Why so few balanced phono inputs?

I feel that I should point out that the type of connector (RCA vs. XLR) does NOT determine whether or not a connection is balanced. What determines this is equal impedance to ground from the two signal carriers. Both for the source and the input. With single ended connections one signal carrier is at ground potential (zero impedance to ground). If you have an input transformer and the primary winding (input side) is left "floating" (no connection to ground) and the cartridge is not grounded you have a balanced connection, whether you use XLR or RCA jacks. This will be true even if the preamp itself is single ended. As long as it has an input transformer and the input is floating. I use RCA jacks on my fully differential, balanced phono pre for convenience (XLR plugs would not fit through the tone arm pod on my Nott. TT).

^^ The thing you have to be careful about with this statement is the cable used. If a shield is involved and it is tied to pin 3 while pin 2 has the inside connection, its not going to work regardless of the impedance to ground.

IOW the balanced connection should be at the very least a twisted pair. If there is a shield, it should shield both conductors simultaneously.

thank-you all for the great response.
Sean

Hi Ralph, I always have this question in mind and you are probably one of the best persons to answer.

I noticed that some supposedly full balanced phonos, even when offering both XLR and RCA input jacks, actually have single-ended circuit for the first input stage (the XLR jacks are really single-ended connection), and then convert the signal to balanced mode.

I always wonder why the designers are doing this when they have full balanced circuit and have XLR input jacks already. Is the main reason, as you have mentioned, just dealers don't like to change the cable when they are doing auditions, or there are some intrinsic difficulties in making a balanced phono input stage, such as noise or needing very tight match of parts etc.?

The thing you have to be careful about with this statement is the cable used. If a shield is involved and it is tied to pin 3 while pin 2 has the inside connection, its not going to work regardless of the impedance to ground.

Ralph, if you would note my earlier post on 5/08 I already mentioned that a coaxial cable would make for a very poor balanced line. I was trying to make the point that just because some piece of equipment has an XLR jack on its panel, that doesn't necessarily mean it's truly balanced. The definition of a balanced output or input is equal impedance from the two signal lines to ground. As an example, with some OP amp based balanced to single-ended conversion circuits the impedances do not exactly match and are off by a few ohms. Doesn't sound like much but it can seriously degrade the CMRR.