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Old 21st March 2004, 09:59 AM   #1
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Default Slew rate Aleph 5 - opinions!

Hi,
OK by now I uderstood what is the slew rate. One of my friends has built class A amplifier which has 45W, but only 8W in class A. He is convincing me that a slew rate of his amp is about 200V/uS. Is that possible? He keeps telling me that the slew rate in class A amplifier is most important, and he is suggesting me not to build Aleph 5 because the slew rate is too low (30V/uS). In Aleph 5 user manual Nelson says it's not so important. I would like to see your opinions on this subject.
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Old 21st March 2004, 10:27 AM   #2
matjans is offline matjans  Netherlands
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a lm3875 based gainclone has a slew rate of about 11v/uS. As the gaincard by 47labs is supposed to use the same chip, it will have about the same slew rate.

Look for reviews of the gaincard and you will see it is quite a highly regarded amp.
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Old 21st March 2004, 06:22 PM   #3
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I think the importance of slew rate is (was?) a bit overrated.

My Aleph5 is amongst the "fastest" amps I ever heard in my system (I listen to music, not to square waves).

So I wouldn´t bother too much and build one. You won´t be disapointed!

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Old 21st March 2004, 07:20 PM   #4
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OK, I have no doubts anymore. I'll just build an Aleph 5 and see what is going on. I like warm and transparent sound, so after all the reviews I red, I think I won't be dissapointed.
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Old 22nd March 2004, 03:44 AM   #5
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A good amplifier is a balance of considerations, and slew rate
is one of them.

There are good examples of amplifiers with enormous slew rates
which didn't sell, and examples of amplifiers with incredibly low
THD figures that didn't sell, and nearly inifinite damping factors
that didn't sell.

There's nothing wrong with having good specs, unless the
design has otherwise been compromised in order to achieve
them (this is the job of the marketing department )

In real life, an Aleph 5 might be called on to do 1.5 V/uS or
so with music, and enjoys a healthy 10:1 margin, so I didn't
add anything to make it faster.
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Old 23rd March 2004, 08:49 PM   #6
azira is offline azira  United States
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I read this and decided to do a little math to see what an objective answer might be.

Assuming 20 KHz as the fastest signal we can hear and therefore the limit. This corresponds to a 50uS period.

V=Vp * sin( 2 * pi / 50 uS * t )

To find the maximum slew rate for a 20KHz sin wave, we need the point where the signal is changing the fastest. That is at 180-deg (or pi rads). And we need the rate that the signal is changing at that point. Therefore we need the derivative.

dV/dt = Vp * 2pi/50us cos( 2pi/50us * t)

Since the point we're observing is at 25us or 180 deg, or pi rads, the cos term is -1.

dV/dt = -Vp * 2pi/50us.

So..

@ 50V peak: slew rate = 6.3V/us
@ 10V peak: slew rate = 1.3V/us

I would be inclined to say that anything above 5V/us is quite satisfactory, 10V/us is a pretty healty margin.
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Old 23rd March 2004, 09:23 PM   #7
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Yeah I was going to say something similar that the slewrate, all things being equal, is just a representation of the maximum frequency an amplifier can produce accurately.

If higher slew figures lead to better THD figures then so be it, but most amps slew fast enough to give high freq extension to please passing bats.

Besides this is Nelson Pass's amp and I think he knows what he is doing!
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Old 24th March 2004, 08:58 PM   #8
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Well, the speed simply reflects the native capabilities of the
semiconductors, as I didn't do anything to either speed it up
or slow it down (that would have been at least one more part).
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Old 24th March 2004, 09:07 PM   #9
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Quote:
Originally posted by azira


@ 50V peak: slew rate = 6.3V/us
@ 10V peak: slew rate = 1.3V/us

I would be inclined to say that anything above 5V/us is quite satisfactory, 10V/us is a pretty healty margin.
That is my experience, although I think 5 V/uS is getting light.
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Old 25th March 2004, 10:29 AM   #10
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Quote:
Originally posted by Nelson Pass


That is my experience, although I think 5 V/uS is getting light.

Listening through the X ZEN sometimes I though : It shines by its own light ...



edit : ZV7R quite revealing article BTW
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