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Old 15th February 2008, 12:45 AM   #1
vladn is offline vladn  United States
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Default square wave testing and slew rate

While recovering from a minor surgery I've been toying with various amp configurations in LT Spice (note - I make my living as an EE but analog amp design is not really my specialty, more of a curiosity). I have some trouble understanding the limits/envelope of square wave testing the amp especially when using difficult capacitive loads.

Let's say you have an amp with fairly deep NFB limited to a healthy 100V/us at the VAS stage and no output coil.

1. Does it make sense to test it with an input signal demanding a higher slew rate (bypassing front end HF filter)? Obviously the the input signal will generate huge error signal in the front end. Even with anti-saturation circuitry everywhere the output will not look pretty.

2. With a perfect square wave input a 100V/us slew rate amp will immediately trip over-current protection at very low output voltages (few volts or less) when presented with a capacitive load in uF range. Overriding the SOA protection in Spice is safe but not very informative since beta of the output stage is seriously drooped at this point and it is not clear how to interpret the results.

It is obvious that a good design should recover reasonably fast and not enter continuous oscillation under these extreme conditions (since it may happen during clipping and such). However do you really expect a clean output when test fixture exceeds slew rate and/or current limit of an amp ?
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Old 15th February 2008, 04:26 AM   #2
tedr is offline tedr  United States
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I know more about mixers than amps but I can give some hints about the situation. In many ways audio is a nasty application for electronics. Power amps WILL be abused, input overload, wrong line voltage, wrong load impedance, thermal overload, mechanical damage, output shorts to ground, and at the end of the day the amp that is still working despite the abuse has a lot going for it. It is also preferred that things degrade gracefully and recover spontaneously.

My answer to question 1 is No.

My answer to question 2 is maybe. Loudspeaker loads may include significant capacitance, but it is a well known torture test to add a few microfarads to see what happens, which should be non-catastrophic self protection.

Input overload in voltage terms is much more common than exceeding slew rate, especially since everything off CD has nothing above 22kHz anyway. Live mics, synths, Cd playback, it is all slow stuff, nothing much to worry about in slew rate terms. It is normal for an amp to be band limited, this means the output square wave will show the HF roll off due to all the stages combined. Applying the "degrade gracefully" concept the band limiting may be applied where it does the most good, which probably means before or in the input stage. You will get much better detailed technical advice from some very knowledgable folk here, I just wanted to get some basics over from the practical side.
Degrade gracefully, recover spontaneously, the load is connected all the time and someone (maybe a lot of people) is listening to it. Anything nasty that triggers protection should not only protect the amp from itself but more importantly protect the speaker (costing $$$$) from the amp.


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Old 15th February 2008, 07:14 AM   #3
Bonsai is offline Bonsai  Taiwan
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Try reading this to get a handle on th e slew rate thing:-

http://waltjung.org/Classic_Articles.html

It was written in th e late '70's so a lot of the stuff we take for granted now (e.g. LTP emitter degeneration/JFET input, importance of high slew rate, open loop linearity, etc) as applied to power amp design were still being discovered or developed.
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Old 15th February 2008, 09:06 AM   #4
gootee is offline gootee  United States
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No. It makes no sense to use too-high slew rates at the input. I would definitely keep the RF filter in place, for that kind of testing.

As has been said, by some famous amp designer, you can make the performance of an amplifier driving squarewaves into a capacitive load look as good as you want, by just adjusting the input's slew rate. So, conversely, I guess, if it's too high then any amp would look bad.

The magnitude of the maximum rate of change of a voltage sine wave is:

slew rate max of sine (in volts per microsecond) =

[(2 x Pi) x (freq in Hz) x (amplitude in volts)] / 1,000,000

So, for example, for a 10V 0-to-Peak 20 kHz sine, the maximum slew rate is about 1.257 V/us. For 20V 0-P it would be about 2.513 V/us.

So, for the rise and fall times of squarewaves used for amplifier testing or simulation, for example, IF we assume that we don't need to have slew rates at an amplifier's output that are outside of some maxfreq range, then the maximum slew rate for the input would be 2 x Pi x peak output voltage x maxfreq / gain / 1000000, in V/usec.

That would mean that for a gain of 20, and a 40v p-p squarewave output, with maxfreq = 22kHz, the input would be a 2v p-p squarewave that would only need rise and fall slew rates of <= 0.138 V/us, i.e. about 14.47 usec risetime and falltime, minimum, giving a maximum slew rate of about 2.764 V/us for a -20v to +20v squarewave transition at the output.

However, I was told, here, by Bob Cordell (Post # 1985, here: http://www.diyaudio.com/forums/showt...t=#post1230123 ), that it is probably a good idea to shoot for at least 50 V/us, at the output (for a discrete-component amp). On the other hand, some chipamps will only slew at a maximum of about 10 V/us, and some even less than that. So, obviously, you wouldn't want to try to exceed that, with them, except maybe for pathological-type tests.

P.S. HINT: For driving large capacitive loads with squarewaves, you can get down to basically zero overshoot and ringing, but still with very high slew rates for almost the whole edge (of the square wave), if you have a large excess current-dumping capacity (and just the right feedback and output networks, etc, of course). When using the TI OPA541 (E version) model with LTspice, I got the excess current capacity by simply paralleling several of them, for example. (But I only got really good performance after I enclosed them all in an fast opamp's feedback loop, and came up with some well-tuned compensation schemes.)
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Old 15th February 2008, 09:12 AM   #5
Bonsai is offline Bonsai  Taiwan
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Tom, I agree with your comments about input slew rates. In my post I was refering to general slew rate requirements of an amp.

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Old 15th February 2008, 10:46 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
following on from Tom's excellent summary.

Take a hypothetical power amp with a slew rate of 30V/uS allowing a 100Vpp (150Winto 8r0) signal @ 100kHz, and the high frequency response set by the input filter were 3db down @ 200kHz (RF=0.8uS).

Would increasing the slew rate higher than 30V/uS be audible?

Or asked another way, should the slew rate match the input filter?
What factors determine this requirement if any?
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Old 15th February 2008, 11:03 AM   #7
roender is offline roender  Romania
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An high SR amplifier always mean a very short GNFB path ?
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Old 15th February 2008, 11:40 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by roender
An high SR amplifier always mean a very short GNFB path ?
physically short or very few components in the string?
Is there a rule for slew rate to GNFB path length?
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Old 15th February 2008, 11:48 AM   #9
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Quote:
Originally posted by roender
An high SR amplifier always mean a very short GNFB path ?
It's the amp that has to prevent the signal from slew limiting. The fb is just (most times) a passive divider. I can't see how the feedback physical arrangement can make a slew limiting amp un-limit. Nor how the physical fb arrangement can make a fast amp start to slew limit.

We built these amps to reproduce audio. Audio signals go up to 20kHz in frequency.

There may be stuff in the input signal that is above 20k, like DAC switching artifacts, EMI and noise, but we don't want that to got to the speaker so hopefully the input filter will attenuate it.
The only reason to test the amp with higher freq components would be to make sure it doesn't upset the circuits to harm the actual audio.

So, for instance, if you have a non-oversampling DAC with no or little output filtering, you'll have a lot of problems in your power amp unless it is supersonic

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Old 15th February 2008, 12:00 PM   #10
roender is offline roender  Romania
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Sorry, the question was wrong ... I mean with few stages. As you know, an FC amp has a very high SR

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