Class A Headphone Amplifier II

Posted on Nov 2, 2012

I built this headphone amplifier for dynamic headphones based on my rules of proper audio design. People who know my designs will realize that this amplifer is much more than just a headphone amp. 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. 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.

Class A Headphone Amplifier II
Click here to download the full size of the above Circuit.

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. The schematic for the standard (non-bridged output) version of the headphone amplifier is shown in figure 1. The open loop gain of the amplifer is about 35. Even with the feedback removed, the THD is less than .01%. That's important because the more linear an amplifier is without feedback, the more the THD, IM and TIM distortions are reduced to unmeasurable levels with feedback added. Stage 1 is a dual FET fully-differential fully-balanced front-end. The idling current is 2mA total per dual FET (1mA per FET) and 4mA for the complete front-end stage which consists of both dual FETs. The dual FETs generate the bias which runs the second stage, and keeps it and the resulting output section in class A at all times. The FETs are ultra low noise dual units specifically designed for audio uses. The total voltage gain of the first stage is 50. Stage 2 is the driver...

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