Balanced in phono stages preamp?

Hi Jeff_jones, we wondered the same thing so we put the same circuit into a more budget priced preamp to see how it works. The same advantages appeared and the phono section of our MP-3 is quiet with cartridges of 0.2mV, while at the same time having only 2 stages of gain and passively equalized. IOW the approach works on a budget too.

>>the way differential amplifiers work is that ground is ignored<<

I'm ok with calling a cartridge 'differential' or 'floating single-ended'. But not 'balanced'. That is where we disagree. In my opinion 'balanced' implies more. It requires 3 terminals.

jh

Hi Hagtech, when you get a chance, take a look at the schematics for the Ampex recording equipment- something tube like the 351 electronics.

What you will see is that the output has an *optional* switched center tap on the output matching transformer. The input, which is also balanced, has no center tap at all. Note that the input is conventional single-ended circuitry with a transformer to effect the balanced input. The ground is handled in a way almost identical to how it is done with a balanced phono setup. Of course, going balanced differential without the transformer offers the possibility of higher performance.

If you look at solid state gear, opamps are often used as a balanced (differential) input; the technique is common.

The point is in all these examples that balanced, even if it is not differential, does not require a center tap. However there is almost always three terminals (the example I gave above with two terminals was in an industrial situation) as you mentioned. In a phono system that third 'terminal' (ground) is shared between both channels.

In any event, the system works. When I did this for the first time in my home system as an experiment, about 1987, I was startled at how straightforward everything behaved. Over the years I've used a variety of interconnects, some shielded, some merely braided and all have operated without a trace of hum. Grabbing the braided cable and squeezing it with your hand made absolutely no difference in the noise floor; the system seemed to all accounts impervious. The technology to operate this way has existed for decades; I am at a loss to sort out why it had not been done sooner!

>>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.