At your own risk.
It should be easy to make as it is a minimal signal path for great sound quality.
Class A operation means electrons are flowing all the time trough the active amplifying device.
In this particular case they flow all the time trough the loudspeaker voice coil too, so before connecting your precious speakers test everything is OK using other small speakers. The normal current is 100mA but watch out it may increase by time as transistors warm up. This may lead to thermal runway and destruction of the speakers. Heat sinks help and the fact that it is a low power design.
Each channel uses:
1x 2N3055 transistor
1x 25KΩ biasing resistor from collector to base
1x9V battery
That's all folks.
It should make an unbelievably hi end sounding amplifier for late day listening when things around are quiet and we want the loud sounding juice of analog class A amplifiers without disturbing the neighbours.
For volume control a variable series resistor from source out to transistor base is used. No electrolytic coupling capacitor is used as the source would have one but pay attention there too. Everything at your risk.
The speaker is connected between collector and Vb=9V.
The 25KΩ biasing resistor is connected from collector to base.
Battery minus and signal ground is connected to emitter.
The sound is amazing as you expect.
The more the amp is pushed the more harmonics it produces instantaneously limiting peaks while making the sound subjectively loud. The small signals are sweet and smooth.
2 channels for stereo should fit a nice box. Bulgin 9V drawer battery holders can be used. Fuse using in series is important for safety. One can play with Vb, Rb etc to improve the sound but all precautions must be exercised. Presumably many 8Ω speakers are connected in series would give a better load and impedance matching for max power transfer. See also the Pleiades open back dipole speaker on euroelectron. With 16 speakers of 8Ω the resultant will be 128Ω. Assuming 100mA flowing the drop would be 12.8V. So a Vb of 24V could be used while having the collector settle at 12V. So excursions of 12V could exist at the output. It may be advantageous to have the quiescent point not midway as non symmetrical clipping produces more 2nd harmonic distortion which is a good thing.
Signal path, setup:
(CD: Ingenue - K.D. Lang) - CD Walkman Sony D-EJ758CK line out L and R connected together for mono - 1KΩ series resistor for reducing volume - Pleiades power amplifier - Sony APM-078
Mono can sound extremely natural, see nearby euroelectron post.
Reference:
2 - Transistor miniature A.F. Amplifier - K. Jones - Audio Amplifiers - edited by J.R. Davies - page 41 - Data Publications LTD - London
Tubes vs Transistors, is there an audible difference? - Russel O. Hamm - JAES
General references:
Electronics, a Systems Approach - Neil Storey
Applied Electeonics - T. S. Gray - MIT
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