Quasi-complimentary; pro / cons?

[Cobra2]

Since my aussieamps N-channel still is my favorite (DIY) amp...
I just wondered why there are so few(?) quasi designs?
What is good and what is bad/difficult with such designs?

Arne K

[Workhorse]

Hi Cobra,

The two main Difficulties Arised when Designing the Quasi amp with N-channel Mosfets are:

Cross-Conduction,
Getting Rail to Rail Swing of output voltage

regards,
K a n w a r

[AKSA]

Kanwar, Arne,

I've not done a quasi-comp yet, but they are beginning to really appeal to me.

R2R Swing: No problem on the bottom rail, since if heavily driven an N-channel vertical mosfet will swing to within a volt of its source.

Top rail is different, however. Perhaps the best way to achieve close to the top rail on the positive half cycle would be to use a bootstrap to lift the midpoint of the BS resistors well above rail. This would easily accommodate the five or six volts bias (and source resistor drop) needed to ensure that Drain-Source voltage is down to about a volt. There is much to be said for an amp with 63V rails which can swing 124Vpp across a speaker!

Cross conduction is caused by asymmetric drive delays between top and bottom of output stage. Perhaps the best way to avoid this problem is to drive each block along identical signal path lengths, and preferably from opposite outputs of the LTP input stage rather than inserting an extra phase shifting stage right at the driver level. This is the usual drive topology, but it's costly of rail efficiency since the driver base (and hence its emitter which is one diode from output level) cannot go lower than the full mosfet drive required, which could be as much as 5 or 6 volts. Quasi, a South Australian designer in the High Power Quasi thread, has done this very cleverly and used a CCS to supply the second LTP for the twin VASs which even accommodates temperature compensation.

Arne, I know Anthony Holton and have heard his N-channel amp. I agree with you, it's a fine amp with a sweet, engaging sound.

Cheers,

Hugh

[Cobra2]

Thanx for the explanations...
Oki, I understand that there is some special considerations, but (exept for the fact that quasi uses only N-ch / npn output devices) what are the good points?
(Well, I know mine sounds good ;-)
And a bit efficiency-loss is no problem?

Arne K

[lumanauw]

Hi, Cobra2,

I've build quasicomp with Vmosfet and the most stable one is something like in patent #5,783,970 (Joseph J Pleitz).
This configuration is very clever, very stable. Look at the arrangement of T6, T7 and R4. And T4,T5, like AKSA said, both legs of differential are used here.

[Cobra2]

@ lumanauw ; do you have a link to the patent?
(btw, nice car-amps! )

Let me rephrase my question; what is the advantage of a quasi amp compared to a N + P type?

Arne K

[AndrewT]

Hi Aksa,

There is much to be said for an amp with 63V rails which can swing 124Vpp across a speaker!

MIRACLE would be my comment unless the condition you conveniently did not state was >=100ohm speaker and signal duration not exceeding 100uS.
Any retraction or am I misinformed again?

[AKSA]

Hi AndrewT,

Nice try!! I don't give retractions because I'm not writing text books or magazine articles here, but if you:

1. Drive the upper output block with a bootstrap, so that input at the gate can easily exceed the upper rail, and thus bring source within a volt of drain, and

2. Can measure 1V drop drain-source on a Class A lower N-type CCS at clip with constant bias, then it can be done with adequate drive (I have made this measurement, BTW, on a working amp).

I see no reason therefore why it could not be done with a stiff power supply, miracle or otherwise, and I feel this is something worth aiming for. Of course, this would be equally possible with dual bootstraps on a fully complementary, too.

Cheers,

Hugh

[lumanauw]

Hi, Cobra2,

You can download the pdf from www.freepatentsonline.com

[djk]

http://patft.uspto.gov/netacgi/nph-P...S=PN/5,783,970