On one leg or two legs?

OK,
Neutral to chassis- 0 v. on all meter settings
Hot- Neutral 120V.
Hot to chassis:
Power cord unplugged 25 V.
Power cord plugged-in, amp. is OFF- 112 V.
Power cord plugged-in, amp. is ON- 90 V.

As usual, I have no clue, how to interpret all this.

Maril555,
You misunderstood what I wanted you to do. All the test measurements are taken at the wall receptacle outlet only. No audio equipment is involved with the test.

In simple terms the test will tell if there is a good electrical continuity between the main incoming service neutral conductor, of your electrical service, and the equipment ground. At some point the electrical service entrance neutral conductor is connected to earth and is also bonded, connected, to the main equipment enclosure as well as equipment grounding bus bar/s. In most cases this is done at the main service electrical panel. Not sure in your case though. I noticed from your pictures the main breaker is not in the main panel so it is more than likely out side by the electric meter. Connection to earth and equipment bonding may have been done there.
So the test is a simple way to look at the thing just from one branch circuit and receptacle.
I tried to explain best I can in layman terms.

So at the wall receptacle outlet only.

Measure from the neutral to the equipment ground, (U shape hole).... Should show 0 volts

Measure from the hot to the neutral.... Should be around 120V to 123V

Measure from the hot to the equipment ground, (U shape hole)... Should be the same as the voltage measurement from Hot to neutral.

And one more measurement if you would. Plug in and turn on one of the power amps and using the other receptacle of the duplex receptacle measure the voltage from the hot to the neutral. This will give a somewhat load condition on the branch circuit for any voltage drop. The voltage should be within about one volt of the reading with the power amp not connected and turned on.
A true test would be to play some music at a moderate level and take the measurement.

Got you,
Hot- Neutral 120 V.
Hot- Ground 120 V.
Neutral- Ground 0 V.

With the amp. ON- Hot - Neutral 120 V.

BTW, thanks a lot for your patience.

The measurements are good......

The bad news is it does not help the hum problem.


From the pictures you supplied of the sub panel what I could see the electrician you hired did a good job. The only code violation I could see was the (4) 30 amp breakers he installed for branch circuit protection instead of 20 amp breakers..... Just guessing he did what you probably requested.
At any rate that would not be causing your hum problem.

You seem to have isolated your hum problem.... The problem exists between the power amps and the speakers.

Could your problem be RFI/EMI on the AC mains of your home's electrical system? Well if you live in an area that is warm at the present time you could shut off every breaker in the main panel except the 2 pole breaker that feeds the sub panel. Then check the amps for the hum.
LOL, take a flashlight with you....

If the hum is gone then start turning back on the breakers one at a time until you find the source of the problem.

Still hum? Then, jmho, I would think that would rule out RFI/EMI noise, interference, on the mains. That is, at least from a source within your home. That does not mean EMI/RFI noise is not coming in from outside your home on the utility power.

==========================
Maybe the problem is the power amps and the sensitivity of the speakers??

By chance do you have another pair of speakers you could try? Less efficient?

You might try a different pair of speaker cables. For a test just some cheapo Home Depot rip cord would do.

Sorry, I am out of ideas......

I did send an email to an EE who posts here on Agon. I asked him to read the thread and post a response if he has any ideas.

Jim

Well,
I already tried turning off all circuit breakers, no help.
Tomorrow I'll bring the amp to the living room to try with another pair of speakers and another outlet.
I have two other pairs of less efficient speakers, so I'll give it a try as well.
BTW, what's your take on the high chassis voltage, higher with the correct polarity?