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AudiocRavings, by Wayne Myers:
4 - Fun With Audio Specs - Power Bandwidth

Power Bandwidth, A New-Old Spec

A Power Bandwidth spec for an audio power amplifier is, relatively speaking, a newer kind of spec than most, but has been around in some form since the 70’s. That was when popular op amps were finding their way into amp designs, and the critical listeners of the world were hearing something very wrong in some designs that should have sounded much better. I was working in Detroit for Robert Bloom’s Audio Designs and Manufacturing (ADM), and our circuit designer, John Monforte (do a Google search, he has gotten around) was trying to demonstrate Transient Intermodulation Distortion (TIM) for us. TIM was a new term, and not everyone in the company believed it was a real thing. So John built two circuits, one with a cheap, generic 741 op amp that had a horribly slow slew rate but measured great at 1 kHz, and the other with a newer op amp that had a much faster slew rate.

We could all hear the difference with the right music. The 741-based amp had a grainy, harsh sound at certain points of the demo track, the other did not. As a result of John’s demo, ADM’s whole line of circuits and products changed for the better.

The Slew Rate of an amplifier determines
the limit of its undistorted output at high
frequencies and high output levels. Here
the triangular line (blue) shows how the
sine input (red) would be affected by
slew rate limiting.

One little problem the industry faced was how to measure it quickly and easily, with repeatable results.

Slew Rate or Power Bandwidth?

Measuring TIM directly was, and still is, somewhat problematic. Slew rate, the fastest possible rate-of-change for a signal out of a given amplifier, is an indirect yet closely-related measure and is easier to accomplish, but still requires specialized test equipment. Power Bandwidth, also an indirect measure, is another way of viewing conditions related to TIM, and is quite easy to measure, How are all of these measurements related?

TIM occurs when some part of the amplifier circuit temporarily falls behind the most rapid movements of the signal, and simply cannot keep up with it. Remember, we are talking about high frequencies and high output levels here. When the feedback-and-control part of the amplifier’s circuit cannot keep up, the output is temporarily out of control. It is like driving a Ferrari F40 fast on a winding road, but with one sleeve of your shirt caught on the gear shift lever and the other caught on the door opener. You will not be able to turn as sharply as you should to stay on the road, so you will run off into the weeds when a tight curve comes along. Staying in your proper lane represents very low TIM when a sharp transient (a tight curve) comes along, occasionally swerving into the other lane or onto the shoulder is somewhat higher TIM distortion, and hitting the weeds beyond that on either side is high TIM distortion. With modern amplifier designs that have a high enough slew rate to support Power Bandwidth to 20 kHz and beyond, your F40 can and should stay squarely in your proper driving lane, your shirt sleeves unencumbered and your arms able to respond to the road with ease.

Stray capacitance can contribute to slower slew rate.
Capacitance intended to stabilize a circuit
can also make its TIM worse.

How Does You One Measure It?

Do not try this at home. Actually, do if you want to, just be sure you use a non-inductive resistive load that can safely handle you amp’s power for several minutes at a time.

Once the amplifier is properly loaded, it is simply a matter of setting the level and amplifier gain for full power output with a 1 kHz input, then running a sine sweep up to 22 kHZ or so from a source like Room EQ Wizard. You might or might not see the frequency response (FR) start to fall off at 20 kHz. If it falls off no more than 3 dB at 20 kHz, then that amplifier passes the 20kHz Power Bandwidth test. This is not a direct measure of TIM, but it tells you that the amplifier will do what is needed to keep up and can play without signs of TIM. A proper Slew Rate spec will differ somewhat depending on power level and load, but a 20 kHz Power Bandwidth number has the same meaning for any amplifier, big or small, 8 or 4 Ohm load, so a Power Bandwidth spec wins the award for convenience and ease of understanding.

The Power Bandwidth test is not unlike having closely-spaced orange cones on a straight stretch of road. If you can keep that F40 on the road while zipping back and forth between the cones at the speed limit, you have proven the maneuverability necessary to stay squarely on the road even through the tightest turns that road will ever throw at you (your amp will have very low TIM characteristics).

This current DAC spec contains Power Bandwidth
of 100 kHz at different load and power levels and
also gives a slew rate spec.

Few are likely to try to run a Power Bandwidth test at home, at least not for fun. Just check the amplifier’s spec sheet. Spec sheets never lie. Right?

Welcome to AudiocRavings, my blog of audio-related thoughts, musings, ideas, discoveries, suggestions, rants, and ramblings. With luck, a portion will be somewhat useful to someone somewhere somehow.

Wayne Myers
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