MagAmp: Choice of transistors

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Hi all,

I'm quite new to the "design" part of DIY, so I've decided to learn more by designing my own class A/B amp. I'll probably make it much like the Leach Amp, just to get the hang of it.

Here is my question:

How do you guys tackle the problem. Do you design the different stages first and then select parts (transistors), or do you let the parts rule the design?

I've got a box full of different transistors that I'd like to use (it's the cheap way to go. I've already got them you know).

Input stage:

* MPSA06 / MPSA56 or
* BC547B / BC557B or
* IRF9620 (FET's from an Aleph project)

Are they any good, or should I get others? I'll probably go for BJT's, even if I got the FET's.

Second gain stage/drivers:

* MJE340 / MJE350 or
* MJ15030 /MJ15031

Output transistors:

Here the choice is wider:

* MJ15003 / MJ15004
* MJ21193 / MJ21194
* MJ11032 / MJ11033

I've got plenty of each, so the decision is not based on availability.

What I want is a small amp. Maybe 2 output transistors (one NPN and one PNP).

Best regards,
There's no reason you can't follow any design methdology that pleases you! ;)

For the projects which I consider most special, they're often built around considering the characteristics of some high grade devices that I want to use- and the design comes from trying to optimize for them.

Other times, it's more like what you describe- I have some transistors or speaker drivers on hand, and it's more of a "let's see what we can do clever with these, without going to too much trouble".

Personally, for output devices, I like the extended beta transistors from Toshiba and the On-Semi copies of them (MJL3281, MJL1302)- their characteristics make possible a very clean output stage before the application of feedback- or just don't use feedback.

I also like JFETs for the input stage, and have done single eneded cascoded differential stages using the NPD5565, and complementary stages using the 2SK389 and 2SJ109-

I've used the MJE340/350 and the 15030/15032 series- just watch the gain fall off on the 340/350's over about 15-20 mA at high temp. I like the Toshiba extended beta small devices also, but even better are some of the Zetx high beta low sat devices, like the FZT696B and FZT796A.

Have fun and enjoy your project!


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