When we published this amplifier in 2007 we were well aware of the low level buzz from the power transformer and we regarded it as inevitable. After all, in a Class-A amplifier, the load current is constant and always high, regardless of how much audio power is actually being delivered. That and the amount of heat produced are the two major drawbacks of class-A amplifiers.
But that constant load on the power transformer means that it is always working hard. In this case, it is delivering over 2A from the balanced ±22V (nominal) DC supply rails. That means the peak rectifier currents can be expected to be at least 10A and it is these heavy pulse currents at 100Hz which cause the audible buzz from the transformer windings. But we judged at the time that the buzz should not be a problem with the lid on the case, even when playing quiet passages of music in a quiet room.
The two 470µH chokes are secured to a piece of blank PCB material using Nylon cable ties. This assembly is then mounted in the chassis on 12mm standoffs and secured using M3 x 6mm machine screws and washers.
And while we judged our prototype to be quite satisfactory, it is apparent that normal production variations mean that some transformers are noisier than some constructors would like. Just recently we have had a number of readers complaining and we were cogitating about the problem: maybe pot the transformer, use pliant mounting or some-such . . .
Then there was a moment of serendipity as one of our staff who had recently been reading one of the articles on Vintage Radio had a sudden thought: what if we tried a choke-capacitor filter for the power supply?
Choke input filters were widely used for the HT (high tension) rails in valve sets because high-voltage capacitors were expensive. And before permanent magnet loudspeakers became common place, the coil for the electromagnet in the loudspeaker did double duty as the choke for the power supply filter. But all of this clever circuitry fell into disuse as high-voltage capacitors became cheaper and more readily available and permanent magnet speakers became the standard. Such is the progress of technology.
So the design of choke input power supply filters has become something of a lost art. In typical valve radios, the iron-cored choke would have had an inductance of around five Henries and be rated for a current of no more than about 50 milliamps. The class-A amplifier has lower supply rails but much higher currents and in any case, we would not want such large values of inductance.
Why not? The answer is that a choke input power supply works quite differently from today’s capacitor input power supply filters. In the latter supplies, the rectifier diodes conduct for only a short time during the peaks of the AC waveform, producing the high current pulses at 100Hz, as mentioned above.
By contrast, in a typical choke input power supply, the rectifier diodes, whether they be thermionic (ie, valve) or semiconductor, typically conduct over most of the 50Hz AC waveform so the high 100Hz pulse currents don’t occur. The result is that the 100Hz ripple on the DC supply is more sinusoidal rather than sawtooth, as it is for capacitor input power supply filters.
Hence, we could expect the addition of chokes in series with rectifier outputs to the power supply board in the class-A amplifier might be able to reduce the transformer buzz. But not so fast. There is much more to choke input power supplies than meets the eye. In capacitor input power supplies, the DC rails are usually only slightly less than the peak value of the AC input waveform. For example, when 16VAC is rectified, you can expect a DC rail of about 22V. But in a typical choke input filter as used in vintage radios, the DC voltage will be a great deal less; about 90% of the RMS value of the AC voltage. That would be unusable in the Class-A amplifier because the reduction in the DC supply rails would greatly reduce the available audio power output.