Fet quantity XA design

[Rob Dingen]

Hi

I’m going to build a XA amp at about 200 W
Now I have some questions how many fet’s should I use.
If you read the manual Nelson uses 4 banks of 8 Fet’s a total of 32 around 19W a piece.
You can easily build them with 4 banks of 6 a total of 24 around 25W a piece.
Why use more you have to drive more capacitance and the open loop bandwidth is lower.
The other thing is that you can use bigger value source resistors so locale feedback is higher.
With a bias of 9A 24 fet’s run on 750mA and a source resistor of 0.68 Ohm and I think the original runs about 560 mA and a source resistor of 1 Ohm.
Also if I use more fet’s is it better to increase the current in the diff pair to get a better open loop bandwidth?

Rob

[Nelson Pass]

Running more current through the diff pair will not substantially
alter the high frequency performance because the actual
transconductance of the input devices will only increase
marginally, and you will have to lower the load resistance on
them which will lower the gain of the diff pair, so you will lose
more than you gain at some point. We feel that the values
we use represent the sweet spot. Of course you are free to
do as you like.

With regard to using fewer output devices, you certainly can
do that. The distortion a lower frequencies will increase slightly
and will decrease slightly at higher frequencies. The amp will
experience a slight spectral shift and sound a little bit brighter.

If that's what you want, then Bob's your uncle.

[Rob Dingen]

Hi Nelson

Thanks for the answer clear talks for me I go for the 32 devices you should have a good reason for that.

Rob

[Clarkcr]

I just got off the phone with Kent at PL and was encouraged to pursue an X amp for my Martin Logan Monoliths instead of an Aleph 2 or even 1.2. Although they would be okay, he explained, the X and XA Class AB backing up the Class A is a better choice for the high current demanded by these electrostatics......a long way of asking for help in building a pair of these myself.

Are the boards for these XAs available? Are the board templates available somewhere....I could etch my own....never done it, but then again I've not done a lot of things!!


Thanks


Chris

[Clarkcr]

I meant the x1000 not the XA. After reading much of the written material on the X and XA, it is the X topography and Class A/AB I'm looking for.

Any ideas?

Chris

[Duck-Twacy]

I might be wrong, but isn't an Aleph 1 not also very good in driving difficult (low imp) speakers??

http://www.passlabs.com/pdf/aleph/a1man.pdf
http://www.passlabs.com/pdf/aleph/a1serv.pdf