Class A amplifier, by Nelson Pass


Specifications (factory) :

Power: 30W/8 ohm
THD: 0.2% (3OW/8 ohm)
Freq. range: -0.5dB (2Hz), -1.5dB (100kHz)
Input impedance: 23kOhm
DC offset: under 100mV

Service manual with schematic:

(364 kb, PDF)

Schematic of speakers DC protection
and delayed attachement circuit :


Front view Inside Board in time of tunning
and detail of end MOSFETs

Experience with building and tweaking of the amplifier and links to other class A amp schematics were published in czech e-zine AMP .
My modification uses the same number end MOSFETs (6x IRP240) as Aleph 30 does. Bias was set to cca 2.6A per channel, input differential pair current to cca 20mA. I added speakers DC protection and delayed attachment circuit ( board on opposite side to transformer). I suggest to try "current mirror" for input pair. THD declines and whats more it cuts DC offset on the output (less than 10mV). R114 value (in original schematic) is reasonable set according to minimal distortion up. I use 580 ohm. Who likes experiments that can remove this resistor and convert amplifier to pure single ended ( without Pass's dynamic current modulation) machine. Impact on sonic performance is very interesting.
Measurements: Power and freq. range better than producer assigns, THD higher than factory model, cca 0.6% at full power. More you can find on pages of the AMP e-zine.(10.9.2003)



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