The inventor of the one transistor power amplifier for music

The earliest reference so far found is this: (earlier references are welcome at the comments section).


2 transistor miniature AF amplifier by K. Jones [Audio Amplifiers - Davies]


This elegant class A (electrons flowing all the time) design has the speaker itself instead of a (collector) load resistor.


(Such brilliant idea can be seen in hearing aids, vintage books on electron tube radios with one electron tube and headphones instead of a load resistor (at your risk). The Pleiades Electra headphone amplifiers use this idea too.)


In the Jones inteligent circuit an AD140 powe transistor is used for driving the 16Ω speaker. An OC44 emitter follower transistor for driving the AD140 base. Bias and negative feedback is from AD140 collector to OC44 base by a 33KΩ resistor. An 6.8KΩ base to ground resistor is connected to OC44 base. A 10μF input capacitor. A -6V battery (At your risk always use a fuse in series for fire hazard protection etc).


Immediatly impressing sound quality of this circuit.


Driver transistor was blowing all the time, possibly due to lack of heat sink.


All Pleiades lab did was to remove the driver transistor and possibly the feedback resistor connecting bias resistor to Vb. not sure yet if emitter follower is needed but modern signal levels from CD player seem enough.


Through the years the AD140 was replaced by 2N3055 and of course changing polarity to +6V as 2N3055 is npn. 2N3055 gives more treble but there is something nice about the germanium transistor sound. Possibly electron mobility? An electron behaves as lighter in Germanium than Silicon.


Changing supply to 9-12 volts and adding a french key heat sink (at your risk a proper heat sink should be used for safety). Recent change is using the BD139 as an output transistor which gives even better sweet extended treble.


A recent schematic is:


(As of writing this the Pleiades BD139 feeds a Zeiss Ikon cinema cabin fullrange speaker to great sound quality around many rooms. CD playing is a Foreigner compilation including I Want to Know what Love is).


https://euroelectron.blogspot.com/2019/05/pleiades-one-transistor-emitter.html


Reference:


Audio Amplifiers - J. R. Davies - Data Publications Ltd - London - copyright 1968