For some time we have been looking for a good headphone amplifier as a supplement to our BUF634 based one but this time preferably triode based, without output transformer and capable of driving low impedance headphones from 16 ohm and up.
We ended up with an Aikido 2006 design from TubeCad journal: Aikido Low-Impedance Headphone Amplifier which looks promising and can be configured in 3 configurations:
1) 12AY7 and 6H30 with feedback, and suitable for headphones from 16 ohm and up
2) 6CG7 and 6H30 without feedback, high gain and suitable for headphones from 32 ohm and up
3) 12AU7 and 6H30 without feedback, normal gain and suitable for headphones from 32 ohm and up
The quiescent current of the output triode 6H30 should be approximately 40 mA (close to maximum for this triode), and is created by a voltage drop across 2x1N4148 connected in series. We found that this circuit was giving too different results between left and right channel, and we chose to improve and replace the diodes with a constant current source LT3092 configured to 40 mA, a modification that we highly can recommend.
The high-power supply is built around a LR8N3, and a TIP162 high voltage transistor, we have used similar versions of this circuit and are pleased with it, the only disadvantage is that the voltage drop across LR8N3 need to be bigger than 12 V. We have also incorporated a PIC18F1320 in the design to
a) receive and decode infrared signals from our Apple remote.
b) operate the motor driven Alps volume potentiometer.
c) delay and connect the high voltage after 30 seconds warm up time.
The filament is driven with alternating current direct from the transformer and the potential is raised by a voltage divider to eliminate hum. In spite of this the constructions have a little bit of hum in configuration 2) and 3), and we have not been able to remove this 100%. Regarding the filament for the two 6H30 and the two 6CG7, it is a good idea to check that the triodes have approximately the same power consumption (current) at 6.3V, due to our power supply design where the 6.3V filament is connected in series.
My brother and I have both chosen configuration 1) of the finished amplifier, and we are very plesed with the result, good full-bodied sound with suitably bass on our Sennheiser Momentum 2 - simple over ear cable only version without bluetooth, a very recommendable headphone.
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