In gear that really does offer both a SE and a separate balanced input, the gain claimed by the manufacturer is usually quoted as being different for the two types of input, with the balanced circuit gain being higher. Why is that or is it a false claim?
@lewm Let's assume a transformer coupled balanced input. The signal is applied to either end of the winding of the transformer. Now ground one side without changing anything else. The gain remains the same because the same Voltage is applied.
Now if the balanced source is simply two single-ended sources with one out of phase with the other, if you run that single-ended you only use one of the two sources. This method works but is not AES48 compliant. When both sources are being used the signal Voltage is twice as much so there is a 6dB increase. That is not how its supposed to work if you want the most performance out of the balanced connection.
In the PE505 owners manual, the data sheet shows the gain is the same via either input type. I took this to mean that cleeds is correct in his assertion that the RCA inputs are hooked up for balanced operation.
It can mean also that the input is a differential amplifier, such as an opamp. The CMRR (Common Mode Rejection Ratio) is high enough that the opamp won't really care if you drive one input or both. As long as the input signal is the same Voltage either way the output level won't change. IOW the opamp amplifies what is different between its inputs; if one input is at ground its fine with that; the overall gain is the same.
Do you know why the owners manual for the PE505 says the XLR inputs are for MC cartridges only?
Its a high gain input so can be overloaded with a high output MM cartridge.
You often quote AES48. What does AES48 say?
AES48 is part of the balanced line standard regime (the other part being the dBm level the source can drive). Essentially its a connection standard; in it we see that the ground circuit (such as the arm tube of a tonearm) is not carrying any signal return (as the shield of an RCA connection might). Instead the balanced line signal is generated entirely with respect to pins 2 and 3 of the XLR connector and carried by the corresponding conductors in the interconnect cable.
This means it is possible to have a balanced connection with no shield at all, since ground isn't part of the balanced signal. The shield is there for shielding only and nothing else.
There seems to be fair amount of high end audio balanced equipment that is not AES48 compliant. Such equipment usually has dual single-ended outputs with one of the outputs out of phase with the other, but both referencing ground and therefore fully functional if ground is used as a single-ended signal return with one of the two outputs.
Put another way a phono cartridge is a balanced source that is AES48 compliant. If it were non-compliant it might have 6 output connections instead of 4, the extra two connections being center taps of each coil (for ground). Its not done that way because a center tap can never be exactly center of the coil and so can significantly degrade CMRR. By allowing it to float CMRR is as ideal as possible.