Output inductor in power amps - pro and con
 

I simulated output impedance of two of my power amps, TT amp and LittleGem, in both cases before and after an output inductor.
This has leaded me to a conclusion that output inductor has an additional function beside to protect an amp against capacitive loads.
Long loudspeaker wires can pickup electromagnetic disturbances, and the loudspeaker drivers generate EMI and we don’t want that all this enter the amp, but to minimize it.
Some audio designer are against use of the output inductor in power amp, but in my opinion there is more advantage then disadvantage to use it.
I am using the output inductor in all my amps, but in JLH MOSFET amp were is used 0.22 ohm resistor instead.
From the simulation could be seen that output disturbances are suppressed from from 50 dB(LittleGem- double EF) up to 70 dB(TT amp, TRIPLE EF).
To this should be added closed loop gain(28 dB in my case) to get the level of output disturbances suppression at a GNFB input.

Did I get it right??
dado

I think that protection against capacitive loads would not be the primary function as much as forming a barrier between the antenae (speaker wires) feeding RF ingress into the input stage via the feedback loop. Typically this coil is located at the speaker terminals of the amp enclosure, while the Zobel should be close to the output stage.

few uH series L are included in amps, explained by the designers as isolating the output stage from potentially destabilizing C load

possible RF ingress isolation has be recognized as a possible side effect, its also been shown how to rearrage Zobel components to improve the RF filter aspect

all you have to do is read "coventional" audio amp design books, articles by Self, Cherry, Cordell

RF rejection of alternate Zobel?
 

Well, Self is completely dismissive of any alternate connection of the Zobel after the inductor. It looks like a deliberate snub of Cherry's work.
But Cherry's article in Electronics World July 1997 doesn't provide any analysis of RF rejection in his proposed alternatives.
Bob Cordell's book is the best on the subject but there is still no real analysis.
Intuitively I expect that the Zobel after the inductor would have better rejection but I would like to check that.
Have I missed a quantitative analysis?

And thanks to Dadod for his post. RF rejection becomes even more important with some advanced feedback compensation methods so this is very useful.

Best wishes
David

I have all those books, but a role of the zobel and series inductance is not so well explained nor simulated. Next simulation is with added zobel after series L and the load is electrical model of a single speaker unit ia sealed box(from Self book). I don't see any differences with previous simulations. Still very good suppression in the audio band but a low dip at 7MHz. Main reason for this dip is a sharp increase of the power amp's output impedance at that frequency.
dado

Hi Dadod, we cross-posted fairly similar comments.
In your simulation of the downstream Zobel you can leave out the resistor in series with the Zobel capacitor (R15). The inductor damper resistor provides sufficient resistance I think. This should improve the rejection.

Best wishes
David

David, no, it didn't. With no downstream zobel the result is the same, so in my simulation second zobel does not show any improvement in RF rejection.
BR Damir

I don't understand why not, but that's why we check.
Thank you. I need to think about it.

Best wishes
David.

Could be that my simulation is not good enough, that's why I presented it here to get comments and suggestion.
BR Damir

The same intuition tells me that for HF rejection it work better
if connected before the inductance since it form a high frequency
damped L + RC circuit , seen from the speaker cables as being
the generator/antenna.