Valve phono stage

so why do top phono stage designers use RCA inputs?

I’ll give that a shot, pending Ralph’s response.

Turntables came into being before low output moving coil cartridges became popular among audiophiles, and perhaps before any even existed. Moving magnet and other high output cartridges have much higher impedances than LOMCs, and consequently will exhibit cable sensitivity even if the interconnection is balanced. For example, as you are probably aware manufacturers of moving magnet cartridges typically provide a recommended range of load capacitance, to which the cable is usually a major contributor.

Also, when turntables came into being RCA connectors and unbalanced interconnections had become the norm on consumer electronics equipment, for a variety of reasons. So putting XLR connectors on turntables and/or phono stages or preamps would have resulted in a non-standard interface that would have ruled out the use of the RCA-to-RCA cables most people had. The additional physical space required by XLR connectors might also have been problematical in the design of some turntables and/or phono stages or preamps.

So for various reasons a tradition arose in the early days of LP playback, and traditions are often self-reinforcing.

Regards,
-- Al

Edit: This was composed before I saw Lew’s well stated response above.

Infection, **If** the balanced inputs of your fully balanced Wyred 4 Sound STP-SE Stage 2 preamp provide significantly better sonics than its unbalanced inputs (and I doubt that anyone not having experience with that particular design can do more than speculate about that), and if you want to consider the Herron or some other phono stage providing only unbalanced outputs, you may want to consider the possibility of converting those unbalanced outputs to a true balanced pair of signals using a Jensen transformer.

Although I have not used Jensen transformers myself, based on a number of experiences with them that have been reported here by highly experienced audiophiles my impression is that they are quite good. And for that matter, the excellent Allnic phono stage that was suggested earlier uses a transformer to drive its outputs, although it apparently has a fully balanced internal signal path and provides a lower output impedance than the Herron would in combination with a transformer. (The output impedance difference would most likely not matter in your case, though, given the 60K input impedance of your preamp, and assuming that interconnect lengths would not be particularly long).

The Jensen PI-2RX listed on this page ($250) would be a suitable transformer, IMO. An additional benefit it would provide is that it would eliminate the possibility of any ground loop issues between the phono stage and the preamp.

Good luck. Regards,
-- Al