Slew rate and slew rate limiting
On Tue, 12 Feb 2013 22:16:14 -0800 (PST), PStamler
wrote:
On Tuesday, February 12, 2013 9:19:39 PM UTC-6, Arny Krueger wrote:
My calculations show that it takes about 3.55 v/microsecond slew rate to
produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to
50 watts into 8 ohms).
Just about any even halfways competent amp in those days could do that.
That would imply that an amplifier putting out 7.75V rms (+20dBu) would need a slew rate of only 1.38 V/us to produce a low-distortion 20kHz sine wave.
Samuel Groner's measurements of opamps suggest otherwise.
Arny, I think you're assuming that if an amplifier isn't actually slewing, it's okay -- in other words, that slewing is an all-or-nothing phenomenon. Groner's tests suggest, instead, that well before the point of overt slewing, the misbehavior of the voltage amplifier stage is producing real and measurable distortion.
And by the way -- in the 70s we saw a lot of consoles with LM301 opamps, which had abysmal slew rates under usual operating conditions. And those consoles sounded like crap, too.
Peace,
Paul
It is certainly fair to say that if it isn't slew-rate limiting, then
slew rate isn't a problem. Slew rate limiting is the same as any kind
of limiting. It's doing it, or it isn't. There's plenty of scope for
other stuff to be wrong of course.
d
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