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

Figure 9.2: A simple unregulated power supply, including rectifier-snubbing and X-capacitor

In a multichannel amplifier, the power supply will fall into one of three types. In order of increasing cost, and allegedly decreased interaction between channels, these are:

1. The transformer, rectifiers, and reservoir capacitors are shared between channels.
2. Each channel has its own transformer secondary, rectifiers, and reservoirs. There is a single transformer but only the core and primary are shared.
3. Each channel has its own transformer, rectifiers, and reservoirs. Nothing except possibly the mains inlet and mains switch are shared.

For amplifiers of moderate power the total reservoir capacitance per rail usually ranges from 4700 to 20,000 µF, though some designs have much more. Ripple current ratings must be taken seriously, for excessive ripple current heats up the capacitors and reduces their lifetime. It is often claimed that large amounts of reservoir capacitance give 'firmer bass', presumably following the same sort of vague thinking that credits regulated power supplies with giving 'firmer bass', but it is untrue for all normal amplifier designs below clipping.

I do not propose to go through the details of designing a simple PSU at this point, because such information can be found in standard textbooks, but I instead offer below some hints and warnings that are either rarely published or are especially relevant to audio amplifier design.

Mains Transformers
The mains transformer will normally be either the traditional E-and-I frame type, or a toroid. The frame type is used where price is more important than compactness or external field, and vice versa. There are various other types of transformer, such as C-core or R-core, but they do not seem to be able to match the low external field of the toroid, while being significantly more expensive than the frame type.

The procurement of the mains transformer for a given voltage at a given current is simple in principle, but the field of audio power amplifiers always seems to involve a degree of trial and error. This is because when transformers are used in unregulated power supplies for audio power amplifiers, the on-load voltage has to be accurate; the power output in watts depends on the square of the rail voltage. Watts do not have a direct relation to subjective volume, but are psychologically an important part of the written spec.

An amplifier that on review does not quite meet its published power output gives a poor impression. The subjective difference between 199 and 200 W is utterly negligible but the two figures look quite different laying there on the paper. It is therefore normal practice to err on the side of higher rather than lower output power; this should not be taken too far as the amplifier will be running hotter than necessary.

The main reason for output power error is that the voltage actually developed on the reservoir capacitors depends on losses that are not easily predicted, and this is inherent in any rectifier circuit where the current flows only in short sharp peaks at the crest of the AC waveform.

Firstly the voltage developed depends on the transformer regulation, i.e. the amount the voltage drops as more current is drawn. (The word 'regulation' in this context has nothing to do with negative-feedback voltage control – unfortunate and confusing, but there it is.) Transformer manufacturers are usually reluctant to predict anything more than a very approximate figure for this.

Voltage losses also depend strongly on the peak amplitude of the charging pulses from the rectifier to the reservoir; these peaks cause voltage drops in the AC wiring, transformer winding resistances, and rectifiers that are rather larger than might be expected from just considering the mean DC current. Unfortunately the magnitude of the peak current is poorly defined, being affected by wiring resistance and transformer leakage reactance (a parameter that transformer manufacturers are even more reluctant to predict), and calculations of the extra peak losses are so rough that they are of doubtful value. There may also be uncertainties in the voltage efficiency of the amplifier itself, and there are so many variables that it is only realistic to expect to try two or even three transformer designs before the exact output power required is obtained. I have run projects where the transformer was exactly right the first time, but that was maybe 10% of cases, and I might as well be honest and put them down to good luck.

The power output of an amplifier depends on when it starts clipping – a common criterion is that the rated power is given when the THD due to clipping reaches 1%. Given the usual unregulated power supply, clipping is controlled by the troughs of the ripple waveform rather than its peaks, and the depth of these troughs is a function of the size of the total reservoir capacity. Since large electrolytics have relatively wide tolerances, this introduces another uncertainty into the calculations.

Secondly, the voltage losses in the power amplifier itself are not that easy to predict, some of the clipping mechanisms being quite complicated in detail. The inevitable conclusion is that the fastest way to reach a satisfactory transformer design is to make only approximate calculations, order a prototype as soon as possible, and fine-tune the required voltage from there.

Since most amplifiers are intended to reproduce music and speech, with high peak-to-average power ratios, they will operate satisfactorily with transformers rated to supply only 70% of the current required for extended sine-wave operation, and in a competitive market the cost savings are significant. Trouble comes when the amplifiers are subjected to sine-wave testing, and a transformer so rated will probably fail from internal overheating, though it may take an hour or more for the temperatures to climb high enough. The usual symptom is breakdown of the winding insulation, the resultant shorted turns causing the primary mains fuse to blow. This process is usually undramatic, without visible transformer damage or the evolution of smoke, but it does of course ruin an expensive component.

To prevent such failures when a mains transformer is deliberately underrated, some form of thermal cut-out is essential. Self-resetting cut-outs based on snap-action bimetal disks are physically small enough to be buried in the outer winding layers and work very well. They are usually chosen to open the primary circuit at 100 or 110° C, as transformer materials are usually rated to 120° C unless special construction is required. Once-only thermal cut-outs can also be specified, but their operation renders the transformer almost as useless as shorted turns do – it is rarely economic to rewind transformers. The point is that they are required for safety reasons; the transformer will fail in a controlled fashion rather than relying on internal shorting and consequent fuse-blowing, and they are significantly cheaper than self-resetting cut-outs.

If the primary side of the mains transformer has multiple taps for multi-country operation, remember that some of the primary wiring will carry much greater currents at low-voltage tappings; the mains current drawn on 90 V input will be nearly three times that at 240 V, for the same power out.