unusual design of input stage

[thanh]

Hi! Very interesting schematic.
What's it about if R34 and R35 are replaced by a current source?
bye!

[Elvee]

Originally posted by thanh
Hi! Very interesting schematic.
What's it about if R34 and R35 are replaced by a current source?
bye!

It will:
a) Become totally unpredictable because one of the current sources will always have a slightly larger current than the other, pulling the NI node (and consequently Vout) all the way to V+ or V- (the loudspeakers will appreciate).
b) Lose practically all open loop gain if the current sources are of a good quality.

The NI node needs an impedance as low as possible; the Thevenin impedance of R34//R35 is already bad, but with current sources it will become disastrous.

Note that I have nothing against the complementary differential topology; I have used it quite often myself, but using an unusual topology doesn't allow one to dispense with sound design rules.
LV

[ilimzn]

i agree with Elvee, the inverting input is implicit, and sees any difference between + and - rail. At the very least, C8 should be split into two caps in series with the idpoint connected to ground. Even better, a proper stiff voltage reference should be used.

Also, Q8 and Q12 are not really necessary except if the voltage rails are outside the Vcemax or Pdmax of Q11 and Q7.

@Lineup: what is your aversion towards cascodes? (You say you NEVER use them). They don't really do a 1 transistor job, they do a 2 transistor job, since they are 2 transistors - they have their uses. but of course, i do agree with you that people sometimes tend to cascode everything, even when it does not justify the achieved improvement. i also find they are amongst the most trouble-free 2-transistor constructs

[Kuzmenko]

Ok, this is not a problem, here you are!

R36 R37 can be substituted with lighting diodes or zeners. But resistors are cheaper

[Elvee]

Looks much better now.
Resistors may be cheaper than diodes, but the difference is, in my opinion, negligible and anyway, resistor+47µ cap is probably more expensive than a diode.
There is a good reason to stiffly set the voltage at these nodes: any inbalance in the supply rails voltages will cause a DC offset at the output.
LV

I threw it in a LTspice file to have some fun, just in case somebody wants to play around with it ...

Some values differ from the last posted schem and I added the zeners. This could be a good base to illustrate changes to this circuit.
L1 in the feedback network is for easy switching between closed/open loop behaviour without loosing dc-reference. Just delete the short and you are open loop.

Have fun!

[lineup]

Originally posted by ilimzn

@Lineup: what is your aversion towards cascodes?

I have no aversion against cascading, in special.
It is just that I like simplicity so much
It would take very much to make me use 2 transistors
when I can do Well with 1.

Same thing, if adding one cap somewhere
and distortion drops only less than 1 dB.
I find it is not worth it. Because probably this cap can cause some other little trouble.

Cascading means not only add 1 transistor, as you know.
You also add a rail from power supply. And resistors and maybe also filter cap.

One reason, in my opinion, why simple amplifiers performs so well in compare
is they have fewer components.
And fewer rails for injections of troubles.

When you have nothing. This nothing can not cause any disturbance.
As soon as you add something, you have a media for disturbance
as well as for the good you have added.

This is my logic explanation for why simple, which theoretically should be inferior
in practical reality so often turns out so well.
While most complicated amplifiers can be almost impossible to tune
and get right.
It may be too many things to cure.


This is also in line with, why Single ended amplifiers, using only 2 power supply rails
V+ and 0V=Ground
can have such good sound.

While dual supply, 3 rails, have not only several paths for supply currents
but also more paths for unwanted 'feedback' and disturbances.
There are a lot of small currents, garbage flying around in a an amplifier
in motion.
Not only at audio frequencies.


thanks for asking, ilimzn
hope you get my thinking

lineup 'Keeps It Simple' man

[Limhes]

Just use the right tool for the right job, to add another obvious comment...

And, please, as someone above already stated, cascading is when you put more stages after eachother, cascoding when you put transistors 'on top of each other'.

[Ultima Thule]

Nothing "new" under the sun...

Just a symmetrical Rush cascode input stage with two cascodes.

VAS connected both as a CE and folded cascode.

But all together it's a bit unique!

Cheers Michael