Audio Aero Capitole - 6021W subminiature tube ??

Wow! Those are indeed strange readings. Let me first verify a couple of things, which I hope you won't mind my asking:

1)Did you make these measurements with power off, which is how they should be measured? If power was on, voltages such as dc offset, ac leakage, etc., might have confused the meter.

2)When you say "0,45ohm," "0,57ohm," etc., am I correct in assuming that you mean "0.45 ohms," "0.57 ohms" etc? And are you sure that the scale the meter was set to was ohms, not Kohms?

Assuming those answers are "yes," I too am very surprised at the low readings on the cdp outputs. However, the 0.14 ohm reading from xlr pin 1 to ground appears to confirm that pin 1 is ground, as it should be.

Did you also measure from xlr pin 1 on the Clayton to its ground (the shell of an rca connector, or chassis)? Looking at your readings, I'm wondering if perhaps for some strange reason pin 3 is used as ground on its xlr connector, instead of pin 1.

Best regards,
-- Al

1) Yes I did measurements with power off.
2) Yes I meant 0.45ohm

I did some additional measurements:
Clayton:
XLR Pin 1, ground (on chassis) 0.28ohm
XLR Pin 2, ground (on chassis) 0.9ohm
XLR Pin 3, ground (on chassis) 22Mohm and falling

XLR Pin 1, RCA ground 0.18ohm
XLR Pin 2, RCA ground 0.8ohm
XLR Pin 3, RCA ground 32Mohm constant

Thank you for your interest.

OK. Perhaps the very low numbers looking into the Capitole's outputs indicate that its outputs are transformer coupled. If that is the case, dc resistance measurements probably don't tell us very much.

Based on the additional measurements, I agree with you that the Clayton's xlr inputs appear to be unbalanced. It looks like pins 1 and 2 are both grounded, and the only input used for signal is pin 3, perhaps through a capacitor.

Since the xlr interface therefore appears to be essentially an unbalanced interface, it might be worthwhile temporarily isolating the amp's ac safety ground, via a cheater plug. Perhaps a ground loop, to which unbalanced interfaces are susceptible, is causing the noise (although a ground-loop problem would usually involve low frequency hum, as well as high frequency buzz, and I'm not sure why the rca output would be quiet if a ground loop were the problem).

Also, I'm wondering if the Capitole's output circuit may be having problems due to having the signal it outputs onto pin 2 shorted to ground, by the Clayton. If possible, it might be worthwhile trying a specially prepared xlr cable that leaves pin 2 unconnected.

Just some thoughts.

Best regards,
-- Al

So I went in quest for XLR to RCA adapter.
I have found one gold plated which I liked.
But it got PIN3 disconnected and PIN2 connected to center pin of RCA plug.

But looking at my measurement I need one with PIN 2 disconnected and PIN3 connected to center pin of RCA plug.

So appear that XLR wiring is not universal or my Clayton’s quasi-balanced XLR inputs are wired wrong way ?

Or they are two schools of wiring quasi balanced input in power apms?

Usually USA-made equipment uses xlr pin 2 for the positive polarity signal, and pin 3 for the inverted polarity signal. The opposite is done in equipment made in some European and other countries.

Therefore rca-to-xlr adapters that are available in the USA usually connect the rca center pin to pin 2 of the xlr connector.

If the Clayton is indeed designed to use only one of the two differential signals, perhaps it uses pin 3 instead of pin 2 to compensate for a polarity inversion in its internal signal path. Or perhaps its input circuit is somehow defective, or miswired. All I can suggest is trying to contact someone at Clayton, or one of their reps or dealers.

Regards,
-- Al