Inverted Shunt Regulator - Tube CAD Journal

January 2001 ^^ TUBE CAD JOURNAL ^^
Volume 3 Number 1 >>
Note the use of the a plate resistor rather than
a current source in the first circuit; it was used
purposely. It is common knowledge that a pushpull
amplifier will reject the amplification of
common mode noise, and while this statement
needs some fine tuning, it is basically correct.
What is not common knowledge is that noise
leaving a driver stage or phase splitter may not
offer a balanced common mode noise for the
push-pull amplifier to reject. For example, if one
half of the balanced signal is thick with power
supply noise, but other half is wonderfully noise
free because of some power supply
modification, then the noise will be amplified
along with the music. Removing the
modification will have the paradoxical effect of
lowering the output noise by reintroducing the
noise. Noise rejection requires balance in a pushpull
amplifier. (Here is a question: How much
noise can be rejected when one tube ceases to
conduction in a Class AB, B amplifier?)
The long tail phase splitter at first glance
would seem the easy winner in balanced noise
race and the split load phase splitter the clear
loser, as it offers a very asymmetric PSRR on its
outputs. The reality is that as long as we factor
this asymmetry in our calculations, it is not
really a concern. As all of this has been covered
here before, I will jump to the punch line: if the
split load phase splitter's grid receives half of the
power supply noise, then the amount of noise on
each output phase will be equal in both phase
and magnitude. This magic results from the
noise leaving the cathode in phase and equal
with grid and the noise leaving the plate in antiphase
to the power supply's noise: 1 - ½ = ½.
Power supply
noise
balancing in
a split load
phase splitter