Simple question...
 

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!

Depends
 

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.

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?

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.
-Chris

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

Q17 is the sensor, Q18 mounting is not relevant.

:) sreten.

Re: Simple question...
 

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

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!