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Can of worms

Posted by MAD MAN HARJU on December 30, 2004 at 08:27:17

In Reply to: Re: 3rd harmonic distortion posted by Soundmind on December 30, 2004 at 00:34:29:

Do you really want to open that can of worms?
But since you have done it I’m going to answer this as good as I can.

To do this I need to explain my “Design philosophy” and some history. I am an electric engineer and have been fiddling around with electronics and loudspeakers since I was 11 years old I built my first own loudspeakers and amplifiers at the age of 16, so I’ve been learning this for over 25 years and are still in the process of learning.

I also used to think that negative feedback was the best thing since sliced bread, but for me the big eye-opener was when I listened to the Monster sometime around 1988. The Monster is a low feedback (around 20 dB) and low power (8 Watt) class-A amplifier design by Jean Hiraga with a very simple design (Only 8 transistors). I know that Hiraga have made some odd statements (It might be translation errors) but he can design well sounding amplifiers.

At this specific time I had the opportunity to compare the Monster with an class-B amplifier of similarly the same size (25 Watt) but with high feedback and a “normal” power supply AND better measurements at full power (I can’t remember the name of it). The other amplifier sounded grainy and metallic in comparison to the Monster witch had a pleasant sound (And don’t come and talk about euphoric distortion or something like that. It was just more true to the original.) The loudspeakers used at that time had sensitivity around 95 dB/W at one metre, and we were putting out less than 1 Watt on average at our listening levels. And actually the Monster had little lower levels of distortion AND of only lower harmonics (2:nd and 3:rd) that where measurable compared to the other amplifier witch had harmonics extending up to the 20:th visible above the noise-floor. (The noise-floor was approximately similar for both amplifiers) This got me to start thinking that high order harmonics are BAD and need to be as low as possible, because they mess up things even at very low levels.

To me the single most important thing in an audio reproduction chain is the source (i.e. the record) followed by the room, loudspeakers, record-player, power-amplifier and the pre-amplifier. In that order! Witch bring us to your question/statement:

“when the signal has gone through dozens of stages of amplification with negative feedback in the tape decks and recording and mixdown consoles not to mention the distortion of tape heads, magnetic tape itself, and phonograph cartridges? “

There are a BIG difference between good and bad recordings, for instance some of Opus-3’s best recordings are made (If I remember correct) with only 3 microphones (A stereo pair and one for the ambience) one mic-amplifier (with no global feedback) per mic and one mix-stage (with no global feedback) before the mastertaperecorder. That’s only two amplifier stages in total and no global feedback before the recording…

But even if you have a big recording console with a lot of stages that can and will introduce distortion, the recording engineer LISTENS to the result at mixdown and tries to eliminate the distortion or compensate for it in the best possible way. The master-recording will have some distortion, sometimes intentional but mostly just something we have to live with but try to eliminate. This recording is what we have to try to be faithful to in the reproduction chain forward on.

I have personally experienced that the same mastertape, but processed at two different pressing-plants (Yes I still prefer vinyl) can give totally different results. And I know that it is the same mixdown. The “bad” pressing is fairly decent but the “good” pressing have much better S/N ratio better dynamics and give me “goosebumps”. BUT I could not hear this until I changed to my present amplifier. (The distortion is aprox the same as the earlier amplifier, but with only local feedback in my present amplifier.)

The distortion in tape heads and magnetic tape rises with signal. That means that with small signals the distortion is also small. And music spends most of it’s time at signals close to zero. I recommend all to read: "Multitone Testing of Sound System Components-Some Results and Conclusions" JAES Vol 49 No 11 2001 November.
Also you could check out “Transient Intermodulation Distortion and Measurment” by Gerald Stanley AES Preprint # 1308, with contains some probability density functions of music and voice. This should not be news to anybody here, and the result is that Class-B amplifiers and Class-AB have their worst distortion performance where the music signal spends most of its time.

I’m currently working on my thesis on how our hearing works with emphasis on the ability to hear distortion, and how we should design our amplifiers and pre-amps according to this, so I have given this subject some thought. As a part of the thesis I’m designing pre-amp based on tubes and have done some measurements on the prototype: At 1Volt rms out 2:nd harmonic is at –88 dB and nothing else above the noisefloor at –94dB of my present measurement-rig. (But I have to modify it because my new modified and hopefully better prototype will be hard to measure otherwise.)
At 5 Volts rms out the 2:nd is at –78dB and the 3:rd –82dB.
The noisefloor of the pre-amp itself is at –120dB

I have compared the prototype against my current pre-amp of mid-fi design and there is a difference, but not any big one. But the prototype is better and not just in my ears. I did some measurements on my current pre-amp and I could se some peaks of harmonics up to the 6:th above the noisefloor of –94dB. Hmmmmm……..

So the distortion obviously matters.

And to all of you who think that we can measure everything and that the only thing we can do to improve is to get higher resolution (i.e. lowering the noisefloor) I recommend again to read “Multitone Testing…” and think about it. There is a big possibility that we are measuring the wrong thing.

BTW. John I got the papers. Thanks

Enjoy Creating / Mad Man Harju

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