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Quattor 21st June 2004 08:34 PM

Simple question...
2 Attachment(s)
Hi ya all,

Please take a look at the attached image. This is the output stage of my preamplifier. I'm wonderig, which one of those two transistors (Q17, Q18) should I mount on the heatsink of output trannies. My guess is Q17, but I'm not sure. What will happen if I mount both trannies on the heatsink? Better or worse regulation (not that it really matters in this case since this is a preamp, not a power amp, but I want to make it right)?

Thanks in advance!

anatech 21st June 2004 09:11 PM

Hi Quattor,
You can try Q17, if you still have a positve tempco on your bias, mount Q18 as well. If still positive, add some diodes or increase the heatsink size.

Quattor 22nd June 2004 10:12 PM

still interested...
I need this answer since I'm in a phase of making the PCBs. I've made them with both trannies mounted on the heatsink of output transistors, but it's not too late to change this if needed. So, what should I do?

anatech 22nd June 2004 11:57 PM

small pcb
Hi Quattor,
Why not either plan to mount both. You can pull one away if you end up with a negative temperature coefficient. The other method would be to use a small pcb for you bias and mount it higher up. Circuit changes would be even easier then.

Upupa Epops 23rd June 2004 06:03 AM

Steep of regulation you can easy to change by value of R 16. On heatsink you can give both transistors or only Q 15.

sreten 23rd June 2004 12:00 PM

Q17 is the sensor, Q18 mounting is not relevant.

:) sreten.

peranders 23rd June 2004 12:24 PM

Re: Simple question...

Originally posted by Quattor
Hi ya all,

Please take a look at the attached image.

Just wondering, why don't you just use one single transistor and two resistors (plus a pot)? What good does the extra transistor do?

Quattor 23rd June 2004 05:41 PM

Peranders - I didn't want to mess up with the original design that much. It performed so well on the simulator that I just had to built it the way it was given. Anyway, stripping it to one transistor is not that hard once I've sorted this one out :-)...

Thanks guys!

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