Gear Heads, Techs Help Please

Bass-4-God · #1 05-04-2010, 12:18 PM

An amps specs read 1000 watts @4 ohms, 5%THD, If I remember correctly the THD stands for Total Harmonic Distortion. It has something to do with the percentage of power you can actually get from the head before it starts to distort. What I do not remember is what is considered a good number or not. Correct me if I'm wrong and help me out on what is a good ratio percentage.

JimiLL · #2 05-04-2010, 12:52 PM

Thats prob. more like .05% THD.... 5% is crazy

Rick Auricchio · #3 05-04-2010, 01:00 PM

No, 5% is a common snake-oil spec. But it will sound like a fuzztone at that level of distortion.

If indeed the spec is five percent, then the amp can more likely push 200w at decent distortion levels.

megadan · #4 05-04-2010, 01:00 PM

In other words, the amp only puts out 1000W when it is heavily into clipping - 5%.
So in reality this amp is likely to put out far, far less than 1000W.

In any other world we call that lying.

Bass-4-God · #5 05-04-2010, 01:52 PM

That is actually the spec that is posted for the new Ampeg SVT7 Pro 1000 watts @4 ohms, 5%THD. I found it own their web . So are you saying that the head won't do what it is advertising or that 5% is no a good ratio.

throbbinnut · #6 05-04-2010, 01:59 PM

It's got a preamp tube, and it's Class D. 5% THD at 1000 Watts doesn't scare me all that bad. It may be good-sounding distortion. But it probably does 1000 Watts no problem and your speakers will be adding in more distortion than that 5% at 1000 Watts of power. Just my guesses.

PSPookie · #7 05-04-2010, 02:01 PM

I thought 5% THD was a pretty typical spec for an all-tube amp.

Ampeg SVT CL 300W @ 3% THD
Peavey VB-2 225W @ 5% THD

slyjoe · #8 05-04-2010, 02:08 PM

Quote:

Originally Posted by megadan

In other words, the amp only puts out 1000W when it is heavily into clipping - 5%.
So in reality this amp is likely to put out far, far less than 1000W.

THD isn't the same as clipping. It's a measure of how many harmonics of a fundamental tone input are in the output. It needs to specify the amount at each frequency to be meaningful. You can't hear less than about 1%.

Quote:

In any other world we call that lying.

May be, but not for the reason above.

Bass-4-God · #9 05-04-2010, 02:09 PM

So if I want clean tranparent sound this is probally not the head that I want.

slyjoe · #10 05-04-2010, 02:20 PM

Look into a separate power amp (QSC, Crown, etc) - they have very low THD (on the order of 0.05 to 0.1%).

BassmanPaul · #11 05-04-2010, 02:45 PM

When I was a young teenager my dad bought himself a Leak HiFi system. As this was before stereo it was mono. The THD of the entire amplifier was 0.1%. That should tell you something as I'm sixty four at the moment. 5% THD for a SS amp is terrible.

Paul

DukeLeJeune · #12 05-04-2010, 02:49 PM

Apologies in advance for the geekspeak.

Total harmonic distortion (THD) numbers do not correlate well with subjective perception. 30% second-harmonic distortion has been shown to be statistically undetectable, and higher levels of second-order harmonic distortion are detectable but subjectively benign. On the other hand, very low percentages of high-order distortion (particularly odd-order distortion) are quite audible and objectionable. And it depends on where in the waveform the distortion occurs; distortion at the peaks is less audible than distortion as the waveform crosses the zero point ("crossover distortion"). This has to do with a generally under-appreciated perceptual phenomenon called "masking", which is beyond the scope of this post.

I am friends with a pair of researchers (Earl Geddes and Lydia Lee) who published a couple of in-depth papers on distortion perception in the Journal of the Audio Engineering Society, and one of them made the comment to me that their research actually showed a slight negative correlation between low THD and listener preference! In other words, an amp with a higher THD spec may well sound better than an amp with a low THD spec, and this is because the techniques that give rise to low THD numbers tend to push the THD envelope in a direction that is of greater audible consequence.

In my opinion the amplifier industry is measuring with the wrong yardstick when they use THD, but there really is no other yardstick that is generally accepted. As a result, those designers who are really trying to design for ears instead of for test instruments are not rewarded with impressive-looking specifications.

This is not a specific comment on the Ampeg amp in question, which I know nothing about, but a high THD spec in and of itself is not a bad thing in my opinion, and may well be an indication of design choices that place perceptual considerations first. Unfortunately it could also be an indication that they're driving the amp into clipping in order to boost the wattage rating.