Balanced in phono stages preamp?

When you get away from purely technical considerations, and you have the average buyers desire for best bang for the buck in both sound quality and reliability, you wonder which approach outweighs the other at a fixed cost of manufacture?

Hi jh, the way differential amplifiers work is that ground is ignored. Our preamps are fully differential from input to output and so the ground is merely a shield- no currents are passed through it. This helps prevent ground loops.

As I mentioned, differential amps ignore ground. They have two inputs: non-inverting (positive) and inverting (negative). The differential amplifier will do nothing if the signal is the same on both inputs; it will only amplify what is *different* between the two inputs.

Thus there is nothing disingenuous about the tone arm being ground- it *is* at ground and is providing a shield as the signal leaves the cartridge.

Balanced inputs that are not differential are a different story- ground can play a crucial role. Differential amplifiers (a subset of balanced amplifiers) consequently offer higher performance, less complexity and lower noise.

Herman, There is a tendency to try to create interpretations for all sorts of experiences that we have in the world. In the case of the SE/balanced issue, the interpretation that I am referring to is to identify sounds within the realm of SE or balanced when it is neither. Its OK to question this- I did myself for a while until I got that I had to give up the interpretive story. Then I was able to realize that sound simply is what it is and is neither SE or balanced but a series of pressure variances in the atmosphere.

It is when the sound arrives at a microphone that it suddenly appears in the realm of electronics and is subject to being SE or balanced. All dynamic pickups are balanced as are capacitive pickups (although their preamps are often SE) and all the output transformers on microphones are balanced and on and on.

There is merit to the argument of keeping what started balanced in that domain, but sound itself has nothing to do with this. To give you some perspective, SE signals travel in a cable with a single conductor and a ground. The ground connection is the 'return' circuit- where is the return circuit for sound itself? There is none- it is an entirely different phenomena from that of electronics.

On an LP the sound is encoded in grooves due to a 3rd phenomena: a mechanical process and is again neither SE or balanced. However FWIW the cutterhead is a balanced load insofar as the amp is concerned: like all other balanced devices reversing the connections does nothing except invert phase (a property of a balanced device).

If it helps at all, take a look at an Ampex 351 schematic and see how the input and output transformers are used to accept or create balanced operation. Its an eye opener: a center tap is not part of the equation...

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!