Could someone explain this Hafler/Strickland output topology to me?

[SQLGuy]

Hi folks,

Recently I was curious if I could find schematics of the last Haflers (the 9500/9505 specifically). I did, but I don't understand how this output section works - i.e. how the current is supposed to flow through the output drivers relative to the + output without shorting the power supply. Could someone explain it to me?

Thanks,
Paul

http://k-amps.8m.com/cgi-bin/i/Power...afler_915c.jpg

[Zen Mod]

mid junction of filter caps isn't referenced to gnd , as usual ;

amp output is referenced to gnd , and caps are referenced to gnd via speaker .

you can do that with almost any amp

[SQLGuy]

OK. Now I think I see it, but it looks more to me like the caps and power supply are referenced to ground by the outputs, not by the speaker. I.e. when the N-channels are saturated, the + end of the power supply will be near 0V, thus the + output will be about -63V from the - output.

Interesting.

What's the advantage of something like this over a regular common collector / common drain setup?

[bear]

The output stage's configuration isn't really any different.

However the place where the load is, is different.

I am not sure, but you do get some benefit - I think - when driving "wierd loads", and there is no "DC" connection to the load, since you are running through the caps...

The feedback has to come from another place too - loop feedback, since the usual junction is now at ground.

_-_-bear

[SQLGuy]

I don't think so. The caps are not in series with the current path here.

If you draw an equivalent I think you'll see that the output is DC coupled.

The difference seems to be that the outputs vary the coupling of the power supply to ground, rather than varying the coupling of the power supply to the load.

[wg_ski]

QSC likes doing this, too. The only real disadvantage is that each channel must have its own power supply.

[Michael Chua]

Originally posted by SQLGuy
What's the advantage of something like this over a regular common collector / common drain setup?

The caps block DC when outputs fail. No need for relays and sensing circuits.

If you plan on doing this, I suggest using Low ESR caps and a healthy margin in voltage rating. I have quite a few Altec and EV amps with their supply caps blown off. No problems with the QSC though.

Mike

[Zen Mod]

Originally posted by SQLGuy
OK. Now I think I see it, but it looks more to me like the caps and power supply are referenced to ground by the outputs, not by the speaker. .....

I wrote:

Originally posted by Zen Mod
...... and caps are referenced to gnd via speaker .

..........

......where "via" means "through" ( not "with" )

[SQLGuy]

Originally posted by Michael Chua
The caps block DC when outputs fail. No need for relays and sensing circuits.

And I still say, no. If the N-channels were to short, for example, you would have -63VDC on the + output and 0V on the - output. C16 would be in series with the load, but so would the two, forward-biased, rectifiers in the bridge, which are in parallel with C16. F1 would eventually blow, but in the meantime you'd be feeding DC to the speaker.

C16 is a power supply filter cap, not an output decoupling cap.

[wg_ski]

It's DC coupled if the center tap of the trafo is connected to the common terminal between the filter caps. In QSC's older version, there was no center tap, and it is AC coupled.