Balanced Phono RCA Only

BRSHiFi

Previously known as kingden
2008-08-28 4:27 am
Evanston, IL
My turntable has only RCA outputs (no XLR support) that cannot be detached from the unit. I am designing a fully balanced phono stage that works both with MM and MC cartridges, with a variable load feature for universal cartridge support.

An RCA cable has a hot and screen. The screen is a heavy duty shield that encircles the hot and connects to ground. If I treat the RCA cables as balanced cable the shield would become hot. XLR connecters have two hots and a shield.

Will hum be picked up since there will be no shield if I use an RCA cable as a balanced transmission line?

It seems like it would not because the hum would have to not be common to both hots. Anyone have experience with this?

Also I am concerned about capacitance since the cartridge I am using is a Shure MM with a 47k impedance.
 
My turntable has only RCA outputs (no XLR support) that cannot be detached from the unit.

The same as many TTs/arms. :p

I am designing a fully balanced phono stage ...

Why are you bothering to do this? What better SQ do you think you will obtain from having a "balanced" phono stage? :confused:

... with a variable load feature for universal cartridge support.

All you need for this is 2 parallel pairs of input RCAs - 1 for changing load resistance, 1 (for MMs) for changing load capacitance. These RCA sockets take "loaded" plugs.

Regards,

Andy
 
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You can run the cartridge balanced, the gnd wire is then the shield, and a dual cartridge loading sets the midpoint, but this is rare as true balanced phono-stages are quite rare. some phono-stages even totally float the input and use the output gnd for shield. This can be a very very quiet (hum free) solution, but something I have not seen in any commercial designs. Downside on balanced input is the noise penalty, where it suffers compared to single-ended inputs.