Can a very simple class A amp produce great sound?

[Bigun]

Originally Posted by Plecto

[...]Would this then have a distortion of way under 1% and it would be good to go as a high-end amplifier with as good of a sound as you can ever get from an amp?

It's a subjective thing, people like different amps. But again, I would suggest reading about the SEWA amplifier as it has the kind of output you are interested in.

SEWA - Seven Watt Amplifier

go down to post 163:

Originally Posted by Tyimo

[...]The serious amps:
-Hiraga Le Monster: very nice and strong sound, but nothing special, if not that: You can't believe how can this 8 Watt amp to be so strong!!
-JLH1969: this is realy a very good sounding amp! A refference amp for ever. Very musical sound, one of the best.
-ZEN "classic": nice and very pleasant sound for the ears, but the bass, dynamic, details, soundstage imagineing not so perfect than the ....... see later
-ZEN ZV4: nice and very strong and dynamic sound, but also not perfect like the......
-ZEN BALANCED 25Watt: nice and brutal strong dynamic and bass etc. sound! A bit "dark" or gloomy sound. I am afraid I prefer the 2nd harmonic distorsions more....
-Zen Lite: It is a realy nice sounding amp, almost perfect, but the details and the sound stage imagineing and the bass control are not so fine than the....... see later
-Aleph5: very nice tube like sound with brutal power and dynamic.
-SEWA V1: O.K. here comes the winner! Nice, soft, warm etc. sound, but the most important what makes this amp unambiguous better than the others are the very strong and realy(!) detailed, controled bass, mega-brutal dynamic, incredible fine micro details and excellent 3D sound stage. The music simply alive in the listening room!!!
I have never listened or felt such a good presence feeling!

[Plecto]

About an op-amp as an input stage. If the feedback is taken from the output of the amp, won't then the op-amp try to compensate for some of the distortion then?

I'm hoping for an oscilloscope for christmas so I can finally start looking at the output of my amps and getting some THD values If I make an amp like this on a breadboard, will measuring distortion give an accurate value? Or do I have to solder it in order to get valid measurements?

[Fusion916]

Originally Posted by AndrewT

If you try to drive an ordinary 8ohms speaker at -20dB (average level ref maximum power) to avoid clipping then you will just about have to clamp the speaker to your ear to hear it. Am I allowed to exaggerate slightly?

I just spit up my drink rofl

[VladimirK]

Originally Posted by Jay

VladimirK, a DIYA member, have had access to Russian mosfets with better specs. But still, imo one stage is not optimum for average power requirement.
I have never thought an output cap as a bottleneck. Even if it creates distortion, it is of non offending type.

Hello, Jay
I think I should add some comments. In this discussion, the problem is that main terms are not well defined. For me,
- "tuby sound" is simply a sound without "harshness", it is very frequently very far from good sound, because of other sound properties. It can be obtained with SS or tube designs.
- "macro sound picture" is the domain where THD could play some role, with very high THD timbres could start to deviate from proper ones, but that must be really high THD, more than 1%. With many simple designs one can easily stay below 1%.
- "micro sound picture" is the domain where all designs differs most of all, and this sound property has almost no relation with THD. It seems to be more related with phase intermodulation, a kind of "jitter" caused by variation of gate-drain, gate-source, base-collector, base-emitter capacitances with voltages. Tube schematics have some natural benefits in this respect. Apart from "jitter", "micro sound picture" suffers from many other reasons, like PS hi-frequency spikes achieving main schematics, effects of capacitors, resistors, contacts, PCB design and electro-magnetic coupling, etc. Many factor tend to destroy "micro sound picture", but there is no THD among them.

Speaking about simple single stage designs, it is quite obvious that they have almost no reason to produce "harsh" sound, so it will be tuby in that or another way.
However, since they have no measures included against phase intermodulation, I find them all loosy at the "micro sound domain", very far worse than good quality tube amps.
I did attempts to suppress somehow the "jitter", by using special HF LDMOS transistors at power follower, with definite succsess. With proper output "cap", it is surely competitive with best tube amps. However, one is forced to use preamp in addition to follower, and any preamp I tried to use with such an advanced follower, made the combination preamp-advanced follower worse in "micro picture" than good integrated tube amp. I speak about listenings of high quality system, that allows to distinguish tiny differences.

For those who are looking for really top sound quality, integrated amp solution, but with external power supply, is preferable.
So, one stage amp is not a real amp, it is a "semi-amp" and can not determine final sound quality.

Many people put on the top three reqirements, and believe they go in correct way
1) output power
2) low THD
3) no output cap
From my point of view, for audiophile home listening, these requirements are terribly misleading and add enormous difficulties in achiving best sound. Good sound coud be obtained even with these postulates, but with terrible difficulties, bringing costs of a good sounding amp to 20000...50000 USD range.

[VladimirK]

As per my observation, "combined" output cap in NoGNFB or low GNFB designs adds less problems to sound (I do not say distortions, since they usually assosiated with THD) than caps at low-signal positions. Sound effects of "combined output cap" is comparable to effects of a relay contact. Due to this, I stay for a long time with SE class A schematics. Within this approach, I obtain some simplification of PS, no need in servo, no speaker protection, no crossover distortions, no deep GNFB, no sound "harshness". However, good "micro sound picture" is not an inherent benefit of simple class A, it must be achieved by careful selection of low Ciss jFETs, very high Ft output transistors, current type and not deep GNFB.

[VladimirK]

On the top of all said above, I should mention also a "bass signature" of the kind of output transistors used. All the good properties of lateral FETs and LDMOS (very high "effective" Ft) do not make them free from some specific signature at bass domain. There is no chance to remove this signature by deep GNFB and very low output impedans (high damping factor). Maybe this will not be the case with specific high efficiency low-power-tube dedicated speakers, but with typical 3-band speakers one could easy recognize bass features, depending on what kind of transistors is used at the output.
After many comparisons and weighing all pro and contra, now I prefer BJTs for HF applications, with Ft = 300...700 MHz.
SIT transistors are a bit another story, they are also very attractive.

[Plecto]

Thank you for the replys, I have a better understanding now

I haven't learned about fourier analysis and calculating THD values yet so I don't know the math behind it, but are you saying that "tuby sound" and "micro sound picture" can't be expressed mathematically? I found it really attractive to think of a distortion meter that could directly and mathematically compare one amp to another

A little off topic here. Could any of you link a class AB design that is allmost as simple as the class A design I linked in the opening post? An op-amp input stage would be preferable and I really like mosfets. As much as I found class A design fancy, the power dissipation and extra demand on the transformer isn't that appealing I was thinking of looking into class AB designs.

[AndrewT]

an opamp with a pair of "helper" transistors will do your ClassAB simply.

[VladimirK]

Originally Posted by Plecto

I haven't learned about fourier analysis and calculating THD values yet so I don't know the math behind it, but are you saying that "tuby sound" and "micro sound picture" can't be expressed mathematically? I found it really attractive to think of a distortion meter that could directly and mathematically compare one amp to another

I would say, math reflects general features, macro picture, but not the complete truth. Math is based on the model, and the model is definitely simplified compared to all details of the phenomenon (sound reproduction).
Additional advances in math is possible, but, again, up to definite point only, it will not explain all the details again.

[Plecto]

I'm really curious about having an op-amp as an input stage with it's feedback taken form the output of the amp. I was under the impression that op-amps usually have great specs (PSRR, CMRR etc) compared to many descrete amplifier designs. Also with the feedback taken from the output of the amp I can't understand why this won't greatly reduce distortion.

Another question. In my next design I wan't to use a voltage regulator to reduce humming. If I then connect the gate bias resistor directly to the output of the regulator, won't this greatly increase the gain aswell? I remember doing this the first time I made the amp and the gain was insane compaired to after I connected the resistors to drain instead. One problam was that the operating point would shift dramaticly if the voltage or the bias current would shift, but with a regulator the voltage shouldnt change and neither should the current.