hifi engine

Total Harmonic Distortion

Need a little information, looking at a used Hafler amp 350 watts/side. I noticed the specs listed the THD as .1%. I am used to seeing .01-.03% numbers. Why does a quality product have 10X the THD I am used to seeing on other amps? Is the relatively high power output responsible for these numbers?

Re: Total Harmonic Distortion

Could you list a few Power Amplifiers that claim 0.01% THD under FTC Method specifications? Are you positive you are not misplacing the decimal point, such as:

.1% and 0.1% are the same; 0.1% and .01% there is a 10x difference.

Assuming there is no error, then there is a further problem:

There is no way to determine sound quality from a simple THD figure. A complete Distortion profile may offer some clues as to sound quality, but even then care should be taken not to read too much into the value specified.

You can take a given amp circuit, increase the level of Global Negative Feedback, and the result of that change may reduce the overall THD spec tenfold but at the same time adding GNF will increase high order and odd order harmonic distortion which tends to degrade sound quality.

Through overuse of Global Negative Feedback, it is trivial to build a bad-sounding amplifier with an impressive THD specification. In fact, some manufacturers, who are well aware that many consumers rely on a strict comparison of numbers, do it deliberately as standard practice.

Don't sweat it, the Hafler amps are clean enough to compete with their peers.

Finally you should be choosing your components through listening to them. After all, playing music is the job they should have been designed to do, not generating numbers in a lab setting.

Re: Total Harmonic Distortion

You need to also look at how the distortion is measured and under what conditions. If that .1% is at full power into the lowest resistance the amplifier can handle, then the specs may be impressive. If a manufacturer claims his amp will put out 400 watts, but distortion is measured at 200 watts, then full power distortion may look terrible. You need to take into account the frequency of the test signal also; most companies do not test distortion from 20Hz to 20kHz (at least they don't publicize it).
BTW, most people cannot hear .1% distortion.

Re: Total Harmonic Distortion

2-Channel component audio amplifiers sold in the United States are required by law to quote a detailed specification using the "FTC Method" in a font that is equal or larger than any other specification quoted.

The Federal Trade Commission passed into law the requirement in the 1970's. Everybody complied until the home AV market arrived; manufacturers of 3+ channel amps argued the FTC law does not apply to multichannel AV gear.

So your cautions are valid for Home Theatre receivers but not to stereo amplifiers*

After a 1-hour precondition at 50% power, the power amp section is tested at specified wall outlet voltage (eg 120V) and measured under the following:

x watts per channel, both channels driven with a 20~20,000 Hz signal into x ohms at no less than x% THD, for a certain amount of time without problems.

If the amplifier's protection circuit kicks in, then the manufacturer cannot specify any value into that impedance. The safety compliance is measured at the same time; if the device can't operate (perhaps it overheats and shuts down) at the specified power and load, you cannot claim it can output that level of power, even if it can exceed that level for short bursts.

The Hafler amp is covered under the FTC Rule and therefore the power spec should be considered reliable.

* eBay amps sourced outside the USA (typically China) are not covered by the FTC Rule, since they are not actually sold in the USA, instead the eBay Buyer is the importer (and the importer is responsible for measuring the device).