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Joel 26th June 2003 05:45 PM

The look of distortion.
Steve Bench has a nice article with that same title, and it's useful in using the o-scope more as a tool to diagnose distortion. He lays out what the different levels and shape of 2nd, 3rd, 4th, etc look like - but he does not go into evaluating essentially clean signals.
In other words, can someone out there with a good distortion analyzer dial up what appears to be a clean waveform from their amp's output - and then measure what the distortion is at that point?
What I'm concerned with is, how far back do I have to roll the gain below clip to get into the region of 1% or less. Is the statement, "if it looks clean, it'll sound clean", fully accurate?

I like the old style desciptive power figures, ie. "undistorted power output is..." It seems to me that this is a more useful number than "1.7% THD". And I would assume that the 1930's engineers simply clipped the amp, then dialed it down until the sinewave looked clean, no?


dhaen 26th June 2003 06:24 PM


It really depends how the distortion is happening.
With clipping, 1% is easily visible on a scope displayed sinewave.
However, you can have 5% discrepency between the upper and lower sinewave amplitudes, and hardly notice.

So the answer in my opinion is that you can "see" distortion on your scope when it is between 1% and 5% depending on the case.

Hope this helps.


Joel 26th June 2003 06:39 PM

It helps some, John, thanks. But what type of evaluation do you personally do on your amps? Do you even consider distortion?

The brain so easily compensates, doesn't it?

dhaen 26th June 2003 10:51 PM


At home I have 2 distortion analysers, both quite simple, that will check at 1KHz only. It's up to me to look at the nulled output (the distortion products) on the scope to determine the proportion of odd and even. However that is usually predictable from the circuit topology since I only use triodes.

Better equipmnt is available elsewhere, but a valve amp doesn't fit in the briefcase!


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