THD Total Harmonic Delusion?

[nitrate]

Once more i find my self tredding along the long path of audio engineering when anougher set of awkard questions are found buzzing about inside my head that demand attention from the masses.

First the question :-

Does vanishingly small amounts of THD in an amplifier GUARANTEE good sound and fine music reproduction?

Explanation :-

I've been developing a classA amplifier for a while and its been on the test bench for far too long now. The problem is i want it to be my best work to date but i'm convincing myself that it dosn't sound that good. A few weeks ago after the project stalled i thought its about time i did some serious research so i started reading loads of papers about the subject of distortion and its effects. I also bought a copy of Selfs 'Audio Power Amplifier Design Handbook'. After reading the book cover to cover i decided to apply some of Selfs methods for reducing THD across the entire amplifier design. After doing this on the sim my amp went from approx 0.065% / 0.016 THD 50/1 Watt to 0.002 / unmeasurable ( < 0.000 ). I was impressed by this figure ( Well done Self ) and so spent this easter mirroring the improvments in the real world on the test bench amp. Upon power up i started to make measurments. Everything seemed good, no measurable THD on the computer via RMMA ( crap soundcard but it said the previous attempt was not that good lol ), good squares and sines on the scope with no noise, 20v/us rise and fall etc. I am convinced there has been a dramatic improvment just as the sim said there would be. I then swapped test gear for audio gear and had a listen. The thing is, i expected the sound to be crystal clear, much improved on the last time i listened but i was dumbfounded to hear it had hardly changed ( if at all ). Hence the question above, if an amplifier has very low THD then why does it not sound as good as amplifiers i have thrown together in the past that use voltage regulators as current sources and have relativly high THD's? Am i imagining this? Are there other hidden distortion's / imperfections that can have a large impact on the perception of sound?? I'm confused but these questions need answers if i'm to carry on my journey into sound.

Thanks for reading
Leigh

[janneman]

Hi Leigh,


I think most people would agree that the correlation between THD and actual sound quality is very loose at best. Surely, if you have loads of THD, the sound reproduction will suffer, but the opposite is not necessarily true.

Some things that come to mind: If you say it doesn't sound good enough, compared with what? Are you swapping amps and feeling the quality goes down? If so, what amp is that?

What kind of speakers do you use? How's your amps damping factor? Have you tested the amp for oscillatory tendencies with complex (speaker-type) loads? What's the speaker min impedance, and what's the amps max undistorted output (volts, watts?)

Just some thoughts.

Jan Didden

[nitrate]

Hi,

I'm pretty sure the amp is stable, i've done extensive testing with capacitors, very low impedencies and even blown a few car headlight bulbs with the amp ( i shouldnt i know ). No tendencies for any oscillation at all. I'm comparing the amp to other home brew amps i've done and conclude that some of my other rush jobs building with only junk parts sound better, even though they are quite primitive. This is the first time i've paid any real attention to quality of sound so i'm rather frustrated to find how hard it is to make any kind of large impact in this area. The amp in question slews at around 20v/us and is capable of 60V Pk-Pk before clipping the rails. Mode of operation is push pull class A. Intended power is approx 50WRMS into 8/ 100 into 4, Full classA up to at least 50. This is not about the amp tho... its about the theroy that less means more in THD terms.

Regards
Leigh

[janneman]

Originally posted by nitrate
[snip] This is not about the amp tho... its about the theroy that less means more in THD terms.

Regards
Leigh

OK, I thought this was about why your new amp sounds 'bad'...

Maybe you have just proven (again) that THD isn't the whole story. But there are so many other factors that really start to dominate how an amp sounds once the THD is reasonbly small. As I mentioned, a relative low power amp with inefficient/low impedance speakers may clip more than you think. If your other amp has a lot more power, it may sound better even if its THD is higher.

Then there's the whole field of power supply and power supply ground loops that can have a *large* impact.

Can you describe the 'badness' of the sound?
Jan Didden

[nitrate]

I'll have a go... I cannot realy explaine why the sound is worse other than it seems like there is a veil over the sound obscuring the detail. I mean the detail is there and everything is clear but theres somthing thats causing fatigue when i listen. I feel forced to listen hard for all those subtle details. They aren't missing, they just dont leap out. Silly i know but thats as close as i can get to describing why i THINK the sound is wrong. Maybe i'm just convincing myself its bad when it isn't but like i say, i AM convinced theres somthing not quite clear. Seems to me the more feedback i use the worse this phenomenon gets. The comparrison amps in question are all lower power BTW

Leigh

[pinkmouse]

Perceptions.

When I first used the Audax HD3P tweeter, I thought it was dull and not very enthusiastic. As I listened to it more, and did some very basic measurements, I realised what I was used to was the relatively higher distortions in the metal dome tweeters I had been using. My ears had been fooling me.

[jcx]

its hard to convince people of the importance of level matching in listening tests

SPL has to be very, very closely matched or you Will hear differences - and at <1 dB level matching the differences will not be perceived as loudness differences, most commonly the actually louder output will sound "better" - even if you are listening to the same circuit between the 2 tests

http://www.provide.net/~djcarlst/abx_crit.htm

if you are serious about gauging your amplifiers by subjective listening tests it could be a good idea to understand what generations of audio researchers have determined about how to turn subjective impressions into usable data - level matched, blinded listening, controls, with comparisons made within our short term memory time horizon

[kevinkr]

Tell us a bit more about the differences in topologies and complexity of the "blameless" amplifier you are working on and the ones which you are using for comparison.

I went through this phase early in my hifi design odyssey and discovered to my chagrin that the more complex and better measuring the topology the less likely it was to sound good to my ear. Simple direct circuit paths are often the best.

Take a look at what Nelson Pass or John Curl do, their stuff is transparent, and good sounding even to this tub-o-phile.

Perceptions may also play quite a large role here, something that sounds radically different from what you are accustomed to may not at first be recognized as an improvement.

Incidentally I found my nirvana by stepping back into the past. Simple vacuum tube circuits did it for me.

Edit: I meant to comment on the previous poster's comments about level matching. I agree totally. Critically important..

[andy_c]

Leigh,

If you look at the spectral display of the amp's output in RMAA with a sine wave input, do you see any frequency components separated from the main one by 100 Hz? These won't show up in a THD measurement but are distortion nonetheless.

If you do see that, and it's not a grounding quirk of your measurement setup, it can indicate the need for more aggressive filtering of the power supply to the amp's front end.

[Pan]

Kevin,

Take a look at what Nelson Pass or John Curl do, their stuff is transparent, and good sounding even to this tub-o-phile.

How did you come to the conclusion that Pass and Curl amps (and which of them) are transparent?


/Peter