In article , Johny B Good
wrote:


I was designing and building PA amps back in the 70s which absolutley
relied on tight PSU voltage regulation to avoid blowing my chosen output
transistors (Motorola 2015s recovered from Gvt surplus acquired analogue
Computer PSU fan cooled heatsink assemblies - twenty to a fan cooled
heatsink module) which weren't particularly good performers slew rate
wise and only had a max Vce of 60 volts.

By using a 45v 10A analogue voltage regulated supply for each channel
(again, using more of those Motorola 2015s) along with a solid state
version of the polyfuse (yet another 2015) of my own invention and using
a bridge output amplifier design, I was able to make a 200W RMS per 4
ohm load stereo power amplifier (800W PMPO in the language of cheap hi
fi except that it was sustained PMPO as in a total of 400W of 500Hz
square wave into 4 ohm resistor loads on each channel).

[snip]

Now, in spite of the merits of regulated voltage rails for the output
stages of a Hi-Fi amplifier, there's also some merit to the use of
completely unregulated analogue PSUs, provided the output devices have
an ample margin of voltage rating.

Indeed. At the end of the 70s/early 80s I deliberately chose to use
unregulated supplies specifically to let the amp deliver transient peaks
well about the amplifier's 'continouous' rated power.

The drawback is that this can remain hidden when people read adverts or
reviews. So doesn't help 'sell' the amp. But does let it deliver more when
it comes to music. Lets it play undistorted at much higher levels.



IMHO, there's no real need to have a fancy VU meter monitoring the
amplifier's output voltage. A minimalistic clipping indicator is all
that is really required in practice.

Indeed. I stuck a LED on the output of a monostable that triggerred when
the difference in voltage across a longtail pair rose when the amp clipped
or struggled. So if the LED stayed unlit you knew the output was
essentially just a scaled up version of the input with no clipping or
limiting. Even very short 'clips' would cause the LED to light for about
half a sec so you had time to see the event.

[snip]

I take issue with the quoting of 'Damping Factor' figures expressed as
a ratio of speaker impedance to amplifier output impedance, eg DF of 400
on 8 ohm speaker loads, implying an output impedance of just 20milli
ohms. It would be better to simply quote this 20 mill ohms figure (the
lower the better) than to falsely claim that the amp can dampen the
speaker cone movement 50 times better than an amp with a Zo figure of 1
ohm.

The whole thing is a nonsense.

Damping Factor made more sense back in the days when power amps were
valve and almost all of them had high output impedances. But even then it
was a weird term given the details. e.g. Amps having an output impedance
that varies with frequency, level, etc, and generally *not* being
resistive. And speakers also being nothing much like a resistor. Alas we
seem stuck with the term.

The amps output impedance can matter, purely due to any effect by
interacting with the speaker's impedance variations with frequency or
signal level. Not really an issue of 'damping' but of fiddling about the
frequency response in a way that mirrors the speaker's impedance-frequency
curve.

Jim

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