Some harmonic distortion questions.


2010-06-29 2:54 pm
Asking some newbie questions so please be gentle.

1) Amplifiers are designed with as low distortion as possible and it is said second order harmonic distortion is prefered by many. So if distortion levels are low in an amplifier should one bother about whether they are second order or other ?

2) Are these distortion level dependent or frequency dependent ?*

3) Speakers also have 2nd or 3rd order distortions. So what things determine weather a speaker driver will have more 2nd order harmonic distortion than 3rd ?

*I guess amplifier with extremely low distortion in treble range, 2nd order harmonic distortion at midrange, and 3rd order at lower woofer level (clean and tighter Bass ?) would sound good. (?)

Thanks in advance.

1) In general yes, minimizing distortion is good. Some people like tube amps, class A amps, that might have relatively high THD reading compared to some other amplifier. I guess second harmonic dominates third in such amps and thus sounds better than some other design with different distribution of harmonic distortion. Second (even) harmonic is an octave from the root and as such could belong to the musical piece, third (odd) harmonic sticks out more easily (doesn't hit exactly to a note).

2) Both, at least with speakers. See this for example Harmonic Distortion
I don't know how this is with amplifiers.

3) Search for Klippel papers for example for speaker driver distortion causes.

ps. There are some amplifiers that have third harmonic distortion in opposite polarity as the speaker, eg. some of the third order distortions could cancel out. Check out this for example ClassB+a mosfet amplifier. Also, MFB (motional feedback) is a technique to reduce distortions (low frequencies). More over, signal could be preconditioned digitally to cancel out some distortions, check out Gunness Focusing. All of these have the idea of introducing some "predistortion" to the input signal.

I find all kinds of distortions unpleasant, especially with high frequencies. I'm not a pro, just my opinions here :)
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2010-11-18 7:59 am
THD measurements are usually dominated by the 2nd and 3rd harmonic, but these are the least objectionable distortions. Some people actually prefer a little bit of one or the other. There are other distortions which do not sound musical. Intermodulation distortion, for example. The spectrum of distortion frequencies matters. Distortion is more detectable if it is dissonant and farther from the fundamental.
Aside from that, distortion is produced by amplifier misbehavior and clipping. How an amplifier responds at clipping is likely to be more important in real world listening, since loudspeakers are not simple 8-ohm test loads and impedance varies across the spectrum. Even at moderate average volume clipping could occur some percentage of the time if the recording has enough dynamic range.
Distortion in an amplifier is typically both level and frequency dependent. Think of a guitar amp having both "clean" and "lead" channels. Each can be tailored by frequency domain controls to produce the best "clean" sound and the best "overdrive" sound. Obviously, in the "lead" channel, the distortion amount is input level dependent. Around the time I graduated college, I saw a guitarist in a club with a Laney amp. A simple 1/2 turn twist of his guitar's volume control and the amp went from clean to a gorgeous sustaining lead tone. I was sooo jealous...

I've shown that you can dial in 2nd or 3rd or both in an ordinary P-P tube amp circuit. I'm pretty sure you could make that behavior frequency dependent, with a simple RC network. Using a DSP to drive the output tubes any way you'd like regarding phase and frequency - there you go! An amplifier that produces a frequency dependent distortion character any way you like.

Legend also has it that 2nd harmonic tricks the ear into believing there's greater dynamics than there really is. So if you want "punchy" bass? I have no idea what frequency ranges register with the human ear regarding this effect however. FAIK it doesnt work on bass frequencies, only mids / treble...

Make a fully differential (mirror circuit) P-P amplifier. Get one of those 4 output DACs and start with all pass, one output inverted in each pair, even output levels on all 4 outputs. Then play on your computer while listening what introducing different output levels and filters do, on just one of the two outputs driving a channel (Hint - the distortion character of the amplifier will change) Then go get a patent so only you can make such an amplifier in your country!

Good luck with your idea and I hope this helps you attain it. How's that for being "gentle" - and encouraging?
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