A simple amp to build, uses commonly available parts and is stable and reliable.
Note that there is no output short circuit protection, so if speaker leads are shorted while the amp is working (with signal), there is a very real risk of the transistors being destroyed. Since this amp was built commercially, the savings were worth the risk – most of these amps were installed in the speaker box, so shorting was not likely (unless the loudspeaker voice coil shorted as happened a few times). Because of the cost of the devices used (minimal), it is a cheap amp to fix even if you do manage to blow it up.
Basic specs on the amp are as follows
Input sensitivity for 60 W output – just under 1V (1V gives 66W)
Gain – 27dB
Frequency response (-3dB) – 10Hz to 23kHz @ 1W
Harmonic distortion @ 1kHz – 0.05% (maximum typical)
Open Loop Gain – 125dB (no load), 80dB (8 Ohm load)
Input Impedance – 22k Ohm
DC Offset – Less than 100mV (< 20 mV typical **)
Noise – < 2mV at output (-80dB ref 50W unweighted)
Changes made from the original design are …
Reduced the value of the Class-A base resistor to 560 Ohm **
Increased the value of the bootstrap capacitor to 100uF
Reduced stabilisation caps to 100pF (they used to be 220pF)
Added the output inductor and damping resistor (see UPDATES)
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