Audio Aero Capitole - 6021W subminiature tube ??

How do I check that ?

If you have a multimeter, check for continuity between xlr pin 1 and the ground sleeve of an rca connector. Otherwise check with the manufacturers.

However, the most common inconsistency in xlr pinouts is that the signals on pins 2 and 3 are interchanged, which would not affect hiss levels. It would just invert polarity, sometimes referred to as "absolute phase," which would have minor or negligible effects on many or most recordings.

Also, check that continuity does NOT exist between pin 2 and ground, and between pin 3 and ground. A near zero resistance between one of those pins and ground would indicate that the unit does not drive the xlr connector with a differential pair of signals. I would be very surprised if that were the case, but if it were it would go a long way toward explaining the hiss.

Regards,
-- Al

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

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

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