GCSE Amplifier:

While still at school and studying for my GCSE exams some 12 years ago, I decided that a good project for a CDT Technology course would be a new hi-fi amplifier. While this wasn't the first amplifier that I'd built, it would be a significant upgrade to the one I already had...

Looking back, it had some nice features, such as Record-out selection, and logic-controlled relays to switch the speakers and bypass the tone controls. And after a number of tweaks and rebuilds it sounded pretty good, as well. Just before leaving home for university some 2 years later, I rashly decided to blow the money I'd earned in the summer on a replacement amplifier. I was able to compare it directly to a number of budget amplifiers - imagine my surprise at needing to stretch to a Musical Fidelity A1 to hear an improvement!

It's still working, and in daily use - my younger brother is using it along with the loudspeakers made at the same time for a different GCSE course. Recently, I got the chance to take a few pictures of it - it's not pretty inside!

Front panel view - click to enlarge...

The front panel is rather crude - it's just painted hardboard. Also, the chassis is made from thin steel which has signs of surface rust... I wanted to use aluminium, but that was far too expensive for the school!

As you might guess, this has been modified a few times. The preamp used to occupy the space on the left, until I built a different version and placed it directly behind all the potentiometers to shorten the wiring. Also, the power supply has been rebuilt on copper-clad board. I taught myself about earth loops the hard way!

The short black wires that emerge from the middle of the power-amp board go to transistors on the heatsinks. These are the v-bias devices which I neglected to place on the heatsinks originally. Another lesson learned the hard way - that cost me one output transistor and a bass driver!

The power-amp was my first attempt at a DC-coupled design. The output devices are Darlingtons, and these are fed with a very simple Lin design, using a conventional VAS and long-tail-pair input stage. The VAS collector is bootstrapped - I hadn't really learned about current sources at this young age. The smaller board at the bottom is a simple relay-control circuit, and enables the 2 speaker outlets to be switched with tactile switches from the front panel. Despite the mentioned speaker-loss, I didn't take the opportunity to add DC-offset protection...

The MM phono stage is top-left - a simple 3-transistor design, which worked well enough with the basic Shure cartridge that I had at the time.

This shows the main preamp - the sub-board is a standard Baxendal tone-control circuit, based around an SGS-Thompson audio op-amp that was popular in the 1970's. This device clips at modest levels, and is rather difficult to make work. But, back then I didn't really know much about op-amps. A relay bypasses it, controlled by the front panel 'Direct' function. This is logic-controlled, and defaults to 'on' at power-up.

Looking back, I'm amazed it worked! Bear in mind not only my age (15), but the almost complete lack of test equipment (I had an analogue multi-meter). I guess it was just a combination of patience, intuition and luck!

Shortly after that, I was given a broken oscilloscope and a few other bits and pieces. Also, I built a couple of basic bench power supplies. Work started on its replacement, and I'd prototyped power stage that looked pretty much like a standard power amp. I never built that, because, as mentioned, I bought a Musical Fidelity A1 instead. Work is due to resume any day now...