Pioneer A-300X, strange VAS

[GEirin]

Hi all.
Anybody can to explain the VAS of the Pioneer A-300X?
Thanks

[tiefbassuebertr]

Originally Posted by GEirin

Hi all.
Anybody can to explain the VAS of the Pioneer A-300X?
Thanks

first stage: first voltage gain stage = differential amp together with current source (Q201a, Q201b, Q205)

second stage: buffer as beta enhancer (ß-enhancer) with current source and current mirror ref. current (Q203, Q233 resp.255, Q207)

third stage: second voltage gain stage (Q211) with current mirror as phase shifter (Q215, Q217) upper current source (Q231) voltage source/idle current (Q 225) and lower current source (Q219)

following features are visible:

1) the same currents of Q205, Q255 and Q231 through ref diode D201 (ZENER 2V7) and because using of current mirror same currents through Q203 and 211

2) Enhancing of the output resistance and the open loop gain at lower frequencies through second current source Q231 in the loop of the second voltage gain stage

It would be interest to know, how this amplifier sounded.

[GEirin]

Hi tiefbassurbertr.
Thanks you for you explanations.
A doubt:
2º stage: buffer as beta enhancer or a baxandall pair (Q203 - Q211)?
3º stage: second voltage gain stage is Q211 or is cascoding VAS Q217 - Q231?

[GEirin]

Hi all
Questions
Why output differential pair is in second collector transistor? Positive leg?
Why 2 voltage gain stage? advantage?
Thanks

[tiefbassuebertr]

Originally Posted by GEirin

Hi tiefbassurbertr.
Thanks you for you explanations.
A doubt:
2º stage: buffer as beta enhancer or a baxandall pair (Q203 - Q211)?
3º stage: second voltage gain stage is Q211 or is cascoding VAS Q217 - Q231?

baxandall pair? what means this? I have heard only about other things (e. g. tone controls) from baxandal (bass treble control) like this:
Baxandall 2
and about
Patent Retriever - Free patent PDF download
(fill in US 5,046,105)

Apparently, the term "Baxandall pair" respective "Baxandall super pair" is a colloquial term specific use at this forum. I don't know, what it means and I don't find explantations on other pages from the web.

Mean you "Sziklai Darlington" (also called "Current feedback pair" = CFP)?

Q203 was connected with their emitter to an independed actice load (current source) Thus Q203/211 are not a Sziklai Darlington.

second voltage gain stage is Q211. Q215-217-231 acts as a phase shifter.

Basicly Q219 is the current source for Q211
Similar topology I found by Harman Kardon PM650PXI (PM 650PXI, PM650 PXI PM650), please look attached pdf.


Unlike your Pioneer, the second output of the differential amplifier (i.e., first stage) is used for the phase shifter stage (current mirror)

[tiefbassuebertr]

Here the Harman schematic

[tiefbassuebertr]

Originally Posted by GEirin

Hi all
Questions
Why output differential pair is in second collector transistor? Positive leg?
Why 2 voltage gain stage? advantage?
Thanks

two voltage gain stages provides more open loop gain (in most cases) as a single voltage gain stage

Basicly you can use one of both (and also both together) collector as output in each circuit. This depends from two situations:
1) the wished phase shift of the whole amplifier circuit (inverted mode or non inverted mode)
2) the choice of fittet cross point connection of NFB (either base of the first resp. input half or second half of input differential amp, this means either serial or parallel NFB)

[GEirin]

Hi thank for you reply.

My knowledge is:
1 Emitter follower (Self)
2 Baxandall super pair
3 Sziklai

[GEirin]

Baxandall super pair in thread "Baxandall super pair" (Peufeu).

[tiefbassuebertr]

Originally Posted by GEirin

Hi thank for you reply.

My knowledge is:
1 Emitter follower (Self)
2 Baxandall super pair
3 Sziklai

you mean this URL:
Baxandall Super Pair

Nothing of them are to find in the schematic of Pioneer.
If collector of Q203 goes to R227 instead to Vee, then one can say, it would be an Baxandall super pair. But the collector goes to Vee (negative rail). So we must find an other term for this topology, and I am shure, there is such a term. But - as already mentioned - my colloquial english in electronics is very bad.
So I called it as emitter follower with their own current source