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Old 1st May 2010, 05:03 PM   #1
steevo is offline steevo  United Kingdom
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Default anyone built an amplifier Distortion measurement circuit

Has anyone built a distortion meter and signal genrator to measure
amplifier distortion accuratley. I have been looking around the web for circuit designs and in electronics magazines and cant seem to find anything.
Have any of the popular magazines ever published a distortion measrement
instrument. Has anyone come across any.

Cheers
Steevo
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Old 1st May 2010, 05:28 PM   #2
latala is offline latala  United Kingdom
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Yes Steevo I have built a no of such units the first 1 in 1975
Most people build a low distortion oscillator and a notch filter this can measure down to 0.05% without any problem
There is an alternative way though that is to use an opamp of good quality and ensuring that you invert 1 signal add them with the aid of a few pots and cancel the origional signal that you inject all that is then left is hum noise and distortion
I may be able to help you with a few sketches if you are interested
another way is to look on E-bay I have picked up some good gear at a low price
you will also need a scope for it to be meaning full
Regards Trev
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Old 1st May 2010, 05:33 PM   #3
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You'll need a nice low distortion audio signal generator, here's a decent performer

I actually need to build one myself having tested my Tenma function generator & found it severely lacking.

Quote:
another way is to look on E-bay I have picked up some good gear at a low price
Agreed, that's where my HP8903E came from & showed me how distorted the function generator was
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Last edited by event horizon; 1st May 2010 at 05:38 PM.
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Old 1st May 2010, 05:42 PM   #4
latala is offline latala  United Kingdom
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Steevo that oscilator looks good you will probably also need a schmit triger to get square waves usefull for response testing
regards Trev
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Old 1st May 2010, 05:50 PM   #5
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Hello Steevo

You can look on Ebay for a used HP-333 or even better a HP339, they are old gear buy you can have them for cheap on Ebay, but most of the time those distortions analysers was not calibrated since years, still they are great gear if you could find one in a good working condition.

There is also the Bob Cordell diy distortion analyser, but it's not an easy one to built;

My implementation of the Cordell Distortion Analyser

Bye

Gaetan
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Old 1st May 2010, 06:01 PM   #6
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Hello Steevo

There is also this very simple but very low distortions sine wave generator.

You can replace the LT1037 by a low distortion Opamp.

Bye

Gaetan
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File Type: gif ultra-pure sine wave oscillator.gif (29.7 KB, 243 views)

Last edited by gaetan8888; 1st May 2010 at 06:06 PM.
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Old 1st May 2010, 06:04 PM   #7
steevo is offline steevo  United Kingdom
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Hi latala i would appreciate any help, circuit and tips aid in building one.
the low distortion signal generator look good, Do i need to be carefull about the PCB layout for that design to get the performance.

Thanks
Steevo
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Old 1st May 2010, 07:57 PM   #8
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In what frequency range and to which distortion level do you want to measure? 20 Hz-20 kHz to -100 dB is not that difficult but more gets tricky, particularly if you can't live with a single fixed frequency (usually 1 kHz).

Samuel
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Old 1st May 2010, 09:11 PM   #9
Pano is offline Pano  United States
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Why not use a computer and a good soundcard? 192KHz/24 bit is not that expensive these days. Lot's of free software, too.
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Old 2nd May 2010, 06:09 PM   #10
steevo is offline steevo  United Kingdom
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If possible I would like to measure down to 0.01% distortion at 500hz, 1khz, 5khz, 10khz, 15khz and 20khz
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