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What is the importance of THD?

post #1 of 13
Thread Starter 
I have a friend and fellow audiophile that thinks THD is the most important thing in a system. Is that true? I can't imagine that it is the tell all story for amps and preamps. I hope someone can help me.
post #2 of 13
No, I think THC is the most important thing in your system
post #3 of 13
THD is like... SPL, or Freq response....

It doesn't mean much... as long as it is <1% you are fine, and usually they are like <0.05%. Not like it makes a difference to your ears anyway.
post #4 of 13
Thread Starter 
Yeah, THD as in Total Harmonic Distortion and that is what I thought. So, when looking for a preamp or an amp, what should I look for statwise?
post #5 of 13
The only thing is that there are a ton of mini-systems...i think my mom will be getting one soon. Some have THD's of <1%, others, <10%. The latter will sound CRAPPY!

Stating the obvious,

- vij
post #6 of 13
i remember reading somewhere that low THD used to be a big selling point. amp designers would use lots of negative feedback to lower THD to obscene levels, but introduced other problems (TIM and such). a low THD doesn't tell much about an amp.
post #7 of 13
Thread Starter 
This is good info. Duncan, what is TIM? I need to make sure that I have plenty of info so I can more convincingly shoot his arguement about the joy of THD down.
post #8 of 13
THD+N as a measurement is basically useless. THD+N vs. frequency is only slight less useless. The reason is that it is basically impossible to predict, aside from really pathological cases (super high THD+N or incredibly low THD+N), the audible properties of a box from THD+N measurements because the same THD+N number can be derived from very audible phenomena as well as very inaudible phenomena. By crunching all the measurements into a single-number, you're throwing away all the information that may tell you anything about audibility.

THD+N is called THD+N because noise is indistinguishable from distortion in the measurement. This should already tell you something about how bad it is.

--Andre
post #9 of 13
sackley: TIM is transient intermodulation, another kind of distortion spec that describes how clean the signal runs through the zero line (ground level). It has something to do with the linearity of a transitor's amplification - that's where class A operation comes into play: If you add some additional - uhm... what's the English equivalent for Ruhestrom? - constant current, you lift the transistor into the linear part of its amplification curve, but thereby you sacrifice quite a lot of power. Some amps can be switched from regular class B into class A operation - the Marantz PM8000 for example. This can be helpful, if you want to use your amp for both personal Hi-Fi use and as PA-workhorse for parties....

Nevertheless, it's yet another spec: If you don't exactly know how each manufacturer measures, it will only serve as a hint - quite similar to S/N-ratio, THD, wattage... Nowadays even cheap amps have very good specs (on paper, at least), so the specs won't tell you a lot, anymore. Usually I find it more revealing to have a look inside the amp, because the types and dimensions of parts as well as the board layout and cabling can tell you quite a lot if you have a little experience. So better trust your ears - and maybe your eyes, too.

Greetings from Munich!

Manfred / lini
post #10 of 13
I agree that giving a THD as a single figure is meaningless.
What is needed is a total harmonic distribution function.
This is for 2 reasons:
First we are more sensitive to higher order HD and second we perceive even and odd harmonics differently. A classical example is an older tube amp design that can put up to 1% THD without sounding harsh or distorted. Thats because most of it is a second HD. Some SS designs have much lower THD but it is maynly third harmonic and thus perceived as dry and harsh sound.
There is quite some psychoacustics involved here.
post #11 of 13
THD levels also come into play when discerning the power output of an amplifier. Power output figures should be accompanied by a THD number as well. Some boomboxes and car amps fudge their power output figures by quoting them at 10% THD instead of the standard <1% THD. A small car amp for instance claiming a power output rating of 50 watts or 100 watts, is very likely quoting at the 10 % THD level. This means that the amp is really only approximately only 5 or 10 watts at less than 1% THD.
post #12 of 13
Oddness and evenness of distortion really doesn't predict audibility. 6th order distortion sounds pretty darn bad, compared to 3rd order. What's important is comparing the distortion spectrum against human masking thresholds, as that will directly tell you if something is audible or not. 2nd order is hard to hear not because of its inherent evenness, but because it's close enough to the main signal, in frequency, to be masked by the main frequency.

--Andre
post #13 of 13
AndreYew.
You're right that higher order harmonics are easier to hear (as i also said) but I don't think 2nd order is inaudiable due to band overlap. perhaps at very low frequency but at higher freq. the2 nd harmonics are far appart. i guess it's a psychoacustics issue.
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