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Old 29th July 2011, 08:41 PM   #141
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Quote:
Exactly, people put too much emphasis on slew rate.
Yes , my "slew rate failure" amp outperforms the tested 100v/us one in both HF THD and bandwidth.Sounds a little better , too. Both exceed what is needed for full power 100k operation , so it (better slew) does not matter. The "faster" one is a lowly EF2 .. another impediment to performance at high current.

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Old 29th July 2011, 09:50 PM   #142
cbdb is offline cbdb  Canada
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Yes , my "slew rate failure" amp outperforms the tested 100v/us one in both HF THD and bandwidth.
So your slower slew rate amp is actually faster (higher freq) than the fast slew rate amp. Makes my point.
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Old 29th July 2011, 11:18 PM   #143
gootee is offline gootee  United States
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Originally Posted by cbdb View Post
Yes, its similar to switching speeds in gates.



Exactly, people put too much emphasis on slew rate. Its not an accurate test for linear operation. As you say, if you stay "well" below it your fine, but how "well" below depends on the circuit topologies and saying one amp with a higher slew rate has better full power high freq response is not necessarily correct. Amp speed with music signals (linear region) is not the same as slew rate (non linear region). The only correlation is that slew rate limits high power, HF response at some point, but below that point it has no influence on the signal.
cbdb,

Most of what you think seems correct but you have the wrong idea about what "slew rate" means.

EVERY time-changing voltage ALWAYS has "a slew rate", which is measured in volts per unit of time at the output, even if it's only changing at 1 volt per year.

Saying "slew rate" says NOTHING, NADA, ZILCH about linearity.

Would a 1 volt per year slew rate necessarily mean that something is operating in its "non-linear" region? "Not necessarily".

I guess that you must be thinking of "slew-rate limited".

When people talk about the "MAXIMUM slew rate" spec for an amplifier, they are referring to the fastest slew rate for which an amplifier does _NOT_ go into any non-linear region.

So an amp that has a max slew rate spec of 40 V/us would still be operating as linearly as it usually did and sound fine if the signal never made its output change at more than 40 V/us. But if you pushed that same amp to 50 V/us, THEN it might get into non-linear regions and the sound could be bad.

Tom
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Old 29th July 2011, 11:46 PM   #144
gootee is offline gootee  United States
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Originally Posted by cbdb View Post
So your slower slew rate amp is actually faster (higher freq) than the fast slew rate amp. Makes my point.
cbdb,

Not necessarily. Slew rate and bandwidth are only very-loosely related to each other (if at all).

If you wanted to try to relate slew rate and bandwidth, you would have to pick an amplitude, first; probably the maximum output amplitude, since that would be a worst-case condition for the slew rate. Even then, I don't think you can relate them in a very meaningful way (see below).

With a single frequency (sinusoid waveform), the maximum slew rate at one amplitude would be different than the maximum slew rate at a different amplitude. A larger amplitude at the same frequency would have to have a larger maximum slew rate (which, by the way, would always occur at the zero-crossing point, for a sinusoid waveform). See post # 92.

The bandwidth's upper limit, on the other hand, only tells us where (above what frequency) the gain can no longer be sustained and has dropped by 3 dB, which, for a fixed input amplitude, would be when the output amplitude has fallen by 3 dB.

I think it's impossible to really relate bandwidth and max slew rate, very well, since the "bandwidth" definition doesn't specify that we still have to be in a linear operating mode at the -3 dB point.

Getting past all of that, if you look at my equation for the maximum slew rate for a sine wave (given any frequency and amplitude), in post # 92, you will see that the slew rate for audio frequencies (for a single frequency, anyway) gets NOWHERE NEAR the max slew rate specs of the amps in question. BUT, solving backwards for "max frequency while still operating linearly", given the max slew rate spec and the maximum output amplitude, MIGHT, I think, tell us the highest frequency that each amp could perform well at, i.e. still staying linear, with the worst-case maximum output amplitude. So maybe that could be some measure of the "speed" of an amplifier(?).

I would be tempted to guess that an amp that could go "faster", well, could also do "slower" better (i.e. more accurately). But that's just a guess.

Tom

Last edited by gootee; 29th July 2011 at 11:51 PM.
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Old 30th July 2011, 12:27 AM   #145
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But if you pushed that same amp to 50 V/us
You can't "push" it. 40 is max ... period. At 250khz,the output level attenuates by not only the input LP filter (which I Have set below this point) , but by the slew, as well. When this happens , THD rockets and the waveform even looks asymmetrical(or non-sinusoidal) . This same effect happens at about 600k on the faster 100v/us LIN topology amp.
Ps - ridiculous .... we are not after an AM radio broadcast amp !!
Quote:
So your slower slew rate amp is actually faster (higher freq) than the fast slew rate amp. Makes my point.
No , but it retains the lowest THD to the highest frequency of the 2 ... by far.

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Last edited by ostripper; 30th July 2011 at 12:35 AM.
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Old 30th July 2011, 12:27 AM   #146
ChPuls is offline ChPuls  Germany
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Originally Posted by gootee View Post
I would be tempted to guess that an amp that could go "faster", well, could also do "slower" better (i.e. more accurately). But that's just a guess.
It is not necessarily right because fast video opamps (made for hf signals) are getting hot with nf-signals. Possible temperatur drift could affect everything.

But we are drifting here with the topic away.

Let us focus on how an opamp influences or even creates a wide soundstage.
Post #113 gives a hint:
Quote:
Originally Posted by cbdb View Post
An increase in sound stage may be caused by distortion (phase anomolies ( actually used in recording studios to simulate a wider stage)), so I believe a lot of people mistakenly believe a system with more distortion is better.
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Old 30th July 2011, 12:31 AM   #147
gootee is offline gootee  United States
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Originally Posted by ostripper View Post
You can't "push" it. 40 is max ... period. At 250khz,the output level attenuates by not only the input LP filter (which I Have set below this point) , but by the slew, as well. When this happens , THD rockets and the waveform even looks asymmetrical. This same effect happens at about 600k on the faster 100v/us LIN topology amp.

OS
OK. Maybe not THAT amp, but one that didn't have an input filter. Anyway, I think I was only trying to make the point that if the "max slew rate" spec was somehow exceeded, then "bad things" could happen, but below that slew rate it should still be operating linearly.
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Old 30th July 2011, 12:37 AM   #148
gootee is offline gootee  United States
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Quote:
Originally Posted by ChPuls View Post
It is not necessarily right because fast video opamps (made for hf signals) are getting hot with nf-signals. Possible temperatur drift could affect everything.

But we are drifting here with the topic away.

Let us focus on how an opamp influences or even creates a wide soundstage.
Post #113 gives a hint:
Quote:
Originally Posted by cbdb
An increase in sound stage may be caused by distortion (phase anomolies ( actually used in recording studios to simulate a wider stage)), so I believe a lot of people mistakenly believe a system with more distortion is better.
Maybe. Don't know. But I have been assuming (and have seen no good-enough reason to change that assumption) that I am much more interested in using opamps (and other amps) in ways that result in the most-accurate reproduction of the source, at the final output, so that I can experience whatever soundstage is inherent in the source material, as accurately as possible, i.e. whatever was intended to be given to us by the artists and the recording engineers.

Last edited by gootee; 30th July 2011 at 12:40 AM.
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Old 30th July 2011, 12:48 AM   #149
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To elaborate

1 below is fast , can reach 500k ... but with .05% THD. measured 100v slew into a 8R resistive load on CRO.

2 below is "slow" can't reach 500k, but can go to 150k with no thd increase.

It might be more of a topology/total loop gain effect at work here ???
(1 is 115 db OLG and 2 is only 60DB.)

Quote:
if the "max slew rate" spec was somehow exceeded, then "bad things" could happen,
It would just naturally attenuate (I simulated without the filter) , whether that is bad .
OS
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Last edited by ostripper; 30th July 2011 at 12:52 AM.
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Old 30th July 2011, 12:51 AM   #150
a.wayne is offline a.wayne  United States
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Since we are slinging slew rate around , what's a good number? over the years most tend to state anything over 40v/us to be a waste of time ... true/false/maybe ..... ?
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