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Old 23rd March 2013, 09:03 AM   #1
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Default 140V/us

What topology would be able to produce a slew rate of 140V/us?
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Old 23rd March 2013, 09:26 AM   #2
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Current feedback can do the trick and with some efforts also voltage feedback but it depends what you are after. Power amp? Notice also that the slew rate is not a parameter free from other ones. Slew rate is a parameter coming from bandwidth in combination with voltage swing.
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Last edited by peranders; 23rd March 2013 at 09:39 AM.
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Old 23rd March 2013, 10:21 AM   #3
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Yes, it's for a power amp. about 200-300 watts AB.

Here's my thinking, which is a bit arbitrary. I'm looking to design an amp that can provide as transparent a window into the audio spectrum as perceivably possible. I've read that audio system that can recreate sound accurately to 100khz have been chosen over less able systems in documented listening as more more pleasant more realistic to listen. I'm vague on the biological foundation for this result, although it has to do with somehow the transients above the audio spectrum at least to 100khz are perceivable and there absence and be missed in A/B tests where one system has that extended range and one system lacks the extended range.

This is kind of moot at the moment because commercial music fails to encode information that high. Although that's just a matter of time, and one can still designing amps that will produce that range now. Hence the motivation. So, I was thinking 140V/us would allow a fairly good 100khz square wave.

I've been reading both the Self and Cordell books on amp design and they seem to lack any todo about current feedback topologies. Do you know of one?
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Old 23rd March 2013, 12:40 PM   #4
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by syberraith View Post
Yes, it's for a power amp. about 200-300 watts AB.

I've been reading both the Self and Cordell books on amp design and they seem to lack any todo about current feedback topologies. Do you know of one?
Nelson Pass's F5 is a good example of a fast "current feedback" amp. Be careful though, current feedback isn't automatically a ticket to high speed. Strictly speaking, what gives the high speed is a push-pull input stage that can operate in class AB to give high output currents in either direction. That kind of input stage is more common with "current feedback" designs, especially opamps, but can be done with either kind of feedback.

For example power amps like the Quad 303 and JLH's 1969 10W class A design both use "current feedback" but have a single transistor input stage, so don't have any speed advantage.

OTOH, a lot of John Curl's designs use complementary differential input stages which allow high peak currents, and thus high slew rates, even though he uses voltage feedback.

That said, 140V/uS isn't that fast and should be quite possible with more "normal" topologies. For 200-300 watts, a bridged amp may be the best option, in which case you only need 70V/uS on each side, which would be even easier.

I haven't read any of Doug Self or Bob Cordell's books but I am aware that Doug Self thinks high slew rate is a bad idea. AFAIK, Bob doesn't have any such reservations though. I'd be surprised if none of his designs offered high slew rates.
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Old 23rd March 2013, 12:45 PM   #5
Bonsai is offline Bonsai  Taiwan
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Syberraith, there are voltage and current feedback designs on my website (follow the link below). All the designs are discussed in some depth, so this might trigger your creative processes.

Bottom line is you can get high slew rates with VFA or CFA designs.
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Old 23rd March 2013, 01:28 PM   #6
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Originally Posted by godfrey View Post
Nelson Pass's F5 is a good example of a fast "current feedback" amp. Be careful though, current feedback isn't automatically a ticket to high speed. Strictly speaking, what gives the high speed is a push-pull input stage that can operate in class AB to give high output currents in either direction. That kind of input stage is more common with "current feedback" designs, especially opamps, but can be done with either kind of feedback.

For example power amps like the Quad 303 and JLH's 1969 10W class A design both use "current feedback" but have a single transistor input stage, so don't have any speed advantage.

OTOH, a lot of John Curl's designs use complementary differential input stages which allow high peak currents, and thus high slew rates, even though he uses voltage feedback.
I've seen the Krell KSA schematic and I like the dual differential input. I happen to like symmetry. I also noticed that Cordell details how to have a push-pull VAS with a single differential input stage. I like that too.

Quote:

That said, 140V/uS isn't that fast and should be quite possible with more "normal" topologies. For 200-300 watts, a bridged amp may be the best option, in which case you only need 70V/uS on each side, which would be even easier.
In my sim, which started out as a copy of AmpLabs C200, then modified with details from the design books and bits and pieces from threads here, I've already got the slew rate up to about 70V/uS and pretty symmetrical. I had to migrate to a two pole compensation scheme, which allows a remnant of over shoot to persist, although the side bands of the 20khz-19khz fundamentals and the peak at 39khz of the TID test are now 117 dB lower than the fundamentals. Without the two pole compensation, it was more like 80dB down. Reportedly, this would be a noticable improvement.

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I haven't read any of Doug Self or Bob Cordell's books but I am aware that Doug Self thinks high slew rate is a bad idea. AFAIK, Bob doesn't have any such reservations though. I'd be surprised if none of his designs offered high slew rates.
The differences in opinion from these pillars of the audio community is quite impressive. Being eclectic, I find a diversity of opinions to be a good thing. It just gives me more to explore, and more from which to choose what I like.

I have a 2x20V 225VA toroidal power transformer that I bought a long time ago for a old project that's been sitting around. I can use that to build a 2x30 watt class A amp with which to do some tests. I lack any speakers that would do it justice though, just some Stageworks PA monitors, with low efficiency drivers that have a noticable coloration in their tone.

Last edited by syberraith; 23rd March 2013 at 01:37 PM.
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Old 23rd March 2013, 01:56 PM   #7
AndrewT is offline AndrewT  Scotland
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30W+30W of ClassA is too much to ask from a 225VA transformer.
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Old 23rd March 2013, 02:06 PM   #8
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You can double the slew rate of a given single-ended gain stage by incorporating two such stages operating in anti-phase, thereby turning them in to a composite differential amplfier. This topology comes, however, at the cost of double the parts count.
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Old 23rd March 2013, 02:40 PM   #9
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Originally Posted by syberraith View Post
The differences in opinion from these pillars of the audio community is quite impressive. Being eclectic, I find a diversity of opinions to be a good thing. It just gives me more to explore, and more from which to choose what I like.
My recollection is that the maximum slew rate required to reproduce audio band signals is on the order of a few V/uS. There are, however, two factors which argue for more than just a few V/uS slew-rate. Should the input spectrum be wider than that of the audio band, such as with DAC analog stages, then we need a greater slew rate than would be required to simply reproduce audio band signals. This will likely also be true for any practical gain stage not featuring effective band limiting either at it's input, or in it's feedback loop.

The other factor is that, should the load present an significant shunt capacitance to the gain stage, current delivered by that stage, needed to charge and discharge that shunt capacitance, must be commensurately significant. Increasing the gain stage's current delivery capability consequently increases it's slew-rate in to a given shunt load capacitance. So, here, we're not attempting to reproduce a wider input spectrum, we are increasing the current delivery capability for the purpose driving some amount of shunt capacitance. An increased stage slew rate is incidental, in this case.

At least, that's my understanding.
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Last edited by Ken Newton; 23rd March 2013 at 03:01 PM.
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Old 23rd March 2013, 03:24 PM   #10
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Hey Ken, I just realized that if I doubled the input stage and made a pushpull VAS then maybe the slew rate would approach 2x what it is now. That would get it in the neighborhood.

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Originally Posted by Ken Newton View Post
My recollection is that the maximum slew rate required to reproduce audio band signals is on the order of a few V/uS. There are, however, two factors which argue for more than just a few V/uS slew-rate. Should the input spectrum be wider than that of the audio band, such as with DAC analog stages, then we need a greater slew rate than would be required to simply reproduce audio band signals. This will likely also be true for any practical gain stage not featuring effective band limiting either at it's input, or in it's feedback loop.
Well, I'm looking at expanding the the output spectrum to a reasonable amount beyond the audio spectrum as a good thing, with the goal being to be able to reproduce all material withing that spectrum in a consistent manner throughout it, meaning with approximately the same amount of distortion regardless of frequency.

Nyquist theory has it that a sampling rate of 2x the highest frequency is all that is required to reproduce that frequency or lower --in a perfect world. As it turns out electronics are far enough away from perfect that to get good digital effect IMHO, one has to go to at least a sampling frequency of 5x the highest frequency. 96khz is the lowest I like to use, and I'd prefer to have 192khz sampling.

From a quick sine wave differentiation, It looks like the maximum rise rate of a 20khz sine wave to 40V is 5V/uS. So for 100khz that would be 25V/uS. Although audio material is going to have lots of transients that rise significantly faster than that. I view the square wave testing as a way to reveal an amps capacity to handle such non-sine wave material.

Quote:

The other factor is that, should the load present an significant shunt capacitance to the gain stage, current delivered by that stage, needed to charge and discharge that shunt capacitance, must be commensurately significant. Increasing the gain stage's current delivery capability consequently increases it's slew-rate in to a given shunt load capacitance. So, here, we're not attempting to reproduce a wider input spectrum. We are increasing the current delivery capability for the purpose driving some shunt capacitance. An increased stage slew rate is incidental, in this case.

At least, that's my understanding.
If I comprehend you correctly, I noticed that also depends on the compensation method employed. I tried just raising the bias current in the input stage, that did increase the slew rate, although it also required an increase in the Miller compensations capacitor to reestablish stability, an increase of such magnitude that for all practical purposes it nullified the gain in slew rate from the current increase. That's what motivated my switch to two pole compensation. I have a few different schematics of current feedback designs now. I'll have to build some new models.
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