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A Pure Class A Dynamic Headphone Amplifier

A Pure Class A Dynamic Headphone Amplifier

This amplifier can output up to 0.5W into a 32-ohm load. It is a pure class A design containing a new never-before-seen servo loop that is not part of the audio signal chain in any way.
Kevin’s Rules of Proper Audio Design

Capacitors in the audio signal path are BAD. Even the best silver-mica or poly caps exhibit non-linearities at low voltage levels. Capacitors belong in power supply sections and nowhere else. Capacitors used to compensate an amplifer generally mean that the amplifier is otherwise unstable, with poles in the right half plane and is therefore a bad design.

Transformers in the audio signal path are even worse: non-linearities in the gain structure, parasitic capacitance between windings, impedance problems…. Transformers belong in linear power supplies and nowhere else.


Ultra high open loop gain: REAL, REAL BAD!!! That basically means anything with an opamp in it. Opamp circuits with open loop gains of 10,000 or more require large amounts of feedback to make them usable. While this reduces THD, the intermodulation products, and especially the transient intermodulation products are much higher than they should be.

Servo loops MUST NOT be in the audio feedback loop. This rule is also very important. Two of my favorite high-end audio electronics manufacturers put servo loops into the minus input of their amplifiers. Most other manufacturers that use servo loops do the same thing. opamps used for servo loops do not have an output impedance low enough to make them suitable for this purpose. Furthermore the dynamic output impedance of opamps adds non-linearities to the audio when put in series with the gain resistor on the minus input.

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