MOSFET Headphones

Posted on Nov 20, 2012

This headphone buffer is based on Greg Szekeres' MOSFET Headphone Driver. It is a robust and reliable zero feedback, class A circuit. [Editor: And like the Szekeres MOSFET amp, this amp has unity voltage gain. Therefore, the audio source must supply adequate voltage to drive headphones.] The power supply is choke regulated. The circuit is configured for low impedance headphones like my 32-ohm Grado SR60s. Greg's circuit was designed to be driven from the output of a preamplifier, as high frequency rolloff will result when a volume control feeds directly into the IRF510 MOSFET, due to the MOSFET's high input capacitance.

MOSFET Headphones
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If a signal from the volume control feeds directly into an IRF510 MOSFET, there will be a high frequency rolloff due to the MOSFET's high input capacitance. Contrary to popular belief, the output coupling capacitor is not the source of the limited high frequency response. Two unbypassed low impedance 470uF electrolytics were used in parallel, and the response was less than a quarter of a dB down at 50 kHz! The output impedance of the JFET follower is only a few hundred ohms, so it drives the MOSFET with a flat frequency response. Since it has a very low input capacitance, a 100 kOhm volume control can be used without penalty. A flat frequency response translates into less coloured, more realistic sound (though not necessarily the most likeable). The frequency reponse of this amplifier, measured with a real load (Grado SR60s), is 20-20kHz ±0.2 dB. The amp's output impedance is a constant 3 ohms. This amplifier project took more time than I expected. I am more familier with tube circuits, and the learning curve for solid state circuitry was steep. What I had not properly understood was how critical power supply design is, for high current circuits like this. Voicing this amp is all about the power supply. The amp had been at the "good but not perfect" stage for several months. I made changes, improvements even, but I did not nail it until I concentrated on the power delivery channel, (a.k.a. the region between the power...

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