Make Hifi with 3 Triple Sziklai transistors in Simple Class A

[lineup]

What you need, for dual channels:

1 transformer 2x30 VAC, 400VA, with split secondaries
8 power resistors with value 33 Ohm
2 power resistors with value 0.47 Ohm
2 small signal transistors, like 2SC2240, BC550C
2 medium power transistors like BD139, MJE340
2 power transistors like MJL3281A
4 capacitors 4700uF or 10000uF
2 potentiometers 100 kOhm
2 film capacitors 2.2uF for the input
A bunch of normal resistors and a couple of 10uF capacitors

Is it possible to make Hifi Class A with only 3 transistors per channel?
Yes
I will show you how.
Just you ask me .......

Oh, yes.
The max output is 9 Watt Class A into 8 Ohm

[Workhorse]

Ok then show us, Lineup

[Ian Finch]

I am asking nicely....Please, do tell since this should get everyone involved for one reason or another.

3 transistors and Hi-fi you say? Marvellous!

[lineup]

Thanks, guys for your interest.
Especially my friend Workhorse ... what have I done to deserve you???
----

If you look at my first figure below (I call it figure1)
you see what is the most simple power amplifier
using BJT, bipolar power transistor.

Dont try that one at home!!!
It will certainly not be Hifi.
The problems are several. Mainly the high distortion of the MJL3281A.
And the input impedance is too low - it will be hard to drive.

In next figure, figure2, I show how we can overcome the problems
by the help of 2 smaller transistors.

What we have created is a SUPER transistor.
Made up of 3 transistors in Sziklai configuration.
Circuit has basically the distortion of the small signal transistor in front
and the high impedance we want.

I be back with the final real schematic
and some test figures, numbers thast shows it really works.

[DF96]

Damping factor around 1 will cause boomy bass with most speakers. Class A - yes. Hi-Fi - no.

[lineup]

You may be right, DF86.
But we have elsewhere discussed damping factor.
There are quite a bunch that do not below high DF is necessary.

The circuit is very much Nelson Pass.
He has made some 'one MOSFET amplifiers'.
So you do not like the idea.
Well, I guess time will tell if there will be boomy bass.
This is a circuit I would like to build.

[DF96]

High DF may not be necessary, although it depends on the speakers. That does not mean that very low DF is good.

I regard Nelson Pass designs as eccentric.

You may like the sound from this type of amplifier. Just don't confuse people by calling it hi-fi.

[lineup]

We can discuss the term Hifi.
For me it is everything that has low THD, say below THD 0.050%
So this circuit with THD 0.002% at 1 Watt is really Hifi .. in my book.
I wouldnt call anything Hifi if it wasnt so for me.

Dont be so fast calling it non-Hifi.

[wahab]

As pointed by DF96 , with DF (!) at 1 , frequency response wont be linear
and highly speakers dependent , wich is not hifi , by the definition.

[Struth]

Hi Guys

To me "hifi" means "not changing the music noticeably".

There cannot be an arbitrary THD number here, as that number does not reflect the distortion profile or the much worse IM products. Many have found out for themselves that having the same THD number does not mean two products will sound the same or reproduce music the same.

The Pass circuits, the JLH class-A amps, most tube amps and a lot of budget solid-state product is no where close to being hifi. Most of it will sound okay with simple music - which is always what they describe it being good for: solo piano, a vocalist with two piece accompaniment, etc. Try those circuits with modern pop, rock, metal or any other style with a lot going on the midrange and the sound turns to mud or is just garbled.

The above circuit will fall into this same category unless the load is made active. See Self's expose on follower circuits in his "Small Signal Audio Design" book. Intuitively a follower should be the simplest thing to design, but it has hidden flaws.

Have fun
Kevin O'Connor