Audio distortion measurement

R

Rob

Guest
Hi all.
Is there a simple way to measure distortion in a audio amp.
I am looking to measure over a frequency range of say 200 - 2Khz.
It will be a low power amp (intercom) , and obviously not hi-fi quality.
I would just like to get a basic distortion measurement. Does
not have to be anything fancy.Looking for a simple analog design.
ie No computer / capture card / software.
A notch filter method is fine but only works for one frequency , I am
looking to have continuous coverage from 200 - 2000Hz.
I get the feeling that what I need may not be "Simple" , but maybe someone
out there has some ideas.
Thanks
Rob
 
"Rob" <seegoon99NSpam@yahoo.com> wrote in message
news:Xns9579CF99EE304robd00hotmailcom@196.25.240.158...
Hi all.
Is there a simple way to measure distortion in a audio amp.
I am looking to measure over a frequency range of say 200 - 2Khz.
It will be a low power amp (intercom) , and obviously not hi-fi quality.
I would just like to get a basic distortion measurement. Does
not have to be anything fancy.Looking for a simple analog design.
ie No computer / capture card / software.
A notch filter method is fine but only works for one frequency , I am
looking to have continuous coverage from 200 - 2000Hz.
I get the feeling that what I need may not be "Simple" , but maybe someone
out there has some ideas.
Click to expand...

What John W. said. But, some other random thoughts:

If you've got a low-distortion oscillator handy (at least, low enough that
it's lower than what you want to measure), then all you need is a distortion
analyzer. HP 331's and 334's will do the trick, and seem to go for between
$50 and $200 on eBay, sometimes even cheaper, though be careful to get one
that's known to be working. (Heck, I've got one I keep meaning to sell;
contact me offline if you're interested and I'll give you a reasonable
price.)

Or, you could do a couple of notch filters, rather than just one. As John
suggested, there's not going to be much difference between 200Hz and 2kHz;
taking a measurement at 300Hz and one at 1kHz would tell you pretty much the
same thing as a sweep would.

Remember that if your system is bandwidth limited, and you test with a
frequency near its limit, you will see very little distortion. The
distortion products are higher frequency than the test signal (assuming
you're looking for harmonic distortion, rather than intermodulation
distortion), and they get filtered out by the limited bandwidth.

For an intercom system, anything up to about 10% THD is probably going to be
acceptable. With that much distortion, you can just about see it by eye on
a scope; and you can certainly hear it with a trained ear.
 
Walter Harley wrote:

-- snip --
For an intercom system, anything up to about 10% THD is probably going to be
acceptable. With that much distortion, you can just about see it by eye on
a scope; and you can certainly hear it with a trained ear.


Ah -- but how do you train the ear without a distortion analyzer?
Click to expand...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
In article <auKdnQnq5M45uP7cRVn-og@speakeasy.net>,
Walter Harley <walterh@cafewalterNOSPAM.com> wrote:
[......]
Remember that if your system is bandwidth limited, and you test with a
frequency near its limit, you will see very little distortion.
Click to expand...
.... or a huge amount of distortion. Think about this circuit:

VCC
!
Vin ----!+\ !/
! ----+--------+---! NPN
--!-/ ! ! !\
! --- C ! !
! --- ! !
! ! R ! !------ To 8 ohm load
----------+-/\/\/--------+
! !
! !!-
--!! P-MOSFET
!!-
!
Vss

For low frequencies, the distortion isn't too bad. As you get close to
F=1/(2*PI*RC) the distortion gets very bad.


--
--
kensmith@rahul.net forging knowledge
 
On 5 Oct 2004 18:24:22 GMT, in sci.electronics.design you wrote:

Hi all.
Is there a simple way to measure distortion in a audio amp.
I am looking to measure over a frequency range of say 200 - 2Khz.
It will be a low power amp (intercom) , and obviously not hi-fi quality.
I would just like to get a basic distortion measurement. Does
not have to be anything fancy.Looking for a simple analog design.
ie No computer / capture card / software.
A notch filter method is fine but only works for one frequency , I am
looking to have continuous coverage from 200 - 2000Hz.
I get the feeling that what I need may not be "Simple" , but maybe someone
out there has some ideas.
Thanks
Rob
just use your soundcard and
Click to expand...
http://audio.rightmark.org/index_new.shtml
works fine
The idea of using a analogue notch filter and LD oscillator these
days is a bit old fashioned




martin

Serious error.
All shortcuts have disappeared.
Screen. Mind. Both are blank.
 
"Rob" <seegoon99NSpam@yahoo.com> wrote in message
news:Xns9579CF99EE304robd00hotmailcom@196.25.240.158...
Hi all.
Is there a simple way to measure distortion in a audio amp.
I am looking to measure over a frequency range of say 200 - 2Khz.
It will be a low power amp (intercom) , and obviously not hi-fi quality.
I would just like to get a basic distortion measurement. Does
not have to be anything fancy.Looking for a simple analog design.
ie No computer / capture card / software.
A notch filter method is fine but only works for one frequency , I am
looking to have continuous coverage from 200 - 2000Hz.
I get the feeling that what I need may not be "Simple" , but maybe someone
out there has some ideas.
Thanks
Rob
Click to expand...
Maybe simplest is to build a tunable '2 opamp bandpass filter' set with a
sharp 'Q' of maybe 10 or more. Audio tuning over 20:1 range by means of a
radio style 2 gang variable capacitor. 200Hz-20kHz frequency range would be
easily available.
Essentially it would function as a manually tuned, audio wave/spectrum
analyser as each audio distortion component could be measured/examined in
turn.
It's very easy to design using barely any bits and pieces. Jim Thompson has
the 'DAPB' circuit etc on his web site.
regards
john
 
In message <Xns9579CF99EE304robd00hotmailcom@196.25.240.158>, Rob
<seegoon99NSpam@yahoo.com> writes
Hi all.
Is there a simple way to measure distortion in a audio amp.
I am looking to measure over a frequency range of say 200 - 2Khz.
It will be a low power amp (intercom) , and obviously not hi-fi quality.
I would just like to get a basic distortion measurement. Does
not have to be anything fancy.Looking for a simple analog design.
ie No computer / capture card / software.
A notch filter method is fine but only works for one frequency , I am
looking to have continuous coverage from 200 - 2000Hz.
I get the feeling that what I need may not be "Simple" , but maybe someone
out there has some ideas.
Thanks
Rob
For a lo fi measurement employ
Click to expand...
2 channel scope on channel invert and add.
Arrange that input to the amp displaces the scope trace approx the same
as the output then adjust variable scope gain to zero the result on the
screen.
Destortion will show up as the amplitude that cannot be zeroed.

--
dd
 
"Tim Wescott" <tim@wescottnospamdesign.com> wrote

Ah -- but how do you train the ear without a distortion analyzer?
Click to expand...
Er, listen to someone speak?

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer: Electronics; Informatics; Photonics.
Remove spaces etc. to reply: n o lindan at net com dot com
psst.. want to buy an f-stop timer? nolindan.com/da/fstop/
 
I read in sci.electronics.design that dd <dd@ddwyer.demon.co.uk> wrote
(in <9OkWM5AusWZBFwQa@ddwyer.demon.co.uk>) about 'Audio distortion
measurement', on Thu, 7 Oct 2004:

Destortion will show up as the amplitude that cannot be zeroed.
Click to expand...
There is likely to be a residual fundamental due to phase shift in the
amplifier. This is not non-linearity distortion.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
In article <uIYe0xMQkZZBFwjb@jmwa.demon.co.uk>,
John Woodgate <noone@yuk.yuk> wrote:
I read in sci.electronics.design that dd <dd@ddwyer.demon.co.uk> wrote
(in <9OkWM5AusWZBFwQa@ddwyer.demon.co.uk>) about 'Audio distortion
measurement', on Thu, 7 Oct 2004:

Destortion will show up as the amplitude that cannot be zeroed.

There is likely to be a residual fundamental due to phase shift in the
amplifier. This is not non-linearity distortion.
Click to expand...
If the phase shift isn't too big, this method still helps a lot to see
distortion. You can knock the fundamental down by about 5X. This makes
the distortion products stand out more.

If you make a RC low pass filter using a pot, you can line up the
fundamental and get a much greater reduction. The nice thing about doing
that is it also can make the 3rd harmonic in your generator less of a
problem.

--
--
kensmith@rahul.net forging knowledge
 
Nicholas O. Lindan wrote:

"Tim Wescott" <tim@wescottnospamdesign.com> wrote


Ah -- but how do you train the ear without a distortion analyzer?


Er, listen to someone speak?

That gives you "good" or "bad" but it doesn't give you "oh that sounds
Click to expand...
like about 10%". And my "good" may be your "bad".

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
"Nicholas O. Lindan" <see@sig.com> wrote in message
news:6zD9d.12604$gs1.7278@newsread2.news.atl.earthlink.net...
That's what I thought, till I actually tried a double blind test on an
amplifier in my living room.

The setup was a Crown IC150/DC150 amplifier pair (I'm showing my age
here), AR9 speakers, Sheffield labs direct to disc record, Shure V15-???
cartridge and Kenwood 720 turntable (? the one made from concrete and
rosewood).

The DUT was a $500 Kenwood A/V receiver. The Crown's 0.0012% distortion
spec Vs the Kenwood's 0.1% - like who's going to hear that, right? I was
all set to sing the praises of Kenwood...

It wasn't even close - no need for the double-blind switch. No contest.
Dirty eyeglasses Vs clean eyeglasses.

I would not have said the ear was such a good discriminator as an
HP distortion analyzer working at the noise floor. But it does seem
so.
Click to expand...

I have a couple of comments.

First: 0.1% distortion is audible to an attentive listener in an appropriate
environment. I would not say the same of 0.01%. It is relatively easy to
push amp distortion under 0.01% these days. (Speaker distortion is another
story, of course.)

Second: the amps differed on their *specs*, not their measured performance.
Did you actually measure the distortion of the amps under the operating
conditions? Perhaps the Crown wasn't working. Perhaps the Crown's spec was
bogus. Perhaps the Kenwood was conservatively rated.

Third: what HP distortion analyzer? An HP 331A, decent but old machine,
can't measure below about 0.1%. An ear is definitely better than a 331A
where it can be used, although in many situations (such as very high power
or into non-loudspeaker loads or at high frequencies) an ear is not useful.

Fourth: you say you "actually tried a double blind test." But you go on to
say "no need for the double-blind switch." So, it sounds like you *didn't*
actually try the double-blind test?

Fifth: THD+N is one spec on which amps can differ. But it is not the only
one. Frequency response, slew rate, IMD...

Sixth: how did you calibrate the amps to achieve the same output level? A
1dB difference in volume can be perceived as a noticeable difference in
sonic quality, even when it is not perceived as a difference in volume per
se.

Your test does not demonstrate what you seem to feel it does.

-walter
 
"Nicholas O. Lindan" <see@sig.com> wrote in message
news:C1H9d.12764$gs1.11597@newsread2.news.atl.earthlink.net...
The post was about listening, not measuring. Kadzooks man,
unclench that sphincter.
Click to expand...
No, that would be messy.


Perhaps the Crown wasn't working.

I think you have this bass-akwards.

Perhaps the Crown's spec was bogus.

Er, know what a Crown amp is? I assume you do. Kinda dumb statement, eh?
Click to expand...
Yes; sorry. (No need to assume; I am on record, as a bit of Googling will
indicate, as being a big fan of Crown.) But presumably you get the point:
the specs don't mean diddley, if what you're interested in is what level of
distortion people can hear, which was the original point of contention.
What matters is how the two amps were actually performing at the time.


Didn't need to: that was the whole effing point. The difference was
obvious.
Click to expand...
This is where we part ways, I think. There are just too damn many people
who have made the same mistake. If you didn't do the double blind
experiment, then all we are talking about is your expectations. I'm not
trying to deny your experience; but I am saying that it is meaningless and
useless to me, or to anyone else.

To spell it out: You're saying that a well-rated and well-reputed amp
sounded better, in a non-blind situation, than a lesser-rated and
lesser-reputed amp. Big deal. Of course it did. Perhaps for good reason,
and perhaps not.

Look, I personally believe you - I believe that the Crown beat the Kenwood.
If I didn't believe you, then I wouldn't have spent all the time and money I
have on getting the distortion as low as it is on the little headphone amp I
sell, and I wouldn't own all the Crown amps that I do. But the bottom line
is that what you're saying is purely anecdotal, unless there's more to it
than you're revealing.


Nicholas O. Lindan, Cleveland, Ohio
Click to expand...
Hey, I lived in Peninsula for a while... just down the road from the
Richfield Coliseum, when it still existed.
 
I read in sci.electronics.design that Nicholas O. Lindan <see@sig.com>
wrote (in <6zD9d.12604$gs1.7278@newsread2.news.atl.earthlink.net>) about
'Audio distortion measurement', on Fri, 8 Oct 2004:
The DUT was a $500 Kenwood A/V receiver. The Crown's 0.0012% distortion
spec Vs the Kenwood's 0.1% - like who's going to hear that, right? I
was all set to sing the praises of Kenwood...

It wasn't even close - no need for the double-blind switch. No contest.
Dirty eyeglasses Vs clean eyeglasses.

I would not have said the ear was such a good discriminator as an HP
distortion analyzer working at the noise floor. But it does seem so.
Click to expand...
You may have been hearing a difference due to something totally
different from non-linearity distortion.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
On Fri, 08 Oct 2004 21:23:14 GMT, "Nicholas O. Lindan" <see@sig.com>
wrote:

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote
"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:10m69mu7s8ub9ea@corp.supernews.com...
Walter Harley wrote:
For an intercom system, anything up to about 10% THD is probably going to
be acceptable. With that much distortion, you can just about see it by
eye on a scope; and you can certainly hear it with a trained ear.

Use a square wave for a source - damn good distortion analyzer. The
scope trace will tell all.
Click to expand...
A square wave is probably the worst possible signal for showing
harmonic distortion. It will, in fact, perfectly hide gross
nonlinearities and huge amounts of crossover distortion.



That's what I thought, till I actually tried a double blind test on an
amplifier in my living room.

It wasn't even close - no need for the double-blind switch. No contest.
Dirty eyeglasses Vs clean eyeglasses.
Click to expand...
Perfectly Zen: a non-blind double-blind test... "no need"!


I would not have said the ear was such a good discriminator as an
HP distortion analyzer working at the noise floor. But it does seem
so.

Now, I'm no golden-eared prissy - my speakers are hooked up with a long
length of 18ga lamp cord, I use WD-40 as a contact cleaner, you know,
a 'regular' high-fi nut.

This was a sample of one. And, yes, I inhaled in the '60's.
Don't bet any money on this ...

But, as a result I have been converted to the "good enough specs ain't all"
school of engineering
Click to expand...
That's not engineering, that's superstition. You have indeed been
"converted."

And the realization that a human is the only final arbiter.
Click to expand...

John
 
On Tue, 05 Oct 2004 15:57:46 -0700, Tim Wescott
<tim@wescottnospamdesign.com> wrote:

Walter Harley wrote:

-- snip --
For an intercom system, anything up to about 10% THD is probably going to be
acceptable. With that much distortion, you can just about see it by eye on
a scope; and you can certainly hear it with a trained ear.


Ah -- but how do you train the ear without a distortion analyzer?
Click to expand...
Play Slipknot records. :)

--

"What is now proved was once only imagin'd." - William Blake, 1793.
 
On Sat, 09 Oct 2004 01:20:34 GMT, "Nicholas O. Lindan" <see@sig.com>
wrote:


Perhaps the Crown wasn't working.

I think you have this bass-akwards.

Perhaps the Crown's spec was bogus.

Er, know what a Crown amp is? I assume you do. Kinda dumb statement, eh?
Click to expand...
Crown also makes NMR gradient amplifiers. My customer tells me that my
amps are better than theirs by a factor of 70. That probably
represents the difference between listening and measuring; theirs
evolved from audio amps.

John
 
Is there a simple way to measure distortion in a audio amp.
I am looking to measure over a frequency range of say 200 - 2Khz.
======================
Click to expand...
I like the spectrum analyzer idea. Put in a 200Hz sin wave and turn it up till
you see the harmonics popping up 3rd at 600hz, 5th at 1KHz... THD is the sqrt
of the sum of the squares of the harmonic distortion products... I'd say you
can hear 3rd harmonics that are 60dB down.... thats about .01% THD....
 
Umm ... -60 db is only .1%THD and I'd agree that it's at least close to
the audible range. .01% is well below it
==============
Click to expand...
OK, Norm... I agree also.... extra zero.... Thats not as bad as confusing kilo
and mega hertz is it?
 
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