Designed to drive Sennheiser HD-600 headphones to high levels in pure class A operation with a minimum of distortion and noise, freedom from cable interactions and as little 'euphonic coloration' as possible.
Flat to DC, with a LOT less distortion and significantly more headroom than the vacuum tube amplifier I built previously.
A separate chassis was used for the power supply to keep noise levels as low as possible. Care was taken with the design of the power supply to ensure that AC line noise doesn't make it into the supply rails, and that commutation of the rectifier circuit doesn't create any noise on its own. I measured the leakage reactance and capacitance of the tranformer's secondary windings and applied a snubber network that critically damps the resonance. The output stage is biased to 35mA of idle current, which is 10mA more than required to stay class A at full power into 300 ohms. This places the operating point of the finals into a more linear region. The output transistors are mounted to the bottom of the case for heat sinking the combined 2.5 watts they dissapate.
A switchable cross-feed circuit is present at the input of the amplifier to be used to help correct imaging problems, if desired. The operational amplifiers are biased into class A mode using constant-current sinks. Half-power bandwidth is several hundred kHz, risetime is under 500ns, and square waves are reproduced with no signs of ringing. This amp produces 30 volts peak-to-peak into 300 ohm cans in class A (375 mW rms), and will make enough current to melt 32 ohm ear-buds.
A Goldpoint stepped attenuator constructed with 0.1% Holco resistors was used for the volume control. With the exception of the polypropylene cap that gets switched in when the cross-feed circuit is engaged, there are no capacitors in the signal path.
The anplifier has about 20dB of gain, too much for some phones. In the case of my Altec Lansing UHP336 ear buds, WAY too much. Yes... This thing can play stupid loud, even into high impedance phones, but is very quiet and one can never have too much headroom....
