PA1BJ Antennes e.d.: 20 milliwatt audio amplifier with 800 ohm speaker and great sound

Miniature audio amplifier 1 to 20 mW.

This LF amplifier has an output of 20 mW. This at 12 V / 2mA. That sounds rather loud!
At 6 volts it has already an undistorted sound and
volume (about 1 mW) is just adequate to listen about 1m away.
It works great, when you use it with a crystal receiver set. Then amp
already givess somewhat sound on the energy of a (not too small) candle flame,
An 800 ohm speaker is neccessary but they become scarce.

Technical:

It's a push-pull design with differential amplifier.
It automatically stabilizes the voltages, in particular the so-called mid rail.
That means it works between 5 and 24 volts.
The efficiency is high.

By means of the 330 K resistor neg. feedback takes place. The mid-rail voltage
is thus always held at half the supply voltage.
I started designing by dividing components by 100, from a 8 ohm speaker design.

The quiescent current is adjustable with the potentiometer of 10 K from 0.1 mA to 1.5 mA.
The crossover distortion is very slightly less at 1.5 mA. But there's not much difference.
On a oscilloscope no crossover distortion was visible.
The two 'final transistors' have been selected on equal HFE.

Power supply:

The energy provision I do preferably by means of a mini- 12V solar panel but this
can of course be any supply.
Speaker is an 800 ohm type from an old Philips TV. These are pretty sensitive.
LF few volts is sufficient.
In 2000, I had already made a three transistor version but this circuit
yet again works better because it adapts to any voltage between 5 and 24.

Performance:

The amp I made on prototyping board I mounted in the loudspeaker-unit so that it is compact and portable.
The preamplifier is only required for weak signals of about 5 mV. One has to check for oscillations inthe amp. These can be stopped by applying capacitors of about 220 pF. Wiring of the pre-amp.has to be kept as short as posibible. With an FM crystal receiver I could hear about 9 stations on an outdoor antenna.