Technical Description
Circuit Description and Technical Information
The philosophy behind the design of the SET 40 is based on the observation that simplicity of the circuit and the reduction of the number of active devices in an audio amplifier will permit the greatest quantity of information to get through the amplifier. This allows the illusion of music to be most powerful and results in the amplifier 'disappearing' effectively from the playback chain. This
The primary design goal of the SET 40 was to take the idea of the single ended (SE) triode amplifier to its ultimate level, using only two triode stages to keep distortion low and reduce the number of active devices to a strict minimum, while delivering enough power so that a wide range of loudspeaker choices could be made for use in a home audio system. Unlike other high power SE triode amplifiers which use several stages of gain and buffering, the SET 40 takes advantage of the distortion cancellation and reduction which occurs when using just two triode stages. The low distortion figures at typical playback levels of
Care in the choice of active and passive parts results in an audio amplifier which will remain valid in the future as fashion and taste changes. In other words, it is likely to become a 'classic,' retaining its musically satisfying performance for many years to come. This thinking begins at the power supply which uses two high value PI filters with
The output stage uses the 845 triode (a tube which has seen many, many years of use as an audio amplifier in high power radio station
modulators) to drive a custom output transformer which provides superb power handling and bandwidth. The 845 tube is run at maximum power in radio stations for over 8000 hours, you should get similar performance life in the SET 40.
The SET 40 bandwidth is similar to many amplifiers which use feedback and
The use of self bias for the 845 tube allows a higher grid resistance to be used, permitting a single high gain, relatively low power, input/driver stage. This type of bias scheme is more difficult to implement than grid bias schemes, requiring high quality, high value decoupling capacitors, large power resistors and a higher voltage on the main supply. (This voltage is near 1200 V DC on the SET 40!)
The input/driver stage uses two 12AX7s in a modified SRPP stage which provide total gain of 30 dB and 1 K ohm of output impedance. This input stage can supply over 100 Vrms at low distortion into the 400 K ohm grid resistor used on the 845. The SRPP type load provides power supply rejection and reduces the sensitivity of the input stage to power supply variations. It also reduces power supply modulation and avoids the problems inherent in the use of high power/high voltage resistor load devices.
SET 40 Owner's Manual | Page 9 |
