How Do You Perceive THD at HF? - diyAudio
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Old 25th May 2012, 07:54 AM   #1
Jay is offline Jay  Indonesia
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Default How Do You Perceive THD at HF?

How do you perceive THD at 10KHz or 20KHz, especially from simulation? Do these numbers mean anything to you?

My standard minimum for THD at 1KHz is 0.1% at any stage in any amps (depends on the topology). But I cannot do the same thing with THD at 10KHz for example. I cannot relate the number with actual (real life) performance.

So, somebody must be able to "benefit" from this number. Otherwise, why bother measuring it. You might for example (just a guess) use the number to predict certain kind of anomalies in the HF reproduction...

Or, may be somebody have tried to make this measurement to become more meaningfull by lowering the fundamental frequency, or even by adding certain filters? (Just a guess).

I will also find some answers later by simulating as many amps that I'm familiar with and check if there is correlation between this THD and the amp's performance. But from limited simulation so far, I cannot draw a good correlation. I don't even know what should the minimum acceptable distortion be.
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Old 25th May 2012, 07:58 AM   #2
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I believe high THD at high frequencies presents itself as the amplifier having a sharp and harch sound to it and sounding louder on say female voices than it actually is.

In summary unpleasant to listen to at any significant level.
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Old 25th May 2012, 08:19 AM   #3
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by Tekko View Post
I believe high THD at high frequencies presents itself as the amplifier having a sharp and harch sound to it and sounding louder on say female voices than it actually is..
Can you put numbers for quantitative measure? For example, 20% is okay, 100% is unacceptable.

I know that this depends on the bandwidth of the amp, but these numbers must have a good use...

The THD itself might not be the weakest link either...
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Old 25th May 2012, 08:34 AM   #4
PMA is offline PMA  Europe
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I am very skeptical. I have solid state and tube line level preamps with almost same distortion profile, but they sound VERY different. THD is IMO very poor measure for subjective evaluation of sound quality.

Anyway, try to avoid ANY design with distortion rising with frequency. In this case THD is an INDICATOR that something else is just going wrong.
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Old 25th May 2012, 08:35 AM   #5
PChi is offline PChi  United Kingdom
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THD is just an easy way to measure non linearity. One number doesn't give much information on the shape of the amplifier gain versus input level and frequency. So in my opinion it isn't possible to say from a THD number what is acceptable and what isn't. Though I think that the various distortions that Loudspeakers have completely swamp any low level distortion from most amplifiers though.

Non linearity at 10 kHz and 20 kHz matters because it will cause intermodulation products with music signals that have multiple frequencies.
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Old 25th May 2012, 08:45 AM   #6
PMA is offline PMA  Europe
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When I say "THD" I mean FFT spectrum. But even same FFT spectrum with amplitude and frequency (of 2 different audio components) does not result in "same" sound.
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Old 25th May 2012, 08:54 AM   #7
wahab is offline wahab  Algeria
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Quote:
Originally Posted by PMA View Post
Anyway, try to avoid ANY design with distortion rising with frequency. In this case THD is an INDICATOR that something else is just going wrong.
In practice , with NFB amps , this will amount to increase
distorsion at low frequencies to match the high frequency
distorsion level...
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Old 25th May 2012, 09:10 AM   #8
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by PMA View Post
I am very skeptical.
I'm skeptical too right now. But I believe that it is possible to interpret numbers and make it more meaningful with certain methodology.

Quote:
Originally Posted by PMA View Post
I have solid state and tube line level preamps with almost same distortion profile, but they sound VERY different. THD is IMO very poor measure for subjective evaluation of sound quality.
The THD alone might not be very useful, but coupled with other parameters, why not? I'm most interested in the phase parameter actually. But still understand a little about it.

Quote:
Originally Posted by PMA View Post
Anyway, try to avoid ANY design with distortion rising with frequency. In this case THD is an INDICATOR that something else is just going wrong.
I think any amp has this behaviour of increasing THD with frequency. The problem is how much is too much. I guess VFET will have terrible THD at above 10kH. And so are simple circuits.

Usually design is about trade-offs. If we know the "minimum acceptable" of each parameter, then it will be easier. For example, once the thd is already with 2 trailing zeros (even less), instead of lowering it further I will try to get good harmonic spectrum and phase behaviour. But noise floor is very important for good bass. My Mimesis clone has -240dB.

Quote:
Originally Posted by PMA View Post
In this case THD is an INDICATOR that something else is just going wrong.
Something like what for example? Please note that TSSA has very high THD, at least in simulation.
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Old 25th May 2012, 09:17 AM   #9
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by PChi View Post
Though I think that the various distortions that Loudspeakers have completely swamp any low level distortion from most amplifiers though.
Except those distortions related with fatigue. IMD, TIM or whatever the name...

Quote:
Originally Posted by PChi View Post
Non linearity at 10 kHz and 20 kHz matters because it will cause intermodulation products with music signals that have multiple frequencies.
But remember that there is no music above 20kHz.
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Old 25th May 2012, 09:49 AM   #10
PMA is offline PMA  Europe
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Quote:
Originally Posted by wahab View Post
In practice , with NFB amps , this will amount to increase
distorsion at low frequencies to match the high frequency
distorsion level...
Why not consider different possibilities of reduction of distortion, like distortion cancellation. I have not stated that high loop NFB would be my preferred solution
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