Perfect Balanced Topology

[richie00boy]

One aspect of balanced topology is that you can get away with a virtual ground, as significant current will not be flowing in that node.

However, for stability a Zobel network should be used, and this is normally connected to ground at one end. This is not good for keeping the ground 'clean' and avoiding the need for a real ground althogether. Can the Zobel network be connected across the two amps in the balance/bridge, or to a power rail? (+ve for one amp and -ve for the other, to maintain symmetry.)

[arthur]

CONNECT THE ZOBEL ACROSS THE + AND - OP OF THE BALANCED AMP

[djk]

http://www.mif.pg.gda.pl/homepages/t...s/simp200W.gif

[richie00boy]

Hmmm, maybe it's the way it's drawn, but that doesn't look quite right to me. The left side sort of looks OK, but the right doesn't seem to follow the same logic.

[UrSv]

The Zobel is the 220n cap and the 1 Ohm resistor. Looks fine to me. The confusing part I gather is the stuf below which is part of the bridging set-up AFAIK.

[richie00boy]

I gathered that zobel was those components, but the left side was confusing as the same value components are also used, but not quite in a zobel method (extra 220n at 1R/220n node to -ve).

To confirm: the only zobel on that circuit is the one across the load?

[traderbam]

What's wrong with a "real" ground? You just have to be careful about where the currents flow.

Each amp needs to have it's output damped at high frequencies with respect to gound. This is because the difference signal at the input is measured relative to ground. The supply rails are not ground, nor is the output of either amp.

Your challenge is to create a "virtual ground" through which the zobel currents can flow. You have moved the problem up in frequency where the currents are smaller so that's some benefit.

[Nelson Pass]

There is no more requirement for a Zobel with balanced operation
than there is for unbalanced. I don't use them, for example.

[richie00boy]

traderbam,

I see what you are saying, but what about single-supply operation with AC-coupled inputs and outputs? In that situation the Zobel is connected to the rail as there is no midpoint ground. I'm trying to avoid a real ground as I'm trying to work simply with a single secondary winding. Bridged/balanced topology gives you the opportunity to do this without AC-coupled outputs.

Nelson,

I did actually look at some X-type schematics to see how Pass did it and saw no Zobel. There seems to be some controversy (see chip amps Peter Daniels vs Fred Dieckmann) about whether or not Zobel is needed. I have not tested my amp yet and it may well be stable enough without, I'm just trying to do some preliminary investigation. Note: the amp is my own discrete design.

[Nelson Pass]

Originally posted by richie00boy
I did actually look at some X-type schematics to see how Pass did it and saw no Zobel. There seems to be some controversy (see chip amps Peter Daniels vs Fred Dieckmann) about whether or not Zobel is needed. I have not tested my amp yet and it may well be stable enough without, I'm just trying to do some preliminary investigation. Note: the amp is my own discrete design.

The issue comes down to the intrinsic stability of the amp, and
that usually means low or no feedback. The Alephs and X amps
have not needed Zobels because they are very simple gain
stages and don't have a lot of feedback. By contrast, the
chip amps are complex circuits and have relatively huge amounts
of feedback. (I make no reproach here, I'm just relating the
difference.)