Balanced in phono stages preamp?

if I provide two independent floating bias supplies for the input of the preamp, I don't have to have the grid resistors go to a common point.

Perhaps I have no imagination but I don't see how that can work. Everything still has to have a common reference point. The bias is applied from grid to cathode, and the cathodes have to have the same reference in a diff amp, so the supplies would have the same reference and not be floating. Correct? It sure would be nice if you could post a picture on this forum.

Hi Herman, the point is that to operate balanced differential you *don't* need a common reference point. I don't know how that idea got started but it is mythological insofar as differential amplifiers are concerned.

As I have mentioned before, to accomplish balanced differential operation you only need two connections: the non-inverted signal and the inverted signal. Traveling together, the two signals can be remarkably resistant to noise *even without a ground*.

In fact in some extra noisy industrial environments the ground can actually make things noisier; sometimes it is omitted and the signal travels in a simple twisted pair. I have a friend who works in motion control and sees this a lot. As long as the bias requirements of the input stage are satisfied there are no worries. That's why I was mentioning the ability to use dual floating bias references in the exchange above.

I've seen instrumentation inputs that use ground references (non-differential) but they lack noise performance compared to those that ignore ground (differential). Differential amplifiers have a secondary advantage as they are simpler than otherwise 'balanced' circuits.

Any inductive pickup (phono cartridge, tape head, guitar pickup, dynamic microphone) can be operated balanced due to the non-polar quality of the device itself. In guitar pickups this can be a powerful advantage as hum is a big problem with electric guitars.

In a phono system the ground is only a shield and does not provide a reference- neither is it connected to the cartridge. In my previous post, we altered the biasing strategy of the input stage to prevent the input resistors from being an artificial center tap for the cartridge while satisfying the bias conditions that the input section needs. So there is no ground connection to the phono section nor one to the cartridge, yet the setup works fine, because differential amplifiers and balanced sources do not *need* ground references to work: differential amplifiers only amplify what is *different* at their inputs, in this case, the phono signal.

That still doesn't explain how you can establish bias with 2 floating supplies like you claimed earlier. I say you can't.

I’ll say it again. The cathodes are tied to a common point; there must be a DC path from grid to cathode to establish bias so the grids must also be referenced to this point. The supplies can’t float.

As soon as you hook up these differential signals they are referenced to the common connection via the grid-cathode bias circuit.

I need to see a schematic of how you can accomplish what you claim before I believe it.

Don't get me wrong, I am intrigued by the idea of feeding my cartridge into a diff amp and have that on my list of things to try. I just don't think what you described is possible.

Hi Herman, the input of the preamp is differential so the cathodes are tied together. The cathodes are also tied to a 2 stage vacuum tube constant current source whose cathode is tied to a resistor that is tied to B- (-250V).

So my bias references are a set of resistors that go from B+ to B- and have a -1 volt output on each one. One side is adjustable to match to the other network. Although this is not how we normally do it, it works as well.

Interesting. So the voltage on the grids is slightly different and the coil of the cartridge is hooked across the grids? Doesn't this put some DC current through the coil?

Hi Herman, not if the voltages are kept the same. That's what the adjustment is for (in this demonstration case).

In our production preamps, the grid resistors are tied to ground and for the sake of simplicity, double as the input loading (however when other cartridge loading is used the single resistor for that purpose is tied directly across the output of the cartridge and has no connection to ground). We could have done a variety of scenarios here but I favor simplicity whenever possible.

Ralph and Herman are speaking Greek as far as I'm concerned.

I do, however, very much enjoy the sound of the phono stage of my Atma-Sphere MP-3!