First of all, they do not exist for tube equipment, maybe there is a hybrid out there I am not aware of, but for all practical purposes anything with XLR connections is going to be solid state.
@billstevenson This statement is false. You might be interested to know that the first balanced line preamp offered to high end audio was the Atma-Sphere MP-1, which we still make. It has an all-tube signal path. FWIW, balanced line equipment first appeared in the 1950s, although back then output transformers were used to produce the balanced inputs and outputs. Our MP-1 is transformerless- we patented a method of direct-coupling instead.
When using XLR connectors be aware that Japanese audio manufacturers use a different configuration for pin for ground.
@toronto416 Its more likely they have pin 2 of the XLR wired as inverting and pin 3 non-inverting (which is opposite of US products). In all countries pin 1 is always ground.
The only potential adverse impact is that the output impedance of the XLR connection might be 2-3 times high. If you have an amplifier with low input impedance, such as a valve amp, you might run into impedance matching issue.
@lanx0003 Usually XLR outputs have considerably lower output impedance than RCA outputs. Its typical in a balanced line setup to see input impedances as low as 600 Ohms (which most modern opamps can drive) although in high end audio input impedances can be 10K to 100K. Our tube preamps can drive 600 Ohms no worries.
balanced in my view relates to when you have really long cable runs which many of us do not.
@emergingsoul The reason to use a balanced connection should be to eliminate ground loops and cable artifact. A proper (AES48) balanced connection will do that. Its worth noting that a lot of 'high end audio' balanced equipment does not support AES48 even though it's balanced. The benefit is there even if the cable is only 6" long.
My understanding is, by definition, single-ended-triode (SET) operation involves amplifying the entire signal with only one output device. A balanced circuit cannot be built this way. But I may be incorrect.
@dseltz Actually an SET has a differential input (but isn't balanced). Most of the time that input amplifies what is different (hence the name 'differential) between the RCA jack and ground. In case you're wondering, the other input to the input tube is the cathode. One way you can tell the input is differential is that if the grid and cathode are tied together there will be no amplification. If your preamp has a balanced output and has an output impedance low enough to drive the cathode, it is possible for the SET to receive and process both phases of the balanced signal. It won't be balanced, but it will be in the differential domain.
