i'm thinking of using some bipolar juction transistor current sources as
active loads in a tube mic amp circuit. i need some advice regarding
PNP devices that have a BVceo of 200-300v, with low base spreading
resistance, and whatever else a 'low noise' designation implies.
preferably obtainable from somewhere like digikey. thanks.

du

dufus wrote:
i'm thinking of using some bipolar juction transistor current sources as
active loads in a tube mic amp circuit. i need some advice regarding
PNP devices that have a BVceo of 200-300v, with low base spreading
resistance, and whatever else a 'low noise' designation implies.
preferably obtainable from somewhere like digikey. thanks.

On Semi has a line of switching transistors that I think were probably
intended for CRT grid drive, and are actually surprisingly quiet. MPSA93
and MPSA92 would be the PNP ones. I used these to drive electrostatic
headphones, as well as an power amplifiers. I don't know how quiet they'll
be on a front end, though.

What about just using an SRPP stage and using another tube as the current
source?
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

thanks for the recommedation. i remember seeing this transistor type
in
a TLAudio tube mic pre being used as an emitter follower after a
plate-loaded
tube stage. is the NPN version the MPSA42/43?

i was thinking of the BJT current source since it doesn't need to eat
100 or 150
volts to do the job, like a tube current source does. of course you
need enuf room for the signal to wiggle. and i'm kinda worried about
working with 150v rails. how bad is a 300vdc shock?

also, how do you set an output voltage level with an active load
which is
really a current output (in the context of no global dc feedback)?
i've never seen a good clear discussion of this in any textbook. it
seems weird that active loading is resorted to for high voltage gain,
yet it constitutes a current output. all i can think of is to just
slap a divider across it (like say, 500k from B+ to plate/PNP
collector connection, and 500k from latter to ground) that's high Z
enuf to not lose all the gain. in other words, what kind of a
transimpedance stage is there beyond a resistor (without resorting to
an opamp)? a little knowledge is indeed a dangerous thing.

dufus



What about just using an SRPP stage and using another tube as the current
source?
--scott

"dufus" wrote in message
m...

i was thinking of the BJT current source since it doesn't need to eat
100 or 150
volts to do the job, like a tube current source does. of course you
need enuf room for the signal to wiggle. and i'm kinda worried about
working with 150v rails. how bad is a 300vdc shock?

Enough to knock your ass across the room and leave you breathless for
several minutes, if you're lucky. Forever, if you're not.

Peace,
Paul