Balanced in phono stages preamp?

Forgive me, I tire of this thread. What started as an intelligent debate has devolved into childish wordplay. I must be annoying everyone as I keep repeating myself (a sign of insanity) like a broken record. I'm like a homeless bum pushing a shopping cart mumbling to myself. Seriously, didn't I just say:

"...or some other reference voltage similar to ground..."?

And before that:

"...the receiver is providing the common mode reference..."?

Maybe my posts are moderated and nobody else can see them. If so, I apologize. I had thought the see-saw analogy and the fact that the input stage is providing the common mode reference (pivot point) for the cart would make sense.

Why did I bother? It's very simple. I like the truth. I don't like the way marketing can obscure and twist reality. You see, there are a few phono stages out there with XLR inputs that have pin 3 tied to ground. Hence, they actually run single-ended. Clearly that is not the case for atmasphere or bat. Just a few I won't name. The problem, as I see it, is that perpetuating this myth that a cartridge in and of itself is a balanced transmitter gives these folks a pass. Customers end up believing. Hey, it had an XLR input, right?

The tragic irony here, is that the guy doing the most to benefit the disengenuous is someone who makes the effort to do it right.

jh

Hi jh, 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. This way the cartridge does not have a 'center tap'. Yet it still works exactly the same way- no noise running as a balanced source into our balanced differential input. It does in tact satisfy all the requirements of a balanced source.

Inductors being what they are, you can create a balanced output with a transformer that is driven single-ended. And it works fine- the technique has been used for decades. Cartridges are no different in this regard.

You might consider the implications- I took a look at your website and you have some impressive little goodies!

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.