I am looking for resources regarding what to look for in oscilloscope measurements of tube amp output or other amplifier measurements. I have found scattered in the threads several bits of information, but nothing coherent about what different square wave output shapes might mean and how to really assess the amps function. Thanks.
I doubt you'll find a textbook that describes that, but should you come across one, please post author, title, and ISBN here as I'm sure others would benefit.
What most people try to figure out from the square wave testing is how stable the amplifier is. In control systems with feedback, ringing on the square wave as it levels out at the top could indicate that the amp is operating close to instability. A little wobble is normal, but excessive ringing may be an indicator of instability. What's considered "wobble" and "excessive" depends on experience, personal preference, etc.
Another thing that can be deduced from the square wave test is the system's transient response. How well does the amp behave when its input suddenly changes. In this category falls slew rate - how fast does the voltage rise on the output when the input changes instantaneously. If the SR is too low, the max power bandwidth will be lower as the amp cannot reproduce a high-frequency signal at max output power without slewing. For sine wave signals, the max slew rate is: SR = 2*pi*f*Vpeak so if the slew rate of the amp is, say, 0.5 V/us (= 500000 V/s), the max non-slewing sine wave that can be reproduced at 20 kHz is: Vpeak = SR/(2*pi*f) = 500000/(2*pi*20000) = 3.98 Vpeak or about 1 W into 8 ohms.
So search terms for your google search:
Amplifier stability
Transient response
Slew rate
Hope this helps...
~Tom
What most people try to figure out from the square wave testing is how stable the amplifier is. In control systems with feedback, ringing on the square wave as it levels out at the top could indicate that the amp is operating close to instability. A little wobble is normal, but excessive ringing may be an indicator of instability. What's considered "wobble" and "excessive" depends on experience, personal preference, etc.
Another thing that can be deduced from the square wave test is the system's transient response. How well does the amp behave when its input suddenly changes. In this category falls slew rate - how fast does the voltage rise on the output when the input changes instantaneously. If the SR is too low, the max power bandwidth will be lower as the amp cannot reproduce a high-frequency signal at max output power without slewing. For sine wave signals, the max slew rate is: SR = 2*pi*f*Vpeak so if the slew rate of the amp is, say, 0.5 V/us (= 500000 V/s), the max non-slewing sine wave that can be reproduced at 20 kHz is: Vpeak = SR/(2*pi*f) = 500000/(2*pi*20000) = 3.98 Vpeak or about 1 W into 8 ohms.
So search terms for your google search:
Amplifier stability
Transient response
Slew rate
Hope this helps...
~Tom
Oh... Read hi-fi reviews in magazines. I read a bunch of those back in the 80'ies, 90'ies. At least back then, it was common that an amp was reviewed both through measurements and listening tests. Transient response was a common measurement and often accompanied by some language that indicated what the reviewers were looking for.
But I haven't opened a HiFi magazine in years. Things may be different now...
~Tom
But I haven't opened a HiFi magazine in years. Things may be different now...
~Tom
Books on waveform examination
R G Middleton has written quite a few books on this!
Amazon.com: Robert Gordon Middleton: Books, Biography, Blog, Audiobooks, Kindle
R G Middleton has written quite a few books on this!
Amazon.com: Robert Gordon Middleton: Books, Biography, Blog, Audiobooks, Kindle
Building Valve Amplifiers is a bog by Morgan Jones
You can find excerpts on Google Books
(but not the measurement section)
SB
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$45US bucks used on Amazon. Might go for it. 3rd edition is over 600 pages. That should tell me something.
Jrenkin,
While you are sure to find an answer in the quoted books, just to add a little to Tomcr's good explanation. A square wave actually represents all/most of the uneven (3rd, 5th, etc) frequencies upward of the basic square wave frequency. One thus has an instant 'signal generator sweep' at one's disposal, showing at a glance if all is moderately well with a design. Low frequency performance is usually easy to check with the direct use of frequencies. The h.f performance is the worrying thing especially if NFB be used, and as said a square wave gives instant indication of where to expect trouble if any. Blowing up any little oscillation at the cross-over point, will yield the troublesome frequency, where one can go and look with a frequency/phase sweep, often to find a peak there.
But do study the quoted sources.
While you are sure to find an answer in the quoted books, just to add a little to Tomcr's good explanation. A square wave actually represents all/most of the uneven (3rd, 5th, etc) frequencies upward of the basic square wave frequency. One thus has an instant 'signal generator sweep' at one's disposal, showing at a glance if all is moderately well with a design. Low frequency performance is usually easy to check with the direct use of frequencies. The h.f performance is the worrying thing especially if NFB be used, and as said a square wave gives instant indication of where to expect trouble if any. Blowing up any little oscillation at the cross-over point, will yield the troublesome frequency, where one can go and look with a frequency/phase sweep, often to find a peak there.
But do study the quoted sources.
Good place to start: Oscilloscope Techniques.
It's an oldie from the late 1950s, but still very good.
It's an oldie from the late 1950s, but still very good.
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