In all of these discussions about slew rate it seems to be represented as a single figure, that of the maximum rate of change at the output with an infinite transient at the input. In fact when I was designing fast amplifiers (I was one of the first to market a 500 V/uS amp) I was really focusing on a definition of slew rate as the slew rate derived from the maximum frequency at full output with less than 3% THD. The origin of this is that at higher slew rates the amp is no longer operating linearly, its a switch. Marketing people like the higher numbers and they are easier to derive but by then the input and VAS stage are no longer operating linearly. The difference can be as much as 10:1.
And this is also why looking at slew rate without looking at settling time and the myriad other aspects of an amp says little about its performance.
I did have a slew rate meter on my first amp. It was a great disappointment. Configured for 500V/uS max it never lit up, even on a scratched record. I rescaled it to .5V/uS and then it danced. It never went into production.
I would never say you need high slew rate to work. But, for whatever reason very high speed amps that are well designed tend to sound better to me.
-Demian
And this is also why looking at slew rate without looking at settling time and the myriad other aspects of an amp says little about its performance.
I did have a slew rate meter on my first amp. It was a great disappointment. Configured for 500V/uS max it never lit up, even on a scratched record. I rescaled it to .5V/uS and then it danced. It never went into production.
I would never say you need high slew rate to work. But, for whatever reason very high speed amps that are well designed tend to sound better to me.
-Demian
Of course, you are correct Demian. Why I have to defend 50V/us is laughable! You, I, Nelson, Charles, and dozens of other experienced designers have settled this years ago. Hundreds of measurements covering virtually all the existing amplifiers have been done, graphed and analyzed. For a number of reasons, a reasonably high slew rate is necessary to make a good solid state design, in general. It MIGHT be possible to make a pathological design that hard limits at a somewhat lower recommended slew-rate, but which operates virtually perfectly below its slew-rate limit. It might also be possible that you could perhaps make an automoble that was lightning fast in traffic, yet would not go faster than 80mph or so, but it would not be easy. Normally, the best handling cars go faster than 80mph, just because this also helps make their response effortless at lower speeds. It is very much the same thing with slew-rate and amplifiers.
john curl said:
Not really....
http://www.diyaudio.com/forums/showthread.php?postid=531239#post531239
"An amplifier driven to it's slew limit by such spureae is unlikely to simultaneously process audio frequency stimuli......"
Hmm, can you show me any audiofrequency capable of driving an
amp with reasonable bandwidth into slewratelimiting ?
Okay, maybe "slewratelimiting" should be defined clearly ?
Isn't high slewrate only better for lowering errorvoltage for higher freqs ?
Mike
Hmm, can you show me any audiofrequency capable of driving an
amp with reasonable bandwidth into slewratelimiting ?
Okay, maybe "slewratelimiting" should be defined clearly ?
Isn't high slewrate only better for lowering errorvoltage for higher freqs ?
Mike
Of course. But many here really hope to hear your thoughts on the reasons why this is so. Putting slew rate requirements into right perspective wrt to stage behaviour of the amp could help progress understanding of important design approaches for many in this community.john curl said:
Who else if not designers with decades of experience would possess the ability to bring to the discussion truely relevant points that would lead to better understanding why very high slew rate is beneficial to the design.
This thread started from amp_man's strong assertion that 300V/us is a necessity.
I would really like to encourage you smart guys to offer your thoughts on all of the implications that could justify setting 300V/us as a target. IMO that would definitely make much more interesting discussion than arguing how is correct way to calculate slew rate of a sine wave or RF noise and just flatly rejecting any need for SR beyond that minimum.
Is RF really the only thing that mattres? Simple RC filter rids slew rate from dependance on frequency. It then approaches limit of 1V/us per 1V-p2p. By picking RC cutoff suitably, input signal SR can always be made a total nonissue.
What then makes the sonic impact even when it is surely known that input signal is not driving amp into SR limits? And even more importantly, when amp is working far from its power and SR limits.
I figure that when amp hits SR limits, all hell brakes loose. But is it a sharp edge to cross? What is a difference between amp working at 1/2 of its SR limits and amp working at 1/10 of said limit? What amp parameters start to suffer when approaching the SR limit, even though being still far from it?
Eric? Don't I remember that name coming up before?...janneman said:
A suitably high slew rate would prevent glitches caused by erroneous transients, but those should not be getting through with high enough amplitude to cause that problem. With sufficient filtering, shielding and bandwidth limiting the maximum slew rate the amplifier needs to reproduce will not reach hundreds of V/us. Previous posters have cited measurements showing what sort of rates of change to expect.mikeks said:
If you have measurements proving that such extremely high dv/dt occurs in a well built amp then please share them. Or if there is some other reason why such high slew rates are beneficial.
Hmmm... This discussion makes me wonder something: What happens in a speaker when presented with very fast slew rates? Do they exhibit some equivalent of slew rate limiting?
Mr Evil said:
Hmm, i think its only limited by how many amperes you are able
to supply and lightspeed...
Just kidding, yes they have this limit, that's why tweeters need
very light membranes, and woofer can have very heavy ones.
And that's why speakers with light membranes can reveal more
details/dynamics.
Mike
Mr Evil said:
Eric? Don't I remember that name coming up before?...
[snip]Hmmm... This discussion makes me wonder something: What happens in a speaker when presented with very fast slew rates? Do they exhibit some equivalent of slew rate limiting?
Sorry, I seemed to remember your first name was Eric. Evil Eric?
Speaker slew rate: that would be the tweeter's acceleration. I haven't done any calculations, but I bet that accelerating a physical mass would be very, very much slower than any practical slew rate of even an old 741. You bring up an interesting point.
OTOH, an amp the has slew rate limiting will produce gross distortions, that probably don't sound all that kosher. So, even with the speaker limit, it probably pays to keep the amp clean. And with competent construction and up to 100W or so, 20V/uSec would be way high enough.
I found Pavel's (I think it was him) experience illuminating: the slew rate meter on his amp had to be sensitised to 0.5V/uSec full scale to get any deflection on normal use! That's a real world test, and should sober up all those 100V/uSec or more advocates.
Jan Didden
Jan,
I almost agree with you. As I stated in earlier posts, a SR of about 30 V/usec is enough for a 100 W amplifier. But the bandwidth and with that the SR must be fixed by the input filter of the amplifier and therefor the amplifier (100 W) itself must have a bandwidth of 150 - 200 kHz and a slew rate of 40 - 50 V/usec.
Marc.
I almost agree with you. As I stated in earlier posts, a SR of about 30 V/usec is enough for a 100 W amplifier. But the bandwidth and with that the SR must be fixed by the input filter of the amplifier and therefor the amplifier (100 W) itself must have a bandwidth of 150 - 200 kHz and a slew rate of 40 - 50 V/usec.
Marc.
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