Balanced in phono stages preamp?

>>In a phono system that third 'terminal' (ground) is shared between both channels<<

Thank you. That's the point I've been trying to make all along here. The receiver is providing the common mode reference, not the transmitter. In your case, a resistor to ground on each side of the differential input. So in this particular case, the differential circuit *does* care about ground.

It is the same with mic inputs, which uses a 6.8k resistor in each leg to provide the common mode reference. The term 'phantom' power comes from the original use in telegraph lines (before telephones), where they were used in both differential and common (phantom) modes. Truly, the original balanced electrical circuit.

Look, that's all I'm trying to say. A cartridge by itself is not a balanced transmitter. However, with proper receiver circuitry we can *force* it into balanced mode (which does require 3 terminals).

jh

So in this particular case, the differential circuit *does* care about ground.

Well, no it doesn't. In our preamp we have two resistors because we have to have a 47K input impedance and because the tubes require a grid leak resistance- for biasing. We could have set the unit up with a pair of 1M resistors and put the 47K across the input, from pin 2 to pin 3. It would have worked as well, but it would have been more parts.

When we load the cartridge, the load is applied with a single resistor from pin 2 to pin 3 of the XLR. The ground functions only as shield with no signal currents.

>>We could have set the unit up with a pair of 1M resistors<<

Exactly! For the circuit to work you *must* have a common mode connection to ground (the 3rd terminal). Or some other reference voltage similar to a ground. I don't understand why you keep trying to deny this. Just because you make the connection inside the box doesn't mean it is non-existent.

If you can get your preamp to operate in balanced mode without these 47k or 1M resistors connected to ground, then send them to me and I will eat them.

jh

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Hi jh, It sounds like we're not talking about the thing. How about if I create a bias voltage for the input section that is set at something other than ground? I could set it a -1 volt for example. You really want to eat those resistors??