Power Amp Schematic

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#42
forr said:
Here is Linsley-Hood's mosfet amplifier using positive feedback.
Click to expand...

Initialy Hood intended to introduce the positive feedback to provide a double-endedoutput...

As his article progressed, he appeared to forget this, and provided only a generic single ended output to drive his output stage..

Thus, TR4, TR5, and TR8 were completely wasted...as TR4's collector goes begging in this design...😱
 
#44
FORR
"I think McLaren ampifiers have a current mirror load and a differential Vas. Maybe this circuit is due to Doug Self."

JOHNNYX
"Self was not a fan of the differential vas, so I don't think it can be due to him"


Self has been working for TagMaclaren, so I believed he was responsible of this scheme. Despite all respect due to D. Self for its deep work, he is a bit contradictory of not being fan of differential VAS because he is very fan the NE5532 or 34, wich has such a VAS. However, this IC has a three voltage gain stages, and the current mirror in second position does not have to deal with a lot of current or voltage across it.

In fact, I think that TagMacLaren bought Audiolab and that the input-Vas stages I refered in previous posts may be due to Swift and Scotland which were authors of the first Audliolab series. TagMacLaren has now abandonned audio and trademark Audiolab is announced to be back again.

If you would like, I can email you the papers from Tag about their amplifiers. It would be good to see such technically well detailed documentations from other companies more often.


The symasym circuit, with its grounded base transistor in the diode limb of the current mirror VAS, is not at all original. It was used by Sony in an input buffer for an active crossover filter. It consisted of a differential pair followed by a second differential pair loaded by a current mirror, one limb with the grounded base transistor, the other limb being the output.

~~~~~ Forr

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#45
johnnyx said:
There are similarities between this MOSFET amp and MikeB's amp. The thing I was referring to was the use of a transistor to pass the current to the mirror, instead of a resistor, R9. In this way, the VAS collector is held at 0v, and the current source on the input stage sets up the currents for the whole amp. A very nice solution I think.
Click to expand...

Hi johnny, this arrangement works very nice, sadly it is not my invention. The biasing is very stable, and DC-drift minimal, ~+/-2mv. In my amp it changes between 0 and 4mv. No trimming.
But that's not the only advantage of that cascode !

Mike
 
#49
Re; Self and differential VAS

In his articles for EW and his book, Self discusses the problems of that topology, arguing in favor of his "blameless" aproach. From what you said, I was thinking that maybe someone had twisted his arm to abandon blameless and adopt other topologies, but now I find that the differential VAS was due to someone else.
Phew....peace at last.😀
 
#54
JohnW said:
Adding input stage degeneration to an existing design should be done with care as the change OL gain could upset the amplifiers stability criteria.


Slew rate will be reduced, but can be restored by increasing input stage current (reducing the value of R3) – something to play with – but watch the amplifiers stability!
Click to expand...

Hi johnW...

Adding input stage degeneration does not compromise stability....infact it enhances it...if nothing else is changed...


Similarly, merely adding input stage degeneration does not affect slew rate...
 
#55
MIKEKS
"Stocchino's approach to generating push-pull operation in the last stage of gain is ingenious... ....at least conceptually
Do you have the full article...?
I would love a copy..."

Sorry I only have the schematics, not the full article.
I agree, conceptually ingenious. However I am not sure that it really solves the known problems of a differential Vas using a current mirror.

~~~~~~~~ Forr

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