In article <h2pr6r$kn6$1@news.albasani.net>, The Natural Philosopher
<tnp@invalid.invalid> wrote:


> Whereas using a small 10KHz signal could often show ghastly distortion
> of several percent..

> The key is to use modified class AB with bipolars, and run in class A
> for small signals, and also to try never to actually switch one side
> OFF, as that led to issues with time delay switching it back on:

That was true in early days. However by the early 1980s I found that the
main problem with audio power bipolars tended to be the switch-off time due
to carrier storage in the base region.

That said, during that time the good japanese bipolars had pretty well
cracked this and even with designs that delivered over 100wpc you could get
negligable crossover problems even with bias of the order of 10mA per pair.


> A nasty problem of thermal stability was then encountered.

Again, that was certainly a serious problem in early days, particularly if
you used Ge devlces like the beloved <sic> AL102. :-) But provided you
knew how to work out the stability margin and select the emitter resistor
it wasn't so much of a problem by the early 1980s in my experience. By then
makers could produce devices with consistent specs, unlike in earlier days
when every device seemed like a 'one off special'. 8-]

> The adoption of power FETS with much better frequency responses,
> negative temperature coefficients and low switch on delays made that
> almost a non problem.

Must admit I never liked power FETs for audio as the ones I tried years ago
all liked to hoot at HF, had a habit of current limiting, and shoved
capacitance where I didn't want it. But again this was in the 1980s so I
imagine they rapidly improved and have been fine for some years.

Overall, I'd be happy to use amplifiers that employ either bipolar or fet
if the designer has produced a decent result.

> Everyone used feedforward at some point to sharpen up transient
> response, and compensate for overall lag. .


Not sure I am "everyone" then. But my memory is fallible. :-)

Slainte,

Jim

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