R J Carpenter wrote:

"MikeN" wrote in message
...
Not infrequently regulated power supply circuitry is found with NPN
pass transistors like the 2n3055, in the negative supply line,

I'm curious to know what are the pros and cons of putting the pass
transistors in this way, as against the more common topology of
putting them in the positive supply line?

Just think about where you are going to get the base drive if the pass
transistor is in the positive lead. You'll need a voltage considerably
higher than the output voltage.

Could you specify what you mean by "considerably higher"?

This may demand that the rectifier put out
more voltage than you'd really like, which means more drop across the pass
transistor and thus more dissipation (lower efficiency).

It's easy to get the supply for the base of the pass transistor if it is in
the return lead, without increasing the rectifier output voltage above the
minimum required to always overcome the drop in the pass transistor.