B&K ST-140

The B&K ST-140 was introduced at about 1982. Her rated power output was 70W into 8Ω from a pair of 2SK135 and 2SJ56 Hitachi Lateral Mosfets. The latter 140M version uses 4 mosfets in the outputs.

B&K ST-140M Power AmplifierFig 1 – B&K ST-140M Amplifier Schematic

The circuit of the ST-140 is pretty basic (Fig 1). There’s a conventional bi-polar input differential pair but instead of a collector resistors, they opted for a current mirror. This is known as an active load. The benefit is it is capable of high gain compared to using a resistor.

The differential drives the VAS with a Darlington. It’s high impedance will prevent the VAS from “loading” the 1st stage. The VAS then drives the mosfets directly. Note that no drivers are used like in most amplifiers.

Summary

There are ample opportunities to upgrade the sound of the ST-140. However, I think the biggest obstacle is replacing the lateral mosfets if they are damaged. There are no replacements for them. If so, the circuit can be modified to drive a pair of bipolar outputs like the MJ15024/25.