) (- and make the resistor a flame-proof resistor). But it is foolhardy and dangerous if you try to elevate the mains earth connection from the chassis this way.

[R.G.]

Be aware that such an amp will likely not past any regulatory safety regulations.

[loudthud]

Be aware that such an amp will likely not past any regulatory safety regulations.

The safety ground must be connected to the chassis. Use diodes rated at 25 amps and you'll pass any "fault" test that a regulatory agency might run. The RC network isolates ground loops when two or more pieces of equipment are connected together and they have a common safety ground through the power cords.

[Enzo]

And that little network is isolating the circuit common from the chassis. It is not in the way of earth connecting to chassis.

[R.G.]

It depends on who tests your amp for safety concerns. If you use a 1/4" phone jack for the speaker out that also has a metal bushing, then the metal bushing qualifies as "accessible metal" under the standards. After that it's up to YOU to convince the tester that it's safe when it does not show continuity to chassis or third prong.

[R.G.]

The safety ground must be connected to the chassis. Use diodes rated at 25 amps and you'll pass any "fault" test that a regulatory agency might run. The RC network isolates ground loops when two or more pieces of equipment are connected together and they have a common safety ground through the power cords.

I'll have to go look in my IEC600650. I believe that the test current of 25A has to pass with less than a diode-drop of voltage. I'll check. My (dim) memory is that it won't pass.

It's not that 25A diodes won't hold line faults to a diode drop. It's the way the standard is written that is the problem.

[loudthud]

It may have been rewritten since the last time I was forced to read thru one of those standards. My recollection is that the fuse just had to blow when the fault occured. I've noticed that Fender guitars have the CE logo on them now days.

The other thing I wonder about is the diode connected across the 1 ohm cathode current sense resistors. Is this to prevent a high voltage from comming out the test jack if the tube shorts and the resistor opens?