a 12AZ7 buffered volume control

Goal
Build a simple, high-fidelity volume control unit that uses common tubes, has unity
gain, low output impedance, and fits in a 6"x4"x2" chassis.

Description
The circuit is a standard resistor-loaded cathode follower buffering a 100k audio
tapered potentiometer. Because the cathode follower has a gain less than one, it's
input capacitance is very low. This eliminates the problem of high frequency losses
caused by the filter that consists of volume control plus input capacitance. Once
equipped with a control such as this, all your power amplifiers can have a fixed input
impedance of 1Meg, and as much gain in the first stage as needed.

The 12AZ7 is identical to the 12AT7 electrically, except it has better heater-cathode
insulation. This allows for a 200V difference between them, as opposed to the 50V
allowable with the 12AT7. This feature is important in a circuit like this where the
cathode is over 200V above ground.

Construction
Other than minding usual good construction practices, nothing about the circuit is critical, or demands special attention. 5%
resistors, a $2 Alpha potentiometer, and Sprague orange drop caps were used throughout. The unit is housed in a
Hammond aluminum chassis.
More photos: inside, back, and side.

Performance
Gain is ~0.98. Frequency response is flat from 10Hz-1MHz, the limits of my signal generator. 100Hz, 1kHz, 10kHz, and
20kHz square waves show no ringing, rounding, or overshoot. Resistors R3 and R11 were found experimentally to remove
any tendency of the cathode follower to oscillate at IF and RF. They are not optional in this circuit.

Sound Quality
This circuit was an improvement over a similar unit I made using a 6SN7 with a 68k resistive load. It seems to be just a bit
more dynamic.