A cross between A5 and A3

[Stabist]

Hi!

I just need a final confirmation - during to some different reasons (mainly short of $$ ) I've reached a decision to scale down a bit the first of my two A5 projects ...
So - I have soma "spare" transformers with 18V secondaries - that just calls the conversion into A3
I have heatsinks that are capable of dissipate 200W/stereo (but 300W would be probably allready too much for them - and as said no $$$ to buy new at the moment) - again - A3.
And I have allready soldered PCB for A5 - but it's a "flexy PCB" - so the number of input fets can be variated
I have enough PSU capacitors (at least something I'm not short off )

So now I have two choices:
- direct scaled down A5 with 12 output transistors and just adjust necessary resistors because of lower PSU voltage
- make a sort of balanced input Aleph3 with 8 transistors, etc ...
I find second option better - I guess there is no need for those 2 additional pairs of transistors considering lower PSU rails ...

And if I'm correct - it's just important to match appropriate values for bias setting and voila - my "crosser" will be born

[MikeW]

Build an A-30. Use all your transistors and crank up the bias.
I built one with 8 outputs per side and a 21-0-21 800va transformer with a C-R-C-R to drop the voltage to 26. I can't get it away from my son.

[Stabist]

Hmm - is there big difference (I know - I will check it out in schematic) between A5 and A30 - because as said - A5 PCB is allready assembled together ...

[MikeW]

http://www.passlabs.com/downloads/ol...t/a30manr0.pdf

[Nelson Pass]

Originally posted by Stabist
Hmm - is there big difference (I know - I will check it out in schematic) between A5 and A30 - because as said - A5 PCB is allready assembled together ...

Not so big that you can't tack one on to the other's board.

[Stabist]

Hmm - what I like most at A3 is 60W/4R - because my speakers have 5R ... I see A30 has "only" 40W ...

Well - what needs to be done is a little more deep look into things (now you've give me another option ) so ...

[Stabist]

Thanks mr. Nelson - I've made a short comparison of A3, A30 and A5 schematic - in the topology itself there are really just few differences - and Ok, I would have (Ok that I would do anyway) to change few resistor values ...

Now I have these Q:
- how come A3 had such high capacitance bank in PSU??
- What limits A30 power in 4R comparing to A3?

[MikeW]

It depends how high you bias it. Wuffwaffs has an excel speadsheet program for Aleph power.

[Stabist]

Just to check it out - biases for original A3, A30 and A5 were??
I have in mind somethin like - A3 around 2,5A, A5 2,1A and A30 ??

[MikeW]

Bias at 2.5 amps with 24 volts at the MOSFETs will give you 45 watts @5 ohms and 50 watts@ 4ohms.