suggestions required on mosfet matching setup for aleph x

[pamaz]

hi everyone in this nice place.
I'm really a newbie in the aleph x, and therefore I'm trying to figure out some things after having seen some ( a lot of ...) threads regarding this amp.

So my first request ( is somebody will be so kind to give me an helping hand) is to understand the matching criteria for aleph x mosfets.
I've already seen the infos ( how to matching devices) by Mr. Pass, but seems to me that this is just the first step in the selection (I mean Vgs selection).
Therefore I would like to understand if there is any more info regarding this matter.
Thanks and hope that somebody will help me to move in this "selva oscura" as Dante is saying in Divina Comedia.

Ciao from Italy

Paolo

 

[BobEllis]

http://www.passdiy.com/pdf/mos.pdf

 

[pamaz]

Thanks Bob, but as I told , I've already read that stuff.

The purpose of my request is to understand if exist a correct setup to make the mosfet matching in its dynamic behavior.

I've read that lot of guys have made the matching also observing the Vgs drift over time while the mosfet itselsf is heating up.

If i don't make errors this seems to be useful to take under control the offset voltage of the amp, while it's warming up.

Therefore my request is : is there any other test method, other than the Vgs matching , that is to be carried on to make the proper selection of the components??

Just to point out the thing one more time: I mean any other method of selecting the mosfets, that is not the selection directly on the completed amplifier.

Thanks and bye.

Paolo

 

[D3]

Tester

Apparantly there is a test circuit available from 'thepcb shop'
apparantly what it does is pulse the mosfet...... at diffent voltages/currents very quickly so heating isn't a problem......then
it plots a curve ....I think to a scope..............so you can match fets
exactly.......

Board + bits costs approx £50

So this may be good if a group member is experianced ......

Hope this helps........................

 

[pamaz]

just found it

Hi D3 .
Thanks to your suggestions I've read the thread related to that kind of mosfet tester, and luckily I've found the related article on November 1993 Elektor issue ( at that time i was subscribed to that magazine).
For anyone interested, it's better to point out that this machine is intended just for MOSFETS testing, not BJT.
To tell you the truth there is also the pcb layout, therefore i will try to set up and test such system. (unfortunately not in the next couple of days )
It's interesting, because this board can show up on your scope, the curve related to two mosfets ( actually up to four if you do the selection for 2 pcs of Nchannel and 2 pcs of P channel ) at once, thus allowing a good matching, without getting crazy in measurements done point by point.
In any case before saying that this is useful for our purpose, i'll try in the next days to set up the thing so to test it.

Furtermore if Mr Pass can give us some more suggestions that can go in deep of the previuos article on this matter, i think lot of "alephX addicted" will thank him one more time.

Thanks and I'll let you know something ASAP

Bye.

Paolo