Let's feed our valve

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Now that we've gone to all the trouble to learn how to convert AC to DC and achieve the correct voltages we need for our equipment, it's perhaps time to send that power into our valve (the amplifier) and turn it into music. I have said since the beginning of this series that the quality of the power in our supply is directly responsible for the the amp's performance. This is true regardless of the type of amplifier: preamp, power amp, source amplification, etc. Because of this fact, we will want to put our best 'power foot' forward when we present our voltage to the amplifier.

If there's noise or vestiges of AC remaining on the supply, the amplifier can pass that along to the loudspeaker or device following, thus adding or subtracting that which is not wanted. To counter this we must make certain the supply is as clean as possible. In the case of a small signal device, such as a preamplifier or source, that task is relatively easy: just regulate the power. But in the case of a device that delivers wattage, such as a power amplifier, it becomes problematic. 99% of all power amplifiers have unregulated power supplies. They have unwanted ripple and noise that gets worse as the power requirements become greater. Certainly there are a handful of amplifiers that have regulated supplies, but they are far and few between. The reason they are not regulated is one of heat and efficiency. A regulator for a power amplifier may be as power hungry as the amplifier itself, since all the power must pass through that regulator. So, what's a designer to do? We regulate the input stage and not the output stage.

The input stage of a power amplifier resembles that of a preamplifier: small voltage in, larger voltage out. It is not delivering much power, therefore it's an easy matter to regulate its power supply. Because all the actual voltage gain is handled in the power amplifier's input stage, all that's left is the output current amplification stage which is many times less sensitive to noise and ripple. So, it's a good bargain.

Understanding where the cleanest power supply matters is essential to making a practical design of a power amplifier.

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Paul McGowan

Founder & CEO

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