APT HOLMAN APT-1

The APT-1 is a vintage power amplifier produced from 1978 to 1985. She was designed by no other than Tom Holman, the founder of THX. To this day, the APT-1 is still regarded as one of the best sounding amplifier..

APT HOLMAN APT-1Fig 1 – APT HOLMAN APT-1 Amplifier Schematic

To begin with, the APT-1 is a fully discrete power amplifier that is exceptionally designed. At the front is a Darlington differential with a Constant Current Source for the standing current. The first leg of the differential drives the VAS which is Cascoded. This is followed by another Darlington for drivers. A total of four TO-3 power transistors are used in the current stage.

What is interesting is the use of a Current Mirror in the other differential leg. The current in this mirror is used to bias the lower transistors in the VAS. So basically, you have a very stable constant current source in the VAS. This is a very elegant use of Current Mirrors from an engineering point of view.

The op-amps are not in the signal path. One is used as a DC Offset Servo while the other is an Overload Detector.

Summary

For those who are fortunate enough to own the APT-1, it is worth restoring. After 50 years, the capacitors are bound to give trouble. The other faults are solder joints fracture due to aging and heat. These are expected from vintages. When fully restored, the APT-1 will sound fantastic with newer, more modern transistors.