Why so few balanced phono inputs?

Well length does matter a lot in professional audio and that's why they use XLR, much lower noise floor and 110ohm vs 75ohm. If your trying to FIX a problem with detail and/or sound quality by switching from RCA to XLR, it dose not always make things better. If your company uses balanced circuts for its audio products that's great. That is a designers choice...But to say that all balanced designs are going to sound better than unbalanced ones is horse shit! Talk to EveAnna Manley at Manley labs who among many others who don't see using unbalanced as a step back in sound quality over balanced. IMO balanced is not the last word in sound quality.

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