"at 0.08 THD"?

[MattG]

When a spec sheet on an A/V Reciever gives a power rating such as "110 watts x 7 into 8 ohms at 0.08% THD" what does the "0.08% THD" mean? How much difference does a 0.03% change make?

This message has been edited by MattG on 08-29-2001 at 05:06 PM

[WMcD]

THD is Total Harmonic Distortion. So you've got some small level of distortion.

This assumes you are putting a sine wave (one pure tone) into the amp. However, at high levels of output, typically the rated level of 110 watts for you, the waveform is distorted in form and clips, or flat tops. This leads to the generations of tones which are harmonics. Harmonics are multiples of the frequency of the input. Just like going up an octave on a musical scale.

You're really getting into a deep question. What is a "figure of merit" for an amp? Is there any single or collection of measurable results, like THD, which will tell us whether it "sounds good." Answer: For the most part, no.

I wouldn't put too much stock in THD at the rated output for a simple reason: We're not operating amps at anywhere near their max level of 110 watts. Particularly with high efficency Klipsch speakers. Rather, we are running amps at a fraction of a watt. (Further, if THD is 0.08 at 110 watts, its is likely to be 0.03 at 100 watts.)

I've found little or no published information from amp manufacturers about performance at the milliwatt level. Yet this is where we're doing our listening.

I have a suspicion that some high power amps have defects when played at very low levels. Therefore it makes some sense to use low power amps. The inference is that they too may have defects when driven at low levels, but everything is scaled down. I.e. now the defects are below our listening level.

Related to this is that amps operating in Class A are going to do well at low levels. Nelson Pass is an advocate of Class A operation and simplified design. His amps consistently get excellent reviews, even from the Stereophile crew, which are otherwise too subjective, wacky, and arbitrary, for my liking.

Bottom line. . . don't pay much attention to THD. These numbers are not figures of merit in actual application.

Regards

Gil

This message has been edited by William F. Gil McDermott on 08-29-2001 at 08:46 PM

[MattG]

Thanks Gil.

I have not yet purchased a receiver, and in fact, probably won't for months if not a year or two, just doing my homework.

I don't think it is wise to spend $3000-4000 on an upper end receiver and speakers with out having heard them and understand all of its functions and ratings.

I am trying to find out the differences between the Onkyo TX-DS898 and the Denon 3802. This THD was one of the diffences, but if that is no big deal then the choice becomes harder because the rest of the main functions are close. One of these days I will have to get out there and listen to these two units side by side.

The amount of info posted on this sight is wonderful! Keep it coming guys!

[Colin]

The audio industry is not as smart as the food industry. With food we measure different kinds of fat as if the parts are less than the sum total of all kinds of fat in food (it isn't). But in audio, we measure Total Harmonic Distortion (THD) on power amps as if it was all the same type of distortion (it isn't either).

Solid-state amps usually do really well here, sporting THD figures of 0.1% or less. Tube amps are belittled by engineers for their high THD figures of 1% or more. But the level of total distortion that is tolerable on SS amps is not even unpleasant on tube ones. A 5% THD on the SS amp is horrible, for example, but the tube amp might actually sound better than the SS amp with that same level. Especially with super sensitive horns.

Yet amplifiers of all shapes and sizes are measured with for total harmonic distortion (THD) specification. It is curious, however, that THD is rarely quoted for loudspeakers. You certainly wont find that sort of measurement in Stereophile Magazine.

The reason is that the figures are pretty bad. There are many speakers out there that routinely exceed 5% THD or even 10% when pushed hard. Thats one of the reasons so many tweaking audiophiles are drawn to big old horns. They dont sound grainy, rough, or harsh when going from soft to very loud. They exhibit an amazing lack of distortion, a magical musical quality and a pure tone that sounds awfuly close to the actual instruments.

I would give both units a long side by side comparison - it is time to play stereo!

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