Learning to measure question

jrenkin · 9th December 2009, 10:32 PM

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.

SY · 9th December 2009, 10:51 PM

There's a pretty good explanation in "Building Valve Amplifiers." I'd start there.

tomchr · 9th December 2009, 11:17 PM

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

tomchr · 9th December 2009, 11:20 PM

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

Merlinb · 10th December 2009, 04:44 PM

Quote:

Originally Posted by tomchr

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.

R G Middleton has written quite a few books on this!
Amazon.com: Robert Gordon Middleton: Books, Biography, Blog, Audiobooks, Kindle

jrenkin · 25th December 2009, 03:58 PM

Hey, thanks everyone! I will start working on some of these resources.

tvrgeek · 25th December 2009, 04:06 PM

Quote:

Originally Posted by SY

There's a pretty good explanation in "Building Valve Amplifiers." I'd start there.

Is that a paper, forum thread, book? Whatever? Also new to tubes.

Svein_B · 04:19 PM

Quote:

Originally Posted by tvrgeek

Is that a paper, forum thread, book? Whatever? Also new to tubes.

Building Valve Amplifiers is a bog by Morgan Jones

You can find excerpts on Google Books
(but not the measurement section)

SB

tvrgeek · 25th December 2009, 05:00 PM

Thanks, will search.

tvrgeek · 25th December 2009, 05:16 PM

$45US bucks used on Amazon. Might go for it. 3rd edition is over 600 pages. That should tell me something.

9th December 2009, 10:32 PM	#1
jrenkin diyAudio Member Join Date: Nov 2007 Location: Denver, Colorado	Learning to measure question 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.

9th December 2009, 10:51 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	There's a pretty good explanation in "Building Valve Amplifiers." I'd start there. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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9th December 2009, 11:17 PM	#3
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	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 = 2pifVpeak 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/(2pif) = 500000/(2pi*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

9th December 2009, 11:20 PM	#4
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	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

25th December 2009, 03:58 PM	#6
jrenkin diyAudio Member Join Date: Nov 2007 Location: Denver, Colorado	Hey, thanks everyone! I will start working on some of these resources.

25th December 2009, 05:00 PM	#9
tvrgeek diyAudio Member Join Date: Dec 2009 Location: Md	Thanks, will search.

25th December 2009, 05:16 PM	#10
tvrgeek diyAudio Member Join Date: Dec 2009 Location: Md	$45US bucks used on Amazon. Might go for it. 3rd edition is over 600 pages. That should tell me something.

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