Has any one done this before?

[keantoken]

Hello.

This is obvious enough it shouldn't be only me who's though of it.

Basically, you sidestep the problem of a capacitor on the output of a class-A amp by taking the feedback signal from point after the main output capacitor.

Here's a quick MS Paint scribble that I coughed up to illustrate my point. It should be readable.

If one is clever in their design, one may not need to include a DC blocking capacitor in the feedback path.

- keantoken

[lineup]

Yes.
This has been done many times in the past.
Both in pre- and power amplifiers.

If coupling this feedback rail with capacitor, you get only AC feedback.

[tschrama]

Nelson did it in the ZEN Revisted 1994 article....It's a interesting topology to lineaize the Cap ...

http://www.passdiy.com/pdf/retofzen.pdf

chrz,
T

[unclejed613]

or another way would be to make a bridged class A with the speaker connected directly to the outputs (no capacitors). there would be no DC offset across the speaker, but the voice coil would be at a DC potential.... panasonic uses a similar scheme with their class D amps which use a single ended power supply.... another added benefit is double the power. another way to do it WITH the cap would be to do a DC level shift to the feedback. either method would keep reactive components out of the feedback loop.

[djk]

http://home.comcast.net/~g.e.dunn/ST120/schem2.jpg

[unclejed613]

dynaco st-80?.......

[djk]

ST-120, but they are more similar than different.

[unclejed613]

duhh..... if i had looked at the URL closer i would have known that....

one could also make a class A amp with split rails and a diff amp, and eliminate the output cap that way. it would provide a lower output impedance too... i think the bridged class A is the best way to do it with a single ended supply. it's pretty easy to make an inverting buffer suitable for the application. as a matter of fact the Dynaco amp could be operated like that without output caps, you just need an ST-120 for each channel....

[KSTR]

Another excellent example (and not only in this regard):

http://www.angelfire.com/ab3/mjramp/simple1.html

- Klaus

[keantoken]

This is great, now I have some more study material.

Are there any downsides to doing this?

- keantoken