Inside power amps

In my series on AC power I had used power amplifiers as the subject for good reason. Power amplifiers have the crudest power supplies and, as such, are more prone to AC power quality issues. A reader asked me a good question. Why? Why do power amplifiers have crude power supplies relative to preamps and DACs? And why don't designers spend more time and money getting supplies in amps right? Great questions. Let's take our own power amplifier, the BHK Signature, as an example. The BHK has two power transformers: a medium sized one for the input stage, a giant one for the output. Part of the amplifier is regulated, part of it is not. And here's the thing. The amplifier weighs close to 90lbs. 90lbs is already heavy–heavy enough that we often hire a moving company to deliver it to people's homes–which is better than FedEx dumping on your doorstep. But I tell you its weight to make a point. There's not a lot more we can cram into the box and still have it manageable by normal human beings. I mentioned part of the BHK is regulated, other parts are not. Most power amplifiers have no regulation. Some, like ours, regulate the input stage and not the output stage. A few, like the Sanders, regulate the whole amplifier, but that is indeed rare (and it is big). And it is rare for a reason. Size, weight, cost and benefit. The biggest benefit of regulating a power supply is in the input where the gain is formed. The output stage, where no voltage amplification occurs, is far less sensitive to the benefits of regulation. A voltage regulator is, in effect, an amplifier. It has an input, output, and typically employs negative feedback. Adding a regulator to a low power circuit, like an input gain stage, does not take a lot of circuit real estate nor does it produce much heat. But try to add a regulator to a massive output stage, like you find on the BHK, and you're likely to increase real estate and heat by a factor of 50% - and the benefits to doing so are minimal at best. Not always the most sensible choice. So, to answer the question first posed, a great deal of time and money on power supplies is already spent on properly designed amplifiers. The BHK, for example, has 8 discrete voltage regulation stages and two power transformers–a healthy power arrangement by anyone's standards. But to fully regulate the cruder, unregulated, output stage would add far more weight and heat sink area - and all for little benefit. Most modern well designed amplifiers have solved the age old problem faced by power amp designers. By a careful blend of regulated and unregulated power, great products that produce gobs of audio power are crafted.