Diode-Compression Power Amplifier

This circuit is an attempt at using diode-compression within a power amplifier. It's based on a design by Bruce Depalma as published here.

The diodes provide peak compression when the amplifier is driven hard. (For hi-fi use, the diodes may be replaced with jumpers and the 470K resistor eliminated. The waveforms below show the difference.)

DePalma basically designed a solid-state version of the Williamson Amplifier, which takes feedback to the input cathode. This arrangement is extremely stable, and the audio quality is excellent.

Circuit Notes:

  • Resistors are 1/4W unless noted.
  • Connect the three points marked "FB" together.
  • 470K controls compression level. (Values between 100K - 470K are acceptable.)
  • The .1uF/1K network alters the gain and frequency response. (The default value is suitable for use with 9V preamp circuits.)
  • Transistors in the second stage need small heatsinks.
  • Vbe multiplier goes between the output transistors on the heatsink (see pictures below).
  • Volume control is a 10K audio taper before the input cap.

    Biasing:

  • Before connecting speakers, trim the balance control to get zero volts at the output.
  • Connect a voltmeter between the output emitters, and adjust the bias to get 75mV. Note: You may have to go back and forth between the two controls, as one can throw the other off.
  • Connect the speaker (or load resistor) and double-check the values again.
  • Let it warm up for awhile, check again, then glue the trimpots into place with silicone adhesive.

    Normal Operation:

    Diodes in Effect:

    Pictures:


  • Copyright 2009 Joe Davisson. All Rights Reserved.