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
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
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
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
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.
I actually need to build one myself having tested my Tenma function generator & found it severely lacking.
Agreed, that's where my HP8903E came from & showed me how distorted the function generator was
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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;
http://www.diyaudio.com/forums/solid-state/154260-my-implementation-cordell-distortion-analyser.html
Bye
Gaetan
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;
http://www.diyaudio.com/forums/solid-state/154260-my-implementation-cordell-distortion-analyser.html
Bye
Gaetan
Hi Steevo,
You can find the original construction article for my THD analyzer (published in Audio) on my web site at Cordell Audio: Home Page. However, as mentioned, it is a difficult build. A good distortion analyzer is quite complex. BTW, the oscillator portion of my THD analyzer is less difficult to build and provides very low distortion.
Alas, all of that was about 30 years ago (time flies), before PC's and soundcards. It is a lot easier to start out with a good sound card and some software. I use a Juli@ soundcard, although I still also use my original distortion analyzer.
Cheers,
Bob
This latter is a very good point. Doing a decent job of measuring 20 kHz THD requires measurement BW out to 200 kHz. Few soundcards, do very well above about 50 kHz, even when sampling at 192 kHz.
Bob
A living example of Bob's design :
http://www.diyaudio.com/forums/solid-state/154260-my-implementation-cordell-distortion-analyser.html
Patrick
http://www.diyaudio.com/forums/solid-state/154260-my-implementation-cordell-distortion-analyser.html
Patrick
Hi guys,
I'm faced with a similar measurement need and I looked about for economical solutions. The professional PC cards packages are excellent, of course, and I wish I could afford one etc.
I thought of a more flexible approach, since I only infrequently need to use it, though it has a flaw; no software for harmonic analysis.
I have this start, though.
1) cheap local kit DSP sig. gen. kit with <.001% distortion.
2) Audio Dac (existing gear) but kit OK (optical links preferred)
3) 16-bit resolution PC scope (Picotech or Handyscope etc.)
4) Usual low inductance loads for things like power amplifiers.
Since I like to measure harmonic content up to, say the ninth, calculating this is real
tedious. I'm not disagreeing with Bob's assessments here, just suggesting that it will suffice for my DIY needs.
I have had the gear working, but it is tedious to read off harmonic peaks to calculate THD. It should be possible in software to do this rapidly from the FFT plot, you might expect. Anyway, I feel confident of measuremets down to .002% or better. I hope to verify this in the coming months.
So, what do you think of this under 500 UK new gear solution?
What hopes for upgrading PICO'S software? (I have asked)
Any (polite) suggestions? Does it help or suggest something better for your own needs?
regards I
I'm faced with a similar measurement need and I looked about for economical solutions. The professional PC cards packages are excellent, of course, and I wish I could afford one etc.
I thought of a more flexible approach, since I only infrequently need to use it, though it has a flaw; no software for harmonic analysis.
I have this start, though.
1) cheap local kit DSP sig. gen. kit with <.001% distortion.
2) Audio Dac (existing gear) but kit OK (optical links preferred)
3) 16-bit resolution PC scope (Picotech or Handyscope etc.)
4) Usual low inductance loads for things like power amplifiers.
Since I like to measure harmonic content up to, say the ninth, calculating this is real
tedious. I'm not disagreeing with Bob's assessments here, just suggesting that it will suffice for my DIY needs.
I have had the gear working, but it is tedious to read off harmonic peaks to calculate THD. It should be possible in software to do this rapidly from the FFT plot, you might expect. Anyway, I feel confident of measuremets down to .002% or better. I hope to verify this in the coming months.
So, what do you think of this under 500 UK new gear solution?
What hopes for upgrading PICO'S software? (I have asked)
Any (polite) suggestions? Does it help or suggest something better for your own needs?
regards I
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I had buided two oscillator circuit,
one based on a wien bridge circuit (inspired by a linear application note or the lm1007 datasheet, i don't remember) stabilazied with 4 small lamps. I can control the frequncy with a uC by a doble digital potentiometer and the selection of the capacitor.
I use this generator as source
the other is a vco inspired by a national semiconductor application note and use this as a frequency source for an AM demodulator of the signal.
The voltage control is generate by a dac connected to the uC.
The uC can adjust in real time both the frequency generators comparing signals with their cristal.
I'm not satisfied at all for the solution, the first generator works well, but the the vco/am block doesn't a clean work because measure more distortion than the reality.
I wish to replace both demodulator and vco, but now i have enough time to follow this project.
one based on a wien bridge circuit (inspired by a linear application note or the lm1007 datasheet, i don't remember) stabilazied with 4 small lamps. I can control the frequncy with a uC by a doble digital potentiometer and the selection of the capacitor.
I use this generator as source
the other is a vco inspired by a national semiconductor application note and use this as a frequency source for an AM demodulator of the signal.
The voltage control is generate by a dac connected to the uC.
The uC can adjust in real time both the frequency generators comparing signals with their cristal.
I'm not satisfied at all for the solution, the first generator works well, but the the vco/am block doesn't a clean work because measure more distortion than the reality.
I wish to replace both demodulator and vco, but now i have enough time to follow this project.
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