so what about slew rate of op amps with digital curent sources ? Would you make a difference between 25V/uS & 2500V/uS. ?
Yes! Thankyou for the right words. I miss not having a scope
"Class B amplifier bias design is such that it operates within the cut-off region for only 180° of the input cycle.
This means that the Q point would have to be within the cut-off region.
Crossover distortion occurs when the input signal voltage is between ±VBE,
during which time the output signal flat-lines along the zero line.
In order to eliminate this crossover distortion,
it is customary to bias each transistor to operate slightly above the cut-off region."
So it's not related to slew rate?
Does some one have clear diagrams of full wave slew rate comparisons that they could post here?
Last edited:
No. They're two different things.
You're making it sound like it takes the amplifier half an hour to "fill the capacitor". That isn't the case. If the amplifier is designed competently the capacitor fills at a rate beyond what's needed to produce a clean signal.
It is true that the intermediate amplifier stages will need to provide a clean signal without slew-rate limiting for the amplifier to have good overall performance. That's been known for nearly 50 years now. I suggest having a look at the articles on Slewing-Induced Distortion (SID) by Jung, Stevens, & Todd from the late 1970s. Walt Jung's article series on the topic in The Audio Amateur starting in January of 1977 is a good introduction as well.
Tom
The seventies was a sort of a 'Spec of the Month!' era. Advertisers glommed onto some particular spec and then manufacturers would then produce dreadful sounding gear with that one stellar spec and all else was just plain awful. Kind of like the guy at the gym who really needs to stop skipping leg day.
Ringing points to stability issues and not to any frequency response or slew rate issues.
A rounded square wave points to a frequency response that drops at higher frequencies.
Neither of these has anything to do with slew rate limiting.
SR limiting is a large signal issue and you can have it while still having a nice rounded square wave.
You will only see SR limiting on a square wave when you output max signal levels, and then the square wave will show not rounded corners but straight line edges. The definition of slew rate limiting.
Jan
Depends on how you quantify success. 😉
There are many designs that measure well below state of the art but sell like hotcakes. Are they successful?
Other designs show stellar performance on many parameters but don't sell for a variety of reasons. Are those successful?
When it comes to slew rate – the topic of this thread – it's pretty clear. If an amplifier has sufficient slew rate to faithfully reproduce an audio signal then it is not slew-rate limited. One can show with math the minimum required slew rate for a given power level and waveform. If the amp has a slew rate that's greater than this requirement, it will not be limited by its slew rate.
But not thereby said that all of an amplifier's performance can be described by its slew rate. I have argued for years that we should look at many parameters and not just one when characterizing audio components.
Tom
There are many designs that measure well below state of the art but sell like hotcakes. Are they successful?
Other designs show stellar performance on many parameters but don't sell for a variety of reasons. Are those successful?
When it comes to slew rate – the topic of this thread – it's pretty clear. If an amplifier has sufficient slew rate to faithfully reproduce an audio signal then it is not slew-rate limited. One can show with math the minimum required slew rate for a given power level and waveform. If the amp has a slew rate that's greater than this requirement, it will not be limited by its slew rate.
But not thereby said that all of an amplifier's performance can be described by its slew rate. I have argued for years that we should look at many parameters and not just one when characterizing audio components.
Tom
Slew rate is an indicator of the response time of the system to changes in input.
Too slow, like LM358 or 747, will give a dull sound, but if used as a thermocouple amplifier, perfect, as fast changes are not needed for a temperature controller.
Too fast, like OPA2134, could overload the amplifier's ability to respond.
So practical limit for audio amplifiers is about 4 (4558 class) to about 40 (2134).
IIRC 5532 is about 8, and TL07x series about 13....enough.
Another example is the turning rate of a tank turret or earth digging hydraulic excavator, it must turn at the right rate till the required alignment is reached...too slow means cannot work properly, and too fast means will miss the target.
In any case, the design parameters are well known since the 1970s, and I do remember the 80s were having a lot of specification based advertising.
But after CD came to the market, all they could say was CD compatible, as the noise / THD was much less than tape or vinyl, and there was little to choose between brands...most had similar circuits with bought out parts!
It was like GM cars...basically similar drivetrains, and different marketing targets for different users, like Cadillac was expensive, Chevy was cheap, GMC for farmers and so on.
Ford also had Lincoln as the expensive brand, and Audi is really a dolled up VW, which will probably show up as a VW model 3 years from the Audi release.
So unless you are a circuit designer, and have access to new production parts, the discussion is useless.
For repairs / improvement, there is a well known set of 'improvements', which are mostly horse dung.
Proceed with caution.
No ties to any names above, as usual.
Too slow, like LM358 or 747, will give a dull sound, but if used as a thermocouple amplifier, perfect, as fast changes are not needed for a temperature controller.
Too fast, like OPA2134, could overload the amplifier's ability to respond.
So practical limit for audio amplifiers is about 4 (4558 class) to about 40 (2134).
IIRC 5532 is about 8, and TL07x series about 13....enough.
Another example is the turning rate of a tank turret or earth digging hydraulic excavator, it must turn at the right rate till the required alignment is reached...too slow means cannot work properly, and too fast means will miss the target.
In any case, the design parameters are well known since the 1970s, and I do remember the 80s were having a lot of specification based advertising.
But after CD came to the market, all they could say was CD compatible, as the noise / THD was much less than tape or vinyl, and there was little to choose between brands...most had similar circuits with bought out parts!
It was like GM cars...basically similar drivetrains, and different marketing targets for different users, like Cadillac was expensive, Chevy was cheap, GMC for farmers and so on.
Ford also had Lincoln as the expensive brand, and Audi is really a dolled up VW, which will probably show up as a VW model 3 years from the Audi release.
So unless you are a circuit designer, and have access to new production parts, the discussion is useless.
For repairs / improvement, there is a well known set of 'improvements', which are mostly horse dung.
Proceed with caution.
No ties to any names above, as usual.
I believe that what you are describing is called "crossover distortion" that occurs with lack of output stage biasing > very nasty distortion.
Interestingly, I have never seen the result of a square wave coming out of a 'zero biased' output stage >
What does that look like ???
This all somehow reminds me of the old but now debated realm of TID > Transient Intermodulation Distortion,
where the simplest solution was to introduce an input LPF so that no stage of the amp was driven beyond its HF capacity > especially feedback signal.
What is a "nice rounded square wave"?
SR limiting creates such a thing 😕
PS.
Regarding "ringing", I was just elaborating [ off topic ] regarding aspects of square waves. but yes, it does relate to instability.
TID and SID are the same thing. You can read more about SID in the references I gave in Post 26.
Not always. Add any series inductance (like a Thiele network) to the output of a 100% stable amplifier and it'll ring with any load capacitance. That's the resonance between the output inductance and the load capacitance and not an indication of instability.
You're right, though, that instability or marginal stability often gives ringing on the transient response, but that doesn't mean that all ringing is caused by instability.
Tom