Why so few balanced phono inputs?

I agree that balanced connection is not per se necessary for superb sound reproduction. But where do you get the "110 ohm vs 75 ohm" idea? The impedance in a typical audio system will be only be that which is reflected by the interconnect, depending upon the output impedance of the driving device and the input impedance of the driven device, no matter how you slice it. Further, I thought "pro audio", where distances are great at times, use 600 ohms.

Further, it has been my experience that those manufacturers who offer true balanced connections are prone to say why it is superior to SE connections. Conversely, those who make only SE devices (like Manley Labs) are prone to diss the notion of balanced connections. So, I would take Ms Manley's opinion with more than a grain of salt. If done well and properly, there is no doubt in my mind that there is an advantage in noise issues associated with balanced connections, especially from cartridge to phono stage, if only because of fewer problems with grounding. That's just a "feeling" based on 3 decades of experience with both types.

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.?