This article is unconnected to the 1980s group Frankie Goes to Hollywood.

A lot of people go out and throw their money away on alcohol, which ends up as part of the contents of the local sewer system rather quickly, on a Friday night, particularly in the UK; but I prefer to stay in and expand my mind: No, not with some new-fangled narcotic substance. – Rather; with study and experimentation in the style of geeky-relaxation:

This particular Friday night in July it was a bit damp outside anyway with somewhat lower than average night-time temperatures for the time of year, so unless I had something amazing planned at some external venue, which I didn’t, I was definitely going to stay indoors. There was some good TV on that evening too, so I’d spend a couple hours watching that and then relax in front of and away from the computer.

I was starting to get a little vacant-minded eventually, and I started to doodle. Now when most people doodle they draw physical things or swirls or patterns or shapes or something similar. When I start to doodle then I usually start drawing an electronic circuit diagram. – Honestly I kid you not. – It’s usually something fairly simple like a Hartley oscillator or a single transistor emitter-follower output stage; but very occasionally I become fully alert while I’m doing it, realise what I’m drawing, and suddenly an idea pops into my head from which I develop something else or it takes me onto another level mentally.

This Friday night was one such event: I’d been contemplating the Darlington transistor in a kind of semi-conscious state, and went on to remark to myself inside my head on the surprising number of hits I’d had on my article regarding a Darlington-pair amplifier circuit. Still in a dreamlike state I put that thought on hold and went on to imagine ways to mix a timebase signal with a direct current to produce an alternating current using a matched pair of bipolar power transistors. – That’s when I realised that I was doodling again; and I’d started to draw a matched pair of bipolar Darlington transistors configured as a high-gain audio amplifier.

I recoiled a little with a start: That was something I’d never thought of before, despite the concept staring me in the face. I thought it might be worth taking further while the idea was fresh in my mind. I started consciously working further on what had been my doodle: I added extra decoupling to the ground points, controlled variable simultaneous negative feedback across both Darlington pairs, 2 sets of potential dividers for biasing the Darlington bases separately…

After faffing about for a while and drawing a circuit diagram with so many corrections it was barely legible, I transcribed the circuit to a fresh diagram in order that it would be legible to anyone else… Then I decided to blog it.

So – fresh out of my mind, totally unrevised and untested, I present to you my idea for a single-channel monaural audio amplifier with gain controlled by means of negative feedback utilising a ganged potentiometer.

I think it’ll work, but I have no idea how well. It’s one of these ideas I draw up that I never actually build, and it remains a theoretical triumph of unstarted construction in my head to times unlimited. Here it is anyway: -

There seems to be an error in the diagram: It appears that I’ve drawn D2 the wrong way round.

If you’re qualified in electronics please feel free to criticise, critique, comment, other words starting with C; even build it and/or improve on the design if you like: ‘Your choice. (I deliberately left the circuit diagram small enough so that you could hopefully get it all in a single browser window in FireFox at a resolution of 1024 x 768 px.)

I didn’t choose any component values other than those of the 10 nanofarad capacitors across the base and emitter of Q1 and Q3: Including them like this does actually increase audio frequency response at bass frequencies. I heard about it somewhere ages ago and have actually tried it to prove that it works: It does; to a limited extent.

Having blogged that I’m now going to get a coffee and do something else. I’ll decide exactly what as I drink the coffee.

Tatty-bye for now.