Balanced in phono stages preamp?

>>And since phono cartridges are balanced<<

This is simply not true. I'm suprised you would continue to actively propogate this myth. If anyone understands the beauty of balanced circuits and signalling, it's you. You get it.

So why pretend a two-wire device is balanced? It's not. A cartridge is floating single-ended. Balanced operation require three terminals: a common (mid-point, average, reference), a positive polarity, and a negative polarity. You don't have that with a cartridge.

Nonetheless, as I pointed out in another thread, a proper receiver can force a cartridge into balanced mode. This is done by using a center tap on the primary of a step-up transformer. The center tap connects to ground. The windings then force the cartridge to act as if it were balanced (but without the 6dB gain). I wonder how many of these so-called balanced phonostages actually do this. And how many simply connect one cartridge tap to ground and pretend?

jh

>>Proof? Invert the phase of one channel of your cartridge in your system and see if it hums<<

You're kidding, right?

>>Balanced is hype<<

There is more to balanced circuitry than is obvious to the casual observer. One significant advantage is PSRR. Or the way it interacts with a power supply. Think holistically for a moment, everything is inter-related to everything else. I can walk through an example here.

What is the best type of regulation? Many will say shunt. Why? Because it offers a lot of line rejection and can be made very fast. It is also class A. What is better than shunt regulation? What if (pretend it is possible) instead of reacting to a load variation, as a shunt regulator does, that you could know in advance what the variation will be? What if the regulator acted precisely at the same moment and without phase delay to a load transient? And without feedback. Would that not be better? Imagine correcting the supply rail perfectly in response to the music. Well, that is basically what a balanced differential stage does. One way to look at it is a single-ended stage mated with a shunt regulator. You can actually run it that way. There isn't any other topology out there that can top that.

Now take it even further. You can add passive line regulation (no feedback). In a balanced phonostage I once designed you could take a variac on the ac line and crank it up and down (simulating line disturbances), and the dc output level from the amplifier doesn't budge. Yes, the power supply rails move in response to the variac, but the output doesn't. Well, at least not at a 1Hz variac rate. The point is, there are tricks you can pull with differential topologies that you can't use anywhere else. It ain't hype. Unfortunately, there is a big price to pay. It takes double the number of devices. Cost goes way up.

jh

>>Everyone here is getting the terms differential and balanced mixed<<

That would be me! Perhaps we are all in agreement technically, just not semantically.

For me, "balance" is like a scale (in the original sense). Or akin to a see-saw. Two ends, and a solid pivot point in the middle. Remove the pivot and ... well ... you have a cartridge.

http://en.wikipedia.org/wiki/Balanced_line

Calling the turntable ground as the "third wire" is disengenuous. As was pointed out above, it is not part of the signalling.

jh

>>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

>>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

>>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

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