Dynaco ST-150

Here is another vintage amplifier I have in my collection. She was introduced in about 1974. Apparently by this time, complementary output transistors were available, hence no Quasi outputs.

Dynaco ST-150 Power AmplifierFig 1 – Dynaco ST-150 Amplifier Schematic

The circuit of the ST-150 is pretty basic. There’s a input differential pair followed by a single ended VAS that drives the positive and negative drivers (see signal flow in Red). Instead of using an active current source in the VAS, the boot-strapped method is used (C8).

Restoring the ST-150 is easy but it would be a shame not to modify her. Once done, she’ll sound fantastic.

Modifications to the ST-150

Fig 2 – Dynaco ST-150 MOD Simplified Schematic

Fig 2 is a simplified schematic of what I intend to do to the ST-150. Basically, I’ve retained the topology of the original circuit, that is an input differential followed by a single ended VAS. That is about the only resemblance to the 1974 design..

There are three mods to this new ST-150. First, in the VAS, there’s a darlington as opposed to a single transistor. This will relieve the “loading” in the input differential when the amplifier is working under high power.

The Bi-polar drivers are now replaced with Hexfets. Their high gate impedance will shield the VAS when the amplifier is worked hard.

Lastly, the boot-strap in the VAS is replaced by a Constant Current Source. These modifications will vastly improve the sound quality of the ST-150.

A new pcb will be fabricated to accommodate these additions. It will be double sided with a ground plane. I am confident that with these mods, the SDT-150 will be a “killer”.