Systems Integration Asia – April-May 2018

TECH TALK

Burst Mode Power

Ampliers:

Getting More Out Than

What You Take In

by Martin M. Bonsoir

Juan Manuel Fangio, arguably one of the finest racing drivers of all
mes, is accredited to have said that the secret to success in
motorsport was to win going as slowly as possible. Some aribute
that saying to Niki Lauda, arguably one of the finest racing driver of all
mes. Regardless of who originally coined the saying, I do believe it is
arguably one of the finest phrases of all mes.

I first heard that axiom as an inquisive teenager and, over the years,
I reverse-engineered it to what I believe was meant: There's no point
going flat out all the me if the environment doesn't require it. What
one must do, is to master your resources and have them in sufficient
supply to meet demand when the me comes.

I always thought that very wise. And as it happens, it is the
philosophy behind burst mode power amplifiers.

Show your Strength

A power amplifier, colloquially known as an amplifier, or amp to
some, is a device capable of increasing the power of a signal. This
feature has varied uses in many fields, and power amplifiers are
found in the most diverse of environments. In the audio industry,
amplifiers are used to increase audio signals (usually a voltage or
current that varies with me) from a smaller to a larger level. These
applicaons include going from microphone level to line level, or
from line level to speaker level.

Of course, when specifying amplifiers to convert line level signal into
something powerful enough to drive one or more speakers, the

output power rang is one of the first items designers look at (quickly

followed by price). But alas, as many things in the AV industry, system

designers are faced with the task of comparing apples to books.

Power rangs are at best unclear, somemes misleading, and always

relave.

Even though the industry has agreed on using a single unit of

measure—the Wa (and this is a major win)—audio power amplifier

manufacturers are usually maligned for the way they rate their

product. Methods used to describe the way output power is

measured include “average power”, “peak power”, “RMS power”,

“connuous power”, “program power”, “peak momentary power”

and indeed, “burst power”. And clearly, different measuring methods

produce different results.

In their defence, I agree that measuring the capabilies of a power

amplifier is complex, as different design philosophies produce devices

with varying capabilies. The real culprit for the confusion is the

audio signal: it just won't stay steady over me.

Most audio content can essenally be reduced to two component

types for amplificaon purposes. There is connuous program

informaon, which maintains a reasonably steady RMS output level

over me (think sine wave, or noise), and peak program informaon,

which is defined by nearly instantaneous transients or spikes within

the material. Transient levels vary depending on the content, but for

most applicaons the peaks can be up to 24dB above the connuous

level. This last type of audio content is by far the most common in the

audio industry, as it includes music and human speech.

Now, the first law of thermodynamics is absolute: the energy of a

closed system is constant—it can't increase. Regardless of the

measuring technique used, no manufacturer is exempt from that. The

maximum amplificaon power can never exceed the total amount of

energy available. In other words, an amplifier can never deliver more

power than that it can draw from its supply...or can it?

Tradional power amplifiers are designed to increase the audio signal

by the maximum amount of power they can get from their supply, be

it direct mains or indirect sources, like PoE or PoE+.

There're variaons on how long the amplifier can deliver maximum

power for, as economics dictate that building an amplifier capable of

performing at its peak for extended periods of me is oen overkill

(remember that the peaks of signals more commonly reproduced are

only temporary?), but if the supply provides 100w, for example, the

maximum theorecal available power from the amplifier is 100w. End

of story. No more energy available.