1970s Design Indulgence

No, but I don’t think you would expect me to!

Once upon a time I was working on top of a structure installing lighting to the workroom below. There were two feeds: these were electric cables of the twin and earth type, and being lighting circuits, were protected by 5 amp circuit breakers.

I had isolated the circuit breaker to the one I needed to cut, so I took my cable cutters and cut through it.

Rather unfortunately I'd identified the wrong cable!

The bang was quite loud. The flash was very bright. And within a split second, the 5 amp circuit breaker to that feed opened and cut off the current.

The cable cutters which were almost as new before the cut, now had two perfect semicircles in each blade that were a few thicknesses larger than the conductors they had cut through. Not only that, but the nice shiny metal had become black, not only with the soot from the rapidly burning PVC coating, but the black scorching went deeper into the metal.

The instantaneous current was obviously at least an order of magnitude greater than the circuit breaker rating, if not two orders of magnitude.

Had it not been for the miles of wiring (and transformers) between the power station and me, with all the resistance that entails, it would have not been just the cable cutters.

Now surely, that explains instantaneous short circuit current?

An output stage diagram by Douglas Self from his book Audio Power Amplifier Design.

I have added the VAS load resistor (top left in blue) and the arrow showing the direction of current through the Vbias spreader.

Call the value of Vbias X ohms, therefore current times resistance gives voltage, OK?

So as X ohms gets larger, the voltage across it gets larger.

The voltage here has to prevent the output stage conducting against itself, which would lead to its destruction.

Therefore Vbias is extremely important!

If the voltage gets too big - bang!

If too small - distortion!

Now also, if the current through Vbias got bigger, then for the value of X, the voltage gets bigger.

So it creeps towards the bang area.

So Vbias is difficult to control when the power supply is unregulated, OK?

Because, a transformer is simply a ratio changer. So if the HT for a 230V supply is 77V, it's going to be 4.3% more on a 240V supply, OK?

So more current is going to flow in Vbias, and the voltage is going to increase toward bang territory.

And if the supply is 220V, less current flows in Vbias, and the voltage reduces toward distortion territory.