The general idea in designing/ inventing anything at all, is to get together some wonderful ideas, build them, and then find out why it don’t work. Unless you use a Computer Aided Design piece of soft brain (sorry, software), in which case you can get it to virtually design it, virtually build it, virtually test it, and come up with something you can then virtually poke around with until you get something new. Except it won’t be, because the intuitive human input has been relegated to accidental poking about and the imagination relegated to accountancy and marketing.
I am not, then, very likely to be a CAD person. In which case I shall have to think a bit. Oh dear.
We’ll start with the power supply. “Why would you do that?” you might enquire. Because if we get that wrong I am not going to get a macaroon. If that part of it is less than it should be, the rest of the amp is unlikely to be worth plugging in. So what do we want from a power supply? At its most basic, some device that takes the ac mains (240-250 volts in UK) and converts it to a useable dc voltage for the electronics. There is little difference in that requirement whether you are supplying a semiconductor (solid state, transistor; call it what you like) or valve circuit. The difference is that semiconductors tend to be higher current, lower voltage, and valves tend to want the opposite. So a power valve will be likely to have several hundred volts at the anode, whereas a semiconductor amp will need voltages of less than hundred volts. The actual voltages depend on the power output of the amp. The power transistors will put out amps, and the power valves, tens (or maybe a few hundred) milliamps.
Clearly an over simplification, and the actual figures will be dependent on the sort of power rating of the amp. But it at least illustrates the general principle.
There are two different possibilities for the power supply, and the main difference between those alternatives is in the way the incoming ac voltage is rectified.
This is a basic arrangement for a full wave rectifier circuit using solid state diodes. It could also be a bridge rectifier arrangement, but the result is the same, so we will stick with this one.
A diode will only conduct in one direction, and the transformer’s job is change the 240 volt input to a useful voltage for the amp to work with. The primary side is the 240 volt input and the secondary is centre tapped, which enables the ends of that winding to act in opposition. That means that when the voltage increases at one end, (with respect to the grounded centre tap) the other end decreases. This enables the diodes to conduct alternately in their forward direction, so producing a dc voltage from the ac input.
All this sounds fine and dandy, but the output is like a rowboat on a ski jump. Not smooth in other words. And therefore not useable for the amp, as it stands, because what you would hear most is a 100hz buzz. Most tunes really need a bit more than that.
So far there is no difference between that which the valve amp requires from a power supply and that of a transistor amp. The difference comes in with voltage/current requirement, and that brings in a big possibility with the valve amp. It can use valve rectification, and the solid state amp does not have that option.
In the next blog we shall consider the why’s of that, and what the differences are.
And just in case you might think I’ve turned sensible, I shall, especially for you, dear reader, come up with some really, really daft ones.
Have a good macaroon…..I mean…..day.