AA334 Input Stage

The AA334s are basic amplifiers, which means they have no volume control, tone controls or input selectors. They are intended to be controlled remotely by a preamplifier or receiver unit which provides the user interface for the audio sysytem.

Pictured here is an early version of the input stage on the test bench.

The 6SL7 is a nice sounding tube. The even spacing between the grid voltage curves is an indication of the tube's linearity. Each tube contains two identical triodes in a glass envelope with an octal (8-pin) base.

I chose the symmetrical totem pole, or SRPP topology primarily because of its inherently low distortion, low output impedance, and its ability to drive our EL34 driver stage. The two triodes in the totem pole are DC coupled and the stage is inverting.

I used Tube CAD software to determine the best operating point for our 6SL7 SRPP input stage. After entering the parameters for the circuit, hit the Calculate button and the program crunches all the numbers.

As our design progressed, eventually an entire amplifier ended up laid out and working on the test bench.

There was high voltage all over the place, completely exposed. You really have to keep your wits about you and one hand in your pocket.