Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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RE: Distortion and Audibility...replay.

Posted by morricab (S) on June 19, 2008 at 11:29:34

In Reply to: RE: Distortion and Audibility...replay. posted by rick_m on June 19, 2008 at 06:22:48:

"I'm curious if you have listened to, for lack of a better description, SET amplifiers where the 'T' stands for transistor rather than tube? Having never heard an SET, I'm really trying to learn vicariously why people like what. Since too many people that should know better prefer them, they can't be dismissed out of hand"

Yes I have heard some of the Pass Labs SET (T for transistor) amps and while I think they were very good in some ways (I am talking about the older Aleph series and not then newer First Watt stuff, which I would love to hear!) I think they were also compromised by using too much negative feedback.

Now my current amps are using single ended (and single devices even!) transistors for the input and driver stage and then feeding a custom designed tube for the output (KR Audio makes their own tube designs including the manufacturing as well as the output transformers.). So there is no negative feedback in the circuit so I am getting in these amps a dosage of JFET and MOSFET Class A "signature" (although this will be a minor component compared to the output stage I think...but I could be wrong). Now FETs run Class A with no negative feedback behave roughly quadratic so one would expect only even and low order harmonics to be present (based on the modeling of such devices by Boyk and Sussman). In other words largely inaudible up to perhaps several % with real music.

So I am not necessarily against transistors per se, I am against unnecessary complexity of circuits that don't lead to greater linearity of the inherent circuit (mismatching, thermal tracking issues etc. etc.). Negative feedback is used to clean these circuits up because without it they would either be completely unstable or have quite high distortion.

You have to be careful with transformer coupled amplifiers (be it tube or transistor). If the output transformer saturates in the bass (and nearly all do from the measurements I have seen whether PP or SET) then you will get highly distorted bass (audibly so). One of the main strengths traditionally granted to SS amps is their bass tightness. This is generally attributed to the high damping factor. However; I know of at least two amps with low damping factor that have superb control over any kind of bass arrangement. The one thing these amps have in common is lack of distortion in the bass, meaning the output transformers are not saturating. This coupled with an article I read in Audioholics about damping factor, which basically shows that damping factors down to 5 are not significantly less controlled than essentially infinite damping factor.

Read this article:
http://www.audioholics.com/education/amplifier-technology/damping-factor-effects-on-system-response

For SETs this is the biggest weakness because they put the biggest demand on the output transformer. It takes a hell of an output transformer not to saturate to avoid the sloppy, slow and weak sounding bass that many SETs suffer from. But its not the circuit itself that is the problem as much as it is this necessary evil coupling device in between.

For an example of what I mean by transformer saturation look here:
http://www.soundstagenetwork.com/measurements/amplifiers/esoteric_a100/
Look at chart 3.

or here:
http://www.soundstagenetwork.com/measurements/amplifiers/hyperion_ht88/
again Chart 3.

and a SET for good measure:
http://www.soundstagemagazine.com/measurements/song_sa300_mb/
Chart 3

Here is a very good sounding SET:
http://www.soundstagemagazine.com/measurements/wyetech_topaz_572b/
Chart 3. Notice that up to about 10 watts the distortion rise is arguably below audibility at these very low frequencies.

And here is one of the champions of tube sound from Lamm:
http://www.soundstagemagazine.com/measurements/lamm_ml11/
Now look at Chart 3. Notice how flat the distortion is even right down to 20hz and also up to 20khz!! Superb and it sounds that way too.
http://www.soundstage.com/revequip/lamm_ml11.htm

"http://www.soundstagemagazine.com/measurements/manley_snapper/"
here you can see the saturation hitting hard at around 30 watts. Below that though not too bad. HF distortion doesn't look so great though.

For a final comparison of tube amps here is a good OTL:
http://www.soundstagemagazine.com/measurements/atmasphere_ma1_mkii2/
Notice also the flat distortion vs. frequency response. The Lamm is just as good even WITH an output transformer...it says a lot about that output transformer (it must cost a fortune).


"http://www.soundstagemagazine.com/measurements/monarchy_se160/"
Nice sounding hybrid amp with flat distortion at both low and high frequencies. Has a SET like distortion vs. power and harmonic structure most likely due to the fact it has zero negative feedback and only active stages in total

http://www.soundstagemagazine.com/measurements/lamm_m12_reference/
Excellent sounding hybrid. Notice modest rise in HF due to a bit of negative feedback in the cicruit. Notice also the harmonic content.



Now look at what most SS amps do:

http://www.soundstagemagazine.com/measurements/redgum_rgm175/
look at chart 3 and Chart 5. See the dramatic rise in high frequency distortion? See the picket fence of harmonics? This IMO destroys natural sound.

http://www.soundstagemagazine.com/measurements/psaudio_hca2/
Here is a Class D mess of an amp (I know I owned it once). Great for bass but sterile sounding. Note all the odd high order harmonics at high levels.

"http://www.soundstagemagazine.com/measurements/threshold_s5000e/"
Notice the worst problem in Chart 3 with HF distortion is at the lowest power! Bye Bye soundstage.

http://www.soundstagemagazine.com/measurements/simaudio_moon_w6/
Nearly Ten fold increase in distortion from 1Khz to 10kHz.


http://www.soundstagemagazine.com/measurements/anthem_statement_p2/
Like the others.

http://www.soundstagenetwork.com/measurements/amplifiers/conrad_johnson_premier_350/

This is just to illustrate that there are some fundamentally different behavior going on with these amps and tube amps (transformer coupled).

"Ultra clean performance at low levels"
For good amps yes.

"-"Built-in" peak compression"
For triode output stages apparently there is a natrual compression that offsets the increase in distortion. Increasing distortion is perceived as loudness and the compressor is working in the opposite direction (ie. making the sound relatively softer compared to the input voltage).

"Mild signal 'enhancement'"
This I don't believe at all.

"Moderate damping factor"
Not the culprit for bass response it is widely believed to be

"Natural sounding clipping"
See above regarding signal compression

"Low EMI susciptibility"
Don't know about this

"No global feedback"
Amp is not susceptible to back EMF distortion nor does it have the effect of biasing in favor (and generating) high order harmonics. IMO, very important in what is seen in the amps differences above. Note that in the case where HF distortion rises rapidly it is almost always accompanied with a sudden drop in damping factor. The negative feedback is no longer performing as it did at lower frequencies and I think that you are beginning to see (on the graph) and hear the inherent non-linearity of the underlying circuitry. This HF distortion is IMO a main culprit of SS sound.

"Easy grid drive for preceding stages"
The driver stage can make a lot of distortion in a tube amp if not done well.

"The warm nostalgic flow of the filament"
Rubbish. My Own amps you basically can't even see the filament glowing unless you look in the tiny slot.
"Watching the blue glow on the envelope"
Same as above

"Being X-Rayed by the secondary emissions"
??

" keep noodling over how good of an amplifier can be built without any global feedback"

Very well in fact.


"But you have moved on because other designs sound more like "live music". In what manner?
"

Mainly two areas, tonally and dynamically. Other amps I have had since were simply tonally more natural sounding. Also, the Threshold (and many other SS amps I have tried) sound, for lack of a better word, constipated as if the sound would burst out if you just turn it up more but it never does let loose like you expect from the live experience (unamplified that is). My current amps, even at low levels, give this alive sensation of dynamic shift that many of my previous amps did not. Maybe they were overdamped in a way.

Also, the highs are simply NOT natural with most amps and now I see from measurements a possible reason why. This I have learned negatively affects soundstage because the distortion makes the highs more prominent (remember distortion increases the perception of loudness) and pushes them more forward. High frequency attenuation is one cue for distance (the further away you are the more attenuated the highs). If the highs are perceptually pushed forward the net result is a foreshortening of the soundstage as things seem closer.

I want to point out that these comments are my thinking to date. As I observe and read more I continually refine what my position is. In other words, if someone can demonstrate to me a ultra high power, high feedback multi transistor amp that sounds as good as the best tube and hybrid amps I have heard then I will be all ears as to the explanation of why. I will also pore over the measured data with great interest. So far, I have heard the best SS amps and been left underwhelmed, although the Edge NL Reference ones were pretty good as was the darTZeel, both despite mediocre measurements but they are both doing some unusual stuff (the darTZeel has no global feedback and only six transistors per channel for example).

The amps I have found to sound the best with regard to the live concerts I have attended and recordings I have made are the ones like the Lamm ML1.1 or the Atamasphere OTL or Monarchy Audio hybrid or my own KR Audio hybrid, which measures quite similar to the Lamm.

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