Summit Audio TPA-200B Dual Tube Mic/Line Preamp - Manual (Page 6)

Circuit Explanation
The TPA-200B is two independent tube preamps with all signal gain occurring in the
tube portion of the circuit. The 990 op amp used for the low impedance output stage is
made of discrete components and then potted for stability. The line and mic inputs are
transformer coupled and are wired for balanced operation. A ¼” unbalanced plug inserted
into the Hi-Z input disconnects the line and mic inputs and feeds the grid of the tube
directly. The input gain control is between the first and second tube stages. The second
stage amplifier has a cathode follower for lower output impedance. The output gain
control is between the cathode follower and the 990 output stage. The tubes are operated
class A with no negative feedback. This design was chosen because it works best in the
“overdrive” mode, and sounds most like tube microphones in the clean mode. The power
supply is all solid state and regulated and the tubes heaters are D.C. operated.
The overload indicator circuit is made of a Hi-Z amplifier that is used for signal pick off
from the first tube stage. The amplifier is designed so it does not affect the operation of
the tube circuit. The sampled signal is then fed to a comparator circuit that turns the LED
on when the signal is –5dBM with respect to clipping. The overdrive indicator works the
same way, but instead of picking up the signal on the plate of the first tube, it picks up the
signal on the cathode of the last tube.