Slew rate calculations - how much do I need?

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#62
peranders said:


Yes... if you manage to get 0.1% at full throttle and 200 kHz. This is, if you don't know it, a real achievement.
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The only problem is the fact, that the outputcoil (~3uH) reduces the
load to the amp below 2 amperes, so it's not really "full throttle".
But with a triple darlington and fast outputdevices not too difficult !
And, it's only "measured" in sims... :sad:

Mike
 
#63
Think it through. How is it that a 100Hz square wave could possibly create slew rate limiting? How much ultrasonic energy is in such a wave. Yes, it can create SID or TIM. You don't need full voltage swing at ultrasonic frequencies, just the bandwidth to do so.
 
#64
What do you think, if my amp can output a fullscale 200khz sinewave with reasonable thd (<0.1%), the slewrate should be more than adequate ?
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Slew rate ability of the amp depends on the load. Amp with short on output has slew rate of 0V/us no matter how smart design. No load and amp shows excellent slew rate. There is a finite grey area between these two extremes.
Dynamic load means dynamic slew rate of amp.
 
#65
john curl said:
Think it through. How is it that a 100Hz square wave could possibly create slew rate limiting? How much ultrasonic energy is in such a wave. Yes, it can create SID or TIM. You don't need full voltage swing at ultrasonic frequencies, just the bandwidth to do so.
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????
I thought it through. If the leading/trailing edges of that square wave rise/fall 10V in 1 uSec, that's 10V/uSec, no?
What does that have to do with the square wave frequency??

Jan Didden
 
#66
Mike Gergen said:
What about symetrical SR? I believe there are 2 thoughts on this; one says that as long as the SR is large enough it doesn't matter if the + excursion and - excursion have the same SR or not, the other says that they both need to be the same.

I would think that if the SR was different for the +/- then the 1/Fb would be different with increasing freq, potentially affecting the sound.

Is this not one of the reasons why full complimentary circuits came about? To attempt to have an equal SR for both sides of 0.
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Well, assuming that jumpy signals can both be going down or going up, you would need appropriate slew rates in both direction. Ideally, you would want them to be symmetrical, because if they are not, the slowest one will be the one limiting your amp.

It's like the strength of a chain, its the weakest link that determines the total strength of the chain, so the most economical chain is one where all the links are the same strength.

Jan Didden
 
#67
thanh said:
hi sam9! yes ,i measure slew rate with squarepulse wave
i'm not sure i understand your words completely but i will try to measure with 1us squarepulse wave .I tested with 20us squarepulse wave
thank you!🙂
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I *think* what Sam9 menas (but correct me if I am wrong) is that you should sim with edges that are slew limiting your amp, only then can you see what the max slew rate is. So let's say you test a power amp with a 40V pk-pk output, and a gain of 20. Then you need an input signal of 2V pk-pk. Take a square wave that has edges of say 50nSec, that's a 40V/uSec input signal (you still with me😉 ).

For the amp to follow that at the output, it would need 20 times that or 800V/uSec which it probably doesn't have (if it is a realistic design). So the amp will slew limit, and the output signal should be a part of a triangle, linearly (in principle) rising of falling to the required output voltage. You *could* in principle measure the slew rate by checking on the graph how many volts it changes in how much time.

But be sure that the amp is in fact slew limiting. But don't overdo it, if you see slew limiting, back off on the input rise time to find where slew limiting starts, then increase edge rise time for solid slew limiting. If you overdo it, you may not test correctly because you may get secondary effects in addition to slew limiting.

Jan Didden
 
#68
john curl said:
Think it through. How is it that a 100Hz square wave could possibly create slew rate limiting? How much ultrasonic energy is in such a wave. Yes, it can create SID or TIM. You don't need full voltage swing at ultrasonic frequencies, just the bandwidth to do so.
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Hmm,
Is a "frequency" given for a squarewave not only an information
of how often it changes from -V to +V ?
ANY squarewave has unlimited high frequencies and will show the
slewratelimits of an amp.
A real squarewave changes from -V to +V in 0.0uS ! (or 0.0nS or 0.0fS),
no matter of it's "frequency".

Mike
 
#70
wimms said:

Slew rate ability of the amp depends on the load. Amp with short on output has slew rate of 0V/us no matter how smart design. No load and amp shows excellent slew rate. There is a finite grey area between these two extremes.
Dynamic load means dynamic slew rate of amp.
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Hi Wimms,

I think the issue is the slew rate of the voltage gain stage, because there is where (generally) the limiting first occurs as a result of too fast input signals. Of course, if you short the output, nothing comes out, but that is not a helpfull situation. Assuming that the output stage has a gain of one, sort of, it will not slew limit unless the protection circuit kicks in. The output transistors will probably blow before slew limiting😀

Jan Didden
 
#71
I have a square wave with a 1ns rise time. This is fast enough for me. However, for general testing, we often reduce the rise time to about 3-10us. This is because we like to be realistic about what actual sources can produce. Slower than this, is not realistic in all cases. Perhaps some of you will never see a risetime faster than 20us, and you can live with a lower slew rate. However, for reasonable worst case, 3-10us, especially with DVD and SACD sources makes more sense, along with moving coil phono cartridges and their effective bandwidth.
 
#72
MikeB said:
So, maybe slewrate should also be measured in Amperes/uS ?
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I don't think it then corresponds to definition of slew rate at all. Closer try would be Watts/us

What I just wanted to point out is that slew rate is not a static parameter of an amp. 2KW into 8ohms or 2KW into 1ohms is quite a different condition for slew rate measurements.
 
#73
john curl said:
[snip] However, for reasonable worst case, 3-10us, especially with DVD and SACD sources makes more sense, along with moving coil phono cartridges and their effective bandwidth.
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John,

Since all those digital sources incorporate filters at the output to limit hf output (in fact the filters are REQUIRED for the reconstitution process) how would DVD or SACD require higher slew rates than, say, CD?

Jan Didden
 
#74
janneman said:
I think the issue is the slew rate of the voltage gain stage, because there is where (generally) the limiting first occurs as a result of too fast input signals. Of course, if you short the output, nothing comes out, but that is not a helpfull situation. Assuming that the output stage has a gain of one, sort of, it will not slew limit unless the protection circuit kicks in. The output transistors will probably blow before slew limiting😀
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Hi Jan,
I think this is where things get complex. It depends, FET output or bjt, GNFB or not, how deep NFB, how much current in vas, etc. All those differences cause different behaviour. Of course I didn't mean short as a meaningful condition, but consider 0.2ohms load impedance. Consider negative load impedance due to backemf. Thats even less helpful.

I'm not sure if you think slew rate here. Its more like continuous ability you consider. Slew rate limiting would happen for very short time, transient. But it is relevant that you note trans blowing before their intrinsic properties limit slew. Its not transistors that limit it, but designer who designs to conform with device SOA.
 
#75
The formula: SR = 2*pi*f*SQR(2*Z*P) is correct.

I remember that one can hear a phase difference of 5 degrees. Given the upper frequecy one can hear (20 kHz), you need a bandwidth uptill 115 kHz (- 3 dB) for a maximum phase difference of 5 degrees at 20 kHz.

This means for a 100 W amplifier a slew rate of 28,9 V/microsec.

The best way to do so, is to design an amplifier with a bandwidth of 150 - 200 kHz and to place a simple input filter of let say 120 kHz at the input.

Marc.
 
#76
peranders said:
Plain me is wondering how your formula looks like, Mike. You gave a reference to a National Leraning brief with the exact formula which I have come up with. This is wrong according to you. Still you mention this document as correct. I'm not following you and I'm sure nobody else is.
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I was refering to J.Curl's assertion that 0.5V/uS/V equates to 50V/uS for 100W. This is clearly incorrect......

http://www.diyaudio.com/forums/showthread.php?postid=530065#post530065

http://www.diyaudio.com/forums/showthread.php?postid=530671#post530671


So get your facts straight..... 🙄......

.........I did not refer to anything you posted....at all... :scratch:
 
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