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A JFET-MOSFET Headphone Driver

A JFET-MOSFET Headphone Driver

The rolloff in the Szekeres amp is audible, but would not worry many listeners, since it tends to make for a smoother, more powerful sonic presentation overall. It is avoided if the MOSFET is driven by a JFET cathode follower with low output impedance.
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.

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