what is an acceptable level of harmonic distortion?

Cristofori

Cristofori

Audioholic
What is an acceptable level of harmonic distortion for a receiver, and how important is THD in the audio chain?
 
M

markw

Audioholic Overlord
Given the levels present in modern receivers I'd say it's a lot more than they are capable of producing, assuming you don't overdrive them.

Odds are you'll never hear it unless you overdrive the amp beyond all logic and reason.

If you really want to get anal, check out some of the harmonic distortion figures in speakers. Compared to that of electronics, they'll make the hairs on the back of your neck stand up.
 
Alex2507

Alex2507

Audioholic Slumlord
... check out some of the harmonic distortion figures in speakers.
You know, I've never run across those numbers ... anywhere. I understand it can be up to 10% in subwoofers without them sounding terrible and up to 5% on regular speakers.
 
highfigh

highfigh

Audioholic Slumlord
Play a vinyl recording and add at least 1.5% to the THD of the speakers. Human hearing doesn't distinguish harmonics as distortion until the percentage is fairly substantial. 1% means the added harmonic is -20dB and that would be masked by the music at most usable levels.
 
TLS Guy

TLS Guy

Audioholic Slumlord
Given the levels present in modern receivers I'd say it's a lot more than they are capable of producing, assuming you don't overdrive them.

Odds are you'll never hear it unless you overdrive the amp beyond all logic and reason.

If you really want to get anal, check out some of the harmonic distortion figures in speakers. Compared to that of electronics, they'll make the hairs on the back of your neck stand up.
Peter Walker when he did the double blind test with "Golden Ears" who denied that solid state amps sounded ad good as tubes, did something naughty the Golden Eared crowd never forgave him for.

He used his fine Quad II tube amps, the solid sate 303 and the solid state 405. He then told them he had a new experimental amp. Peter had resigned the amp to add 2% THD across the board. The Golden Ears doubled blinded could not distinguish any of the amps.

In Kessler's book the Closest Approach, he is asked why bother to make amps with 0.1% or less THD. He said, because it does not cost any more to make an amp with 0.1% THD, and he could not sell an amp with 2% THD!

Having said that Peter added linear harmonic distortion. There is however a non linear distortion in solid state call B amps, called crossover distortion, this is minimized by biasing to class AB. However the cooler the amp runs, the more it is biased to class B and crossover distortion will be higher. This form if distortion is not measured on a lot of distortion analyzers, but easily seen on the scope. The ear is very intolerant of this type of distortion. In good designs it is minimized. It is interesting though, that Peter conceived his ingenious current dumping amplifiers to totally eliminate this type of distortion. The only solid state amps that can make this claim are the class A amps, that have a lot of other issues, the Quad 909 and some amps of Nelson Pass, and he has almost certainly infringed Peter's patents in these designs.
 
BMXTRIX

BMXTRIX

Audioholic Warlord
All amps add a bit of distortion, but when you are reading power measurements, which is the most common way for manufacturers to give their power claims they will say: 75 watts @ x%THD where x is your THD level for that power.

If you look here:
http://hometheatermag.com/receivers/denon_avr-4810ci_av_receiver/index4.html

You will see a receiver like the Denon AVR-4310CI measures between .001% and about 2% distortion levels on the test bench with two speakers driven between 1 and over 175 watts.

So, what is the THD? What is the actual power?

The reality is that you don't notice THD unless it is pretty high, but the manufacturers will use ridiculous numbers (10% sometimes) THD to increase their claimed wattage and will make those claims based upon one channel driven. A quality manufacturer typically makes the claim based upon two channels driven.

Very few do all channels driven THD tests, but Home Theater Magazine does and publishes the results - but at the end of the day, the test is not relevant to normal listening... typically.

The ohm load on the receiver also matters with the test - typically it will be done at 8ohms, though some will make claims based upon 4 ohms, or even 2 ohm loads.

Depending on the context where you are trying to get THD levels from, and why it matters, is when this all comes into play.

A receiver claiming 120 watts per channel, with 10%THD is likely sitting on top of a piece of junk receiver which could hurt your speakers if you try to drive them loud. A quality manufacturer, like Denon, Pioneer, or Yamaha will prevent more than a couple of percent of distortion before shutting down the receiver to help protect your speakers.

The most common way to damage speakers is not by overdriving them with to powerful an amp, but turning up a weak amp way to much and under driving the speakers.
 
Cristofori

Cristofori

Audioholic
The reality is that you don't notice THD unless it is pretty high, but the manufacturers will use ridiculous numbers (10% sometimes) THD to increase their claimed wattage and will make those claims based upon one channel driven. A quality manufacturer typically makes the claim based upon two channels driven.
Isn't the higher number of THD worse? Why would a manufacturer want to advertise a worse THD rating?

I'm guessing here that some less than honest manufacturers know that the higher number of THD will be ignored by most people who don't know any better, and who are only concerned with wattage/power?

But I might think that playing the old "wattage game" wouldn't matter so much anymore today with the indifferent, clueless ipod toting masses, unless it involves car audio. It would only matter to people like us, and we know that wattage isn't the whole story.
 

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