What does this circuit do?

[peterc]

Hi all
I am looking at VT4 and 8 in this circuit from an AWA mic pre, but I have also seen it done in Neumann and Telefunken circuits as well.
What is the purpose of this part of the circuit? Capacitance multiplier? I am assuming it is designed to make the voltage rail stiffer and stronger.
Many thanks
Peter

 

[ccaudle]

Yes, appears to be a regulator type circuit. The voltage on the base is just a little lower than the collector voltage (96% based on the 3.3k/82k voltage divider), so think of the transistors as emitter followers driving that voltage downstream of that part of the circuit.

 

[peterc]

great, thanks CC

 

[ruffrecords]

It is a kind of active smoothing circuit. Any ripple and noise on the supply line is smoothed by R17 and C7 and it is this smoothed version that appears at the emitter. Because the current through R17 is very small you can achieve much higher ripple reduction with modest value R and C than you would need if you just directly decoupled the supply rail (which is what Neve used to do with a 1000uF or even 2200uF capacitor.)

Cheers

Ian

 

[peterc]

Thanks Ian