Aleph-X builder's thread.

[Mark A. Gulbrandsen]

Its funny that Nelson says .01 volt is fine...he's got to be talking output devices...not input. I have a pair for Q-5,7 that are within .007 V of each other. I should have this thing up and running tommrrow night so it'll be interesting to see what my offset actually is. I feel that actual subbing of just one of these devices is probably the best way find a match that achieves really low DC offset.

Mark

[moe29]

it seems to me though, that the only way you can get a dead on
exact match is to have these devices in the circuit and running for
at leat 30 minutes before you can get an accurate reading. I don't
see how you can test them out of circuit and match them to within
less than .01 volt.

[hifiZen]

Don't forget that in the Aleph-X we don't have unity-gain feedback at DC. So, the Vgs mismatch between Q5-Q7 gets multiplied by the amp gain!

[Netlist]

Originally posted by Coulomb

So B32 is the theoretical and B33 is the practical?

Exactly.

So if I jumper these resistors, can't I take the output for my FET boards directly from the FET Pads? Then I would only need one other wire for the current sense line to each board.
Anthony

Do you mean mounting the Mosfet's and resistors off-board?
Then yes the current sense wire goes to the off-board section.
I would also mount the 221 gate resistors directly to the gates and connect a wire to the board from there.

/Hugo

[moe29]

Chad, thanks for the info! I have been reading and re-reading
the WIKI too... 1st time builders should really be taking adavantage
of all the hard work that went in to setting that up... great info!

i set up a jig to be able to swap out one of the differential pairs
in circuit. Finding a good match this way takes longer, but for
some reason i feel a little better that it's being done in it's operating
environment. Of course this means they're not mounted
back to back during testing...

wish me luck!

[tomchaoda]

Thus, it is a two gain stage design, where Q5 and Q7 form the first gain stage (diff. pair), while Q2 and Q11 form the second gain stage on each side. Q1 and Q10 are the primary power transistors for the dynamic current sources with controlling elements Q7 and Q8.

As I study this part in the "The circuit" section, shouldn't the Q7 and Q8 be Q3 and Q8?

Thomas

[protos]

I was also looking to up the bias on the diff pair.What's the best way to do this?How much are they running at now?I thought lowering the 392r would lower the gain?
Another thing about Aleph's and A-X that I don't quite understand - if you increase the ac current gain for better performance into 4ohms what does that mean that for class a operation?Does it become a kind of heavily biased class AB? When does it leave class a operation - when the output stage bias limit is reached or sooner?

[Eric]

Hugo: Thanks for the reply. I went back and re-read the entire thread on the high-powered version. Great stuff!

Grey: I have 2 heatsinks from the original group purchase, and am interested in building a relatively low powered version with dual mono in the same chassis. I have been playing with the AXE-1 spreadsheet and am looking at 15v rails, 3A bias with 4 fets per channel. Seems to require about 90w dissipation per channel making it a good fit to the heatsinks.

I have also been considering the logistics of putting 3 channels in the same chassis (front sound stage for my home theater system), but don't think it will all fit in an 11" cube...

[GRollins]

Regarding the front end differential offset vs. the actual offset at the output: You can use the inevitable slight mismatch of the output devices to counteract the front end differences. Just swap them back and forth. With luck, you can find a pair that will yin to the other's yang and produce a nice, harmonious balance.
protos,
Yes, lowering the value of the front end load resistors will lower the gain...if done in isolation. There are two factors that you want to balance--the gain and the DC offset across those resistors. The gain isn't critical, actually. The DC offset is, however. That DC offset sets the bias for the output MOSFETs; switches 'em on.
If you increase the bias so as to handle a lower Z load, you're ensuring that it reamins in class A. The Aleph output stage does not do class AB gracefully. The current source and the output won't drive symmetrically, and the signal gets kinda lopsided.
Eric,
I haven't looked at anyone's spreadsheets, so I can't vouch for accuracy. I've got one I hamered together a couple of years ago when I was torturing the Aleph circuit to see what it would do. I then went back and modified it to handle the Aleph-X. It's ugly as home made sin, but it does what I need, so I've never bothered to look at the others.
Is the 3A bias per side or for the whole amp? How much power were you shooting for and into what load?
An 11" cube sounds pretty limited for heat dissipation for three channels. Perhaps forced air cooling would be an option.

[Aud_Mot]

OK I am lazy

Does anybody have heatsink information

Make
Model
Supplier in QTY of 2 or 4 peices

I expect will be making the +/- 15V rails at 3.0 - 3.5 Amps bias

If there is several choices, I would like to see them. I would tend to use overkill when it comes to the heatsink.

Reagrds,

Aud_Mot