Is a mini-Aleph using BF862 possible?

[digits]

It is good when you have extra voltage to throw away... (up to the point where dissipation becomes an issue).

[njepitt]

Hi Guys,

I've been thinking a little more about rebuilding this, especially about juma's comments about 2SJ201 or 2SJ313 (paralleled). It looks like I'll be doing another project first, so there's plenty of time to think some of these things through.

I rather like the idea of paralleling the 2SJ313, but they may not be available when I need them (Seems Zhou is running out, and isn't selling any right now...) So I looked at a few datasheets, and am wondering about IRF9510 and IRF9610. Numbers I've found are the following (capacitances from the tables, transconductance approximate values from the graphs for 0.4A or so):

2SJ313: Ciss = 210 pF, Coss = 90 pF, Crss = 45 pF, Yfs = 0.7 S

IRF9510: Ciss = 180 pF, Coss = 85 pF, Crss = 30 pF, Yfs = 0,7 - 0,8 S

IRF9610: Ciss = 170 pF, Coss = 50 pF, Crss = 15 pF, Yfs = 0,7 S

When juma spoke of parasitic capacitance, which of the three numbers is the issue? Crss? In any event, IRF9510 looks pretty close to 2SJ313 by that measure, unless I misunderstood something. (ALways a possibility...)

So:

1. If the issue is linearity, then which of the graphs on the datasheet should I have been looking at?

2. Would paralleling one of the IR mosfets (or Fairchild equivalent) be an interesting idea?

3. Did I miss some other interesting alternative?

Any input very welcome, as always.

Nigel

[juma]

Originally Posted by njepitt

... I rather like the idea of paralleling the 2SJ313, but they may not be available...

Ask buzzforb, he has plenty...

[njepitt]

Hi juma,

Originally Posted by juma

Ask buzzforb, he has plenty...

I may well do that, but I'd still like to understand the stuff I posted above...

Cheers,

Nigel

[njepitt]

Originally Posted by juma

Ask buzzforb, he has plenty...

Oh, and thanks for the info...

Cheers

Nigel

[pidesd]

when paralleling 2sj313, what kind of matching is necessary? i have not found any info on this except that i needed to match the paralleled devices but it benefits only for heat sharing between devices. but i m kind of not sure.

is that correct?

also, would two sj313 in parallel at 500mA be ok for power dissipation with +-15V rails or is it pushing it?

[pidesd]

nevermind about heat...juma said in his ''f5 with 2sj313/k2013'' thread:

''It's very silent amp (even without cap multipliers) but you have to be aware of the heat
I did play with the value of the bias current and, to my ears, these MOSFETs sound best at Id between 400 and 500 mA. If you are brave and lower a PS voltage a bit (say +/-16V) at Id=600mA you'll have additional advantage of zero tempco.
Also, in this case you don't have to worry about low Vds becaus parasitic capacitances are very low (compared to usual HEXFETs) and you just want to drive a single pair.
If you do it please post your results, it would be great to see a lineamp getting hotter than many power amps... ''

still, is there drawback 2 vs 3 outout fets paralleled? i guess not really exectpt for possible higher voltage/bias...?

[pidesd]

Originally Posted by pidesd

when paralleling 2sj313, what kind of matching is necessary? i have not found any info on this except that i needed to match the paralleled devices but it benefits only for heat sharing between devices. but i m kind of not sure.

is that correct?

also, would two sj313 in parallel at 500mA be ok for power dissipation with +-15V rails or is it pushing it?

mosfets in parallel must match themselves. the CS fets dont have to match the driving fets, Mr.Pass suggested in one of his article.If you had searched a bit you would have foun! As to how exactly it affects performance, i dont know.

[njepitt]

Hi Guys,

I'm still collecting parts for my rebuild of this circuit. I've decided to rebuild the PSU also, probably in the chassis, and I brought back a nice toroidal transformer from a recent visit home to the UK, which at 250VA should be enough for a stereo build (I hope). I also bought some thin film SMD resistors and stuff for the front end around the BF862s.

I've been thinking about pisesd's question about matching. I bought exactly the 12 2SJ313s I needed, not having thought about matching too much, and not wanting to buy too many, since they aren't that cheap. I'm hoping I can match adequately within the 12 I have, but am wondering how closely the triplets should match. Anyone have ideas?

Cheers

Nigel

[pidesd]

first of all, what i would do is maybe substitute only the mosfets, with best matching with what you have, and see i'd hear an improvement. You could also build with pairs and not triplets to get better matching...?

or maybe you could build an f5 . Buzzforb matches the fets specifically for this amp