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The Benefits Of Switch-Mode Power Amplifier Technology

How a switch-mode power supply actually works, how a switch-mode amplifier processes the signal, and more

By Bruce Bartlett February 15, 2011

It’s an answer to a sound company’s prayers: low-weight, cool-running power amplifiers with low current draw.

Look for switching power supplies in power amps, and switch-mode power amplifiers.

Compared to a standard amplifier power supply, the benefits of a switch-mode (switching) supply are lower size and weight. Compared to a standard analog power amplifier, a switching amp tends to run cooler and draw less current because it is so efficient.

In addition, the amp can be smaller and lighter because of compact circuits and less-massive heat sinks.

A switch-mode power supply creates DC from the AC mains power using transistor switches and other components. A switch-mode amplifier turns its output transistors on and off at an ultrasonic rate when it amplifies the audio signal.

A conventional amp with a switching power supply is often called a “switching” or “digital” power amplifier. But so is an amplifier that uses switching in its audio amplification.

It’s hard to tell what you’re getting, so you’ll need to check the product literature for details.

To sort out the differences, let’s explain how each system works.

Switch-Mode Power Supply
As we know from experience, amps of high power tend to be quite heavy. One of the main factors in that weight is the iron in the power transformer, which is part of the amp’s power supply.

To avoid core saturation at the 50 Hz or 60 Hz frequency of the AC mains, this transformer must be large and massive. That characteristic leads to minimal inductive reactance at those low frequencies.

It would be great if the mains frequency were a few hundred kilohertz. Then a smaller, lighter power transformer could handle the AC mains without saturating. That reduces the weight of the amp as well.

In a switching power supply, the 50 or 60 Hz line frequency is converted to about 100 kHz so that a junior-sized transformer can be used. The filter capacitors can be smaller too.

See Figure 1, below. From left to right in the diagram, this is the signal path:

1. The 60 Hz mains power is fed to the switching power supply.

2. The supply rectifies and filters the incoming AC power to convert it to DC. (This is the same principle used in any AC-to-DC power supply).

3. Connected to the DC output, transistor switches turn on and off at an ultrasonic rate. They chop the DC into pulses, which effectively creates a mains power supply at a high frequency. In other words, they convert DC to ultrasonic AC square waves.

4. Next in line is a low-weight isolation transformer. The square waves pass through it by magnetic induction.

5. Finally, another AC-to-DC circuit rectifies and filters the waveform to produce DC for the power amplifier circuitry.

Figure 1: Block diagram of a typical switching power supply. Click to enlarge

A control circuit (not shown) senses the output voltage of the transformer and adjusts the switching circuit in real-time to regulate the final DC output voltage.


As you can see, switching power supplies are more complicated than conventional supplies.

A benefit of this scheme is high efficiency. That is, the amplifier wastes very little power in the form of heat. Most of the incoming AC power is converted to amplifier output power.

How does that happen? Well, remember from Ohm’s Law that voltage times current equals power. When the voltage or current in a device is zero, so is the power.

The switching or “chopping” transistors produce no current when they are switched off, and produce no voltage when switched on, so they generate very little power as heat. This results in a highly efficient power supply.

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About Bruce

Bruce Bartlett
Bruce Bartlett

Recording Engineer
AES and SynAudCon member Bruce Bartlett is a recording engineer, audio journalist, and microphone engineer. His latest books are “Practical Recording Techniques 5th Ed.” and “Recording Music On Location.”


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