Audio amplifier power supply design - Part 1: Power supply types & transformer considerations

• Has most of the advantages of linear regulated supplies, as listed above.

• Ripple can be considerably lower than for unregulated power supplies, though never as low as a good linear regulator design; 20 mV peak to peak is typical.

• There is no heavy mains transformer, giving a considerable saving in overall equipment weight. This can be important in PA equipment.

• Can be bought in as an OEM item; in fact this is virtually compulsory in most cases as switch-mode design is a specialized job for experts.

• Can be arranged to shut down if the amplifier develops a dangerous DC offset.

• Can be specified to operate properly, and give the same audio output without adjustment, over the entire possible worldwide mains-voltage range, which is normally taken as 90 – 260 V.

Disadvantages

• Switch-mode supplies are a prolific source of high-frequency interference. This can be extremely difficult to eradicate entirely from the audio output.

• The 100 Hz ripple output is significant, as noted above, and will require the usual PSRR precautions in the amplifiers.

• Much more complex and therefore less reliable than unregulated supplies. Dangerous if not properly cased, as high DC voltage is present.

• The response to transient current demands is likely to be relatively slow.

• Their design is very much a matter for specialists.

On perusing the above list, it seems clear to me that regulated supplies for power amplifiers are a bad thing. Not everyone agrees – see, for example, Linsley-Hood [2]. Unfortunately he did not adduce any evidence to support his case.

The usual claim – in fact it is probably the closest thing to a subjectivist consensus there is – is that linear regulated supplies give 'tighter bass' or 'firmer bass'; advocates of this position are always careful not to define 'tighter bass' too closely, so no one can disprove the notion. If the phrase means anything, it presumably refers to changes in the low-frequency transient response; however, since no such changes can be objectively detected, this appears to be simply untrue.

If properly designed, all three approaches can give excellent sound, so it makes sense to go for the easiest solution; with the unregulated supply the main challenge is to keep the ripple out of the audio, which will be seen to be straightforward if tackled logically. The linear regulated approach presents instead the challenge of designing not one but two complex negative-feedback systems, close coupled in what can easily become a deadly embrace if one of the partners shows any HF instability.

Before everyone runs off with the idea that I am irrevocably prejudiced against supply regulation, I will mention here that the first power amplifier system I ever designed did indeed have regulated power supplies, because at the time I was prepared to believe that it was the only way to achieve a good hum performance. Remarkably, considering that the only test gear I had was an old moving-coil test-meter, it all worked first time and without any misbehavior I could detect. I still have it in the cellar. However, I did take away from the experience the conviction that if the power supplies were more complex than the amplifier, something was wrong with my design philosophy.

The generic amplifier designs examined in this book have excellent supply-rail rejection, and so a simple unregulated supply is perfectly adequate. The use of regulated supplies is definitely unnecessary, and I would recommend strongly against their use. At best, you have doubled the amount of high-power circuitry to be bought, built, and tested. At worst, you could have intractable HF stability problems, peculiar slew-limiting, and some expensive device failures.