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Measuring distortion

jaymanaa

jaymanaa

RIP 1961-2018
Does anyone use anything besides a scope to measure harmonic distortion? I'm trying to put together a Heathkit setup to do it. I have (and have serviced) an IM-58 Harmonic Distortion Meter and a IGW-47B square/sine generator. Even after buying the manuals I have less than 100 dollars invested so if it doesn't work out, it won't be the end of the world, but I'm hoping it will. I beleive it's Rozenblits that describes how to tweek tube circuits using this kind of setup (I'll have to find the right book). I'm just trying to get some practical advice on the best way to go about this. Thanks in advance for any do's and don'ts, or tips in general. I'd really hate to release any magic smoke from the meters or amplifiers that I'm fooling with. I can't ever get it to go back in.:D
 
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Fred and I use the IM 58 at work. He built it from the kit and it is an excellent tool for tweaking the phase inverters to minimum distortion. You'll need to calibrate it in order to get accurate readings.

THe sine/square generator is nice especially with the use of an o-scope and a dummy load. This way you can watch the output of a sine going into the amp under a workload and see if it comes out clean. Most problems that could be audible will be easily seen on the scope.
 
Sounds good. I guess I need to get over my fear of O'scopes and hook one up. I recently acquired one in a "tube buy" that looks pretty nice. It's an RCA from the 80s I'd guess. I think I'll try to find a manual for it. I wouldn't even begin to know how to hook it up without one.
 
jaymanaa said:
Sounds good. I guess I need to get over my fear of O'scopes and hook one up. I recently acquired one in a "tube buy" that looks pretty nice. It's an RCA from the 80s I'd guess. I think I'll try to find a manual for it. I wouldn't even begin to know how to hook it up without one.
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The pros use HP distortion analyzer, audio generator and scope. Basically you connect a hi-power load to the amp, inject an audio CW tone, turn up the GAIN to desired output, TUNE the distortion analyzer to NULL the fundamental, and then measure/record the TOTAL harmonic levels on the DA meter. You might need to set or record a reference level before nulling. The THD can be read or computed from the DA meter in dB or %. You can also see what's going on pretty well with the scope. Do this for at least a Low, Middle and High audio frequency.

-Gregory
 
jaymanaa said:
Sounds good. I guess I need to get over my fear of O'scopes and hook one up. I recently acquired one in a "tube buy" that looks pretty nice. It's an RCA from the 80s I'd guess. I think I'll try to find a manual for it. I wouldn't even begin to know how to hook it up without one.
Click to expand...


We use a spectrum analyzer. You can't really measure distortion with just a scope. You wouldn't be able to differentiate signal from noise. You need a distortion analyzer or a spectrum analyzer and a good audio signal source.
 
I'm pretty low tech here in the ol' tube dungeon. I had read something about using a distortion meter to dial in pentodes. Basically the guy (Jones or Rozenblitz) said to install variable resistors on the pentode, then watch the meter as you adjust the values of the pots. When you get the lowest distortion (measured by the swinging needle), unhook everything and measure the values of the pots and install resistors of the same value. I haven't tried it yet but plan to soon. Does this make sense to you guys?
 
I gave this setup a whirl, and if I'm doing it right, I show;


6% @ 20,000Hz

8% @ 14,000Hz

10% @ 200Hz and at 500Hz

24% @ 20Hz


Are those readings in the realm of possibility, or am I all wet? According to the scale on my sine wave generator, my hearing stops at 16,000Hz, which is about the same frequency that the dogs started barking.:D
 
jaymanaa said:
I gave this setup a whirl, and if I'm doing it right, I show;


6% @ 20,000Hz

8% @ 14,000Hz

10% @ 200Hz and at 500Hz

24% @ 20Hz


Are those readings in the realm of possibility, or am I all wet? According to the scale on my sine wave generator, my hearing stops at 16,000Hz, which is about the same frequency that the dogs started barking.:D
Click to expand...

That's a lot of distortion. Are you sure you measured it correctly?
 
jaymanaa said:
No, I don't think so. I found the book I was missing that details how to do this, so I'll try again.
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My guess is you aren't nulling the input signal correctly. I don't use a distortion meter directly, but I would assume it just gets a thd+n as an rms value and compares this either to a fixed reference level or to the signal before the bandstop less the thd+n after the bandstop.
That assumes all of the input signal has been cancelled, or at least cancelled to 10dbs or so below the thd+n you are trying to measure.
 
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