A cross between A5 and A3

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Hi!

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

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 :D
 
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?
 
Yep :D

And 2,7A will give me magic 60W :D But also (according to that sheet) around 135W dissiptaion/channel :( Which could allready be a little overdosed for my two heatsinks ...

BTW - another Q - if I use 4 heatsinks (like original Aleph) - BUT I mount transistors only on two (and so avoid long wirings) - but I DO connect all four heatsinks with massive Al "corner blocks" - will it spread the heat pretty proportional or will I get two HOT heatsinks and two medium warm ??
 
Now I don't understand it anymore at all ... :(

I've done some simulations with wuffwaff's excell sheet:
- if I take rail 34V, bias 2,5A and AC current gain 50% - i get 60W/8R BUT only 50W/4R (and the climax at 6R with 75W) ... Hmm - does that mean that in "real life" AC current is somewhere around 62-63% or what's wrong with my calculation??

And also - A3 - 25V rails, 2,5A bias and 50% - 30W/8R, again 50W/4R and climax at 4,5R with 58W.

So from this point of view - it's almost same for my speakers wheter i would made A3 or A5 ... ?!?!? Hmmm ...
 
Primoz,

the formula to calculate the ac current gain should be:

gain = (R1/R2)x(R3/R4)

where:

R1 = output sense resistor (A30: 0R47 / 6)
R2 = sorce resistor of aleph current source (A30: 0R47 / 3)
R3 = senseresistor of source of aleph current source (A30: 1k)
R4 = feedbackresistor of outpt sense resistors (A30: 820R)

=> (0,07833/0,15666)x(1000/820) = 0,5 x 1,2195 = 0,6097

=> ac current gain is ~ 61% with Aleph 30.

Uli

:nod: :nod: :nod:
 
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