A Devious Alternative to Regulated Power Supplies
In the list of the advantages of linear regulated supplies set out above, the one that seems to have most appeal to people is the first. It allows an amplifier to approximate more closely to a perfect voltage source, which would give exactly twice the power into 4 Ω than it gives into 8 Ω. In the not always rational world of hi-fi, this kind of amplifier behavior is often considered a mark of solid merit, implying that there are huge output stages and heavyweight power supplies that can gracefully handle any kind of loudspeaker demand. I disagree, for the reasons set out above, but let's follow the train of thought for a bit, until it derails.
A regulated supply clearly gives a closer approach to this ideal than an unregulated supply whose voltage will droop when driving the 4 Ω load. However, even if the regulated supply is as stiff as a girder of pure unbendium, there will still be load-dependent losses in the output stage that will make the 4 Ω output less than twice that into 8 Ω.
Assume for the moment that we have an amplifier which gives 100 W into 8 Ω. There will be emitter resistors in the output stage, and the lowest value they are likely to have is 0.1 Ω. (There are good reasons why these resistors should be as low as practicable, because this improves linearity as well as efficiency – see Chapter 6.) These resistors are in series with the output and so form a potential divider with the load. Their presence alone, without considering other losses such as increased output device Vbe values at higher currents, and the wiring resistance, will cause the 4 Ω output to be 195.1 W rather than 200 W. That perfect voltage source is not so easy to make after all.
However, to make a rather ambitious generalization (and all generalizations are of course dangerous) it can be said that the power deficit from this cause is rather less than that due to unregulated supply rails drooping, which can cause twice the loss in terms of watts. This factor depends very much on how big the mains transformer is, how big the reservoir capacitors are (because that affects the depth of the ripple troughs, which is where clipping occurs first) and so on – I said it was a generalization. It is therefore perhaps worthwhile to look a little closer at the regulated supply issue.
I was once faced with this situation: the managing director wanted exact power doubling in a high-power design, but I was less than enthusiastic about trying to make heavy-current regulated power supplies work dependably. Time for some thought.
If you accept that there is no problem in making a hum-free amplifier that runs from unregulated and ripply rails – which is emphatically true, as demonstrated in the second half of this chapter – then the function of the regulators is simply to keep part of the supply voltage away from the amplifiers. In effect, the output stage is a giant clipping circuit. So why not do the clipping at the input of the amplifier, where it can be done with a couple of diodes, and go back to an unregulated power supply? The idea is shown in Figure 9.1.
Figure 9.1: Putting a small-signal clipping circuit at the amplifier input to emulate a regulated power supply
The electrical power previously wasted in the regulators is now absorbed by the output devices, perhaps necessitating a bit more heat-sinking, but all the complications of regulators disappear. As with a regulated supply, the clipping will be clean and uncontaminated by ripple – in fact probably cleaner because a small-signal clipping circuit will have no time-constants that may gather unwanted charges during overload.
Now you may think that this is cheating – the managing director certainly did, but even he was forced to admit that what I proposed was functionally identical to an amplifier with regulated supplies, and much cheaper. However, the idea of deliberately restricting amplifier output – and this new approach simply makes it obvious that that is what regulated supplies do – did not appeal to him any more than it does to me, and the project went forward with unregulated supplies. And no hum.
In the foregoing argument there is one point that has been oversimplified a little. Making a small-signal clipping circuit is straightforward. Making a clipping circuit that is wholly distortion-free below the clipping point is anything but straightforward. As I described in Chapter 2, it can be done, with some non-obvious circuitry. You will, I hope, forgive me for not revealing it at the moment, but I rather hope that someone might buy the idea off me.