Best topology for no-feedback ClassAB buffer ?

[Bigun]

I'm looking to make a simple impedance buffer, something that has a high Zin and a low Zout - whilst avoiding the high idle power dissipation of a Class A design. I don't want any feedback (except for degeneration perhaps).

Power requirements are low (e.g. <10W) so cross-over distortion at low power is perhaps the limiting factor.

Candidates include double EF, Krill O/P, Sziklai buffered EF, Diamond buffer.

What would you recommend and why ?

[CBS240]

SE class A?

J-fet input, use the -Vgs to bias a SE class A double EF. You could use a CCS for the J-fet source current, and a CCS to bias the output stage.

If class A is out of the question, then Diamond buffer is the way I would go.

Simple, functional.

[lineup]

I like Sziklai.

If you want a real killer, mainly for power and loudspeaker driving,
use something I call triple Sziklai.
We have normal 2T darlington.
And 3T triple darlington.

A Triple Sziklai is using 3 transistors, too.

The figure shows a Triple Sziklai
that simulates often better than triple darlington and anyhing else.
One other benefit, it has lower saturation voltage.
So can give a bit extra before clipping level.
Input impedance can be very, very high.
Like more than 1 MOhm I have measured.

MOSFETs and Triple Darlington steals voltage.
This is a factor especially when you have lower voltages and want to squeeze out max.
MOSFETs for example, can not work within 4-5 Volt of supply.

It all depends on your application.
For 10 Watt probably a double 2T Sziklai would do very well.



Now we will soon get some really wise guy coming here and discard Sziklai output.
Saying it is prone to oscillate.
Rubbish!
This is no reason against.
Because any amplifier, with any output, can oscillate.
We see this happen all the time under construction.
No. It is all about doing it right.
Whether we use the terrific Sziklai configuration or not.

Regards Lineup

[Edmond Stuart]

Originally Posted by lineup

I like Sziklai.
[snip]
Now we will soon get some really wise guy coming here and discard Sziklai output.
[snip]
Regards Lineup

I'm afraid Bigun himself will discard your proposal, as he "don't want any feedback (except for degeneration perhaps)".

Cheers,
E

[Elvee]

Here is an unconventional suggestion:

It is a half-diamond circlo buffer.

It has a full NPN output, low-Z, good linearity, no crossover distortion, a servoed Iq control, and a decent bandwidth, even with 2N3055s.

[sajti]

Originally Posted by Bigun

I'm looking to make a simple impedance buffer, something that has a high Zin and a low Zout - whilst avoiding the high idle power dissipation of a Class A design. I don't want any feedback (except for degeneration perhaps).

Power requirements are low (e.g. <10W) so cross-over distortion at low power is perhaps the limiting factor.

Candidates include double EF, Krill O/P, Sziklai buffered EF, Diamond buffer.

What would you recommend and why ?

I did lot of research about it. Finally my vote is the triple EF. I use tube VAS, so high input impedance is very important.
The CF has not very linear input impedance, what can be problem if no feedback applied. The diamond is good only for low power, or You have to use one more EF somewhere.

So my amplifier use triple darlington output. I carefully selected the components, and set the quiescent current.

Sajti

[Bigun]

Thanks guys, I like these ideas.

Whilst I agree that the CFP (Sziklai) embodies some feedback, I'm willing to accept a single Sziklai pair as a 'compound device'. It's a bit of a stretch but I think it's a valid option. However, I would reject something like Hawksford as being a form of feedback too overt.

Do you guys have a feel in terms of sound - unwanted harmonics etc. between some of these options ?

I'm also thinking of a tube VAS (JFET is an alternative) so this is really a kind of hybrid. And so maybe I forgot to include Wavebourn Tower design as an alternative ?

[Edmond Stuart]

Originally Posted by Bigun

Thanks guys, I like these ideas.

Whilst I agree that the CFP (Sziklai) embodies some feedback, I'm willing to accept a single Sziklai pair as a 'compound device'.
...................

>some feedback??? Well, of the order of 40dB. That's a little bit more than 'some'.
So what do you want? A decent low distortion buffer (based on feedback) or a high-end distortion generator (without feedback)?

[Bigun]

Edmond - you are absolutely correct !

I better explain a bit more...

I have heard a no-feedback tube amp in my house and I very much like the sound. I don't want to build another global feedback amp after hearing this. Sure, there are some great gnf amps but life's too short to build them all so why not narrow down some choices ?

Whilst I like the sound of the tube amp, for a small desktop amplifier (such as one I might wish to use with my computer) a tube amp starts looking big, with two large output transformers for starters. So I am thinking of a hybrid with a single triode front end and SS output devices. If I do this with Class A I need huge heatsinks. So I am thinking Class AB SS outputs. I want to avoid feedback - but I simply don't know if Class AB without feedback is going to do the job adequately or if 'local' feedback can be 'allowed' without losing the sound I have heard from a no feedback amplifier. I do believe that if you are going to have any feedback at all, you have plenty of it and avoid the middle ground of just a little. The Sziklai has plenty of local feedback so perhaps it's OK, but perhaps it is not and the only way I could justify my feelings on this was to consider it a 'compound' device where the feedback loop is sufficiently 'local' as to be benign. But I have no experience to judge this. So I'm seeking opinions. I may simply stick with an all-tube amp if this is the best option.

I didn't want my first post to be a long read but may be I left out too much explanation, I hope the above helps.

[Edmond Stuart]

Hi Bigun,

First, thanks for explanation.
So you want tube sound. In that case, I would say: just use tubes and an output transformer, which is also part of the game, so to speak. If you need only 10Watts, then the O/P transformer doesn't need to be that big, provided that you are using a balanced output stage (as you know of course, a single ended class-A OPS, needs a much large one).

Cheers,
E.