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Apples Vs Apples: Introducing the Common Amplifier Format (CAF)

Attaining a truly meaningful comparison between various amplifier models

By Pat Brown October 7, 2013

Last year I was contracted to perform a barrage of measurements to be used to help a theme park select between several amplifier brands. They provided the list of tests, and I provided the instrumentation and the sweat.

The amplifiers were similar with regard to their power ratings, but differed in how they performed during the testing. Today’s “smart” amplifiers can behave very differently depending on the test stimulus, load impedance, and especially the duration of the test.

I found myself needing to “tweak” the test differently for each one. In some cases, I was measuring how the amplifier is programmed to respond to an event, making many comparisons “apples versus oranges” and subjective.

After days of testing, I was not satisfied that the measured results produced a meaningful comparison between the various models. This “specification dilemma” had reared its ugly head before, with regard to loudspeaker data. It was solved by the development of the Common Loudspeaker Format (CLF). 

It’s been eight years now since the development of the CLF. It gives sound system designers the information needed to deploy loudspeakers, and this intended purpose kept the format from drifting in scope and becoming bloated.

A hallmark of the format is the use of the Maximum Input Voltage (MIV) for characterizing the maximum allowable drive signal to the loudspeaker. The MIV is crucial for proper amplifier selection. It gives the maximum RMS voltage that the loudspeaker can handle, and it is the basis for determining the amplifier size required to deliver it.

The CLF is a concise format that is appropriate for its intended task. It was intentionally designed to present system designers with what really matters. The CLF has been wildly successful, and is now the main data format for nearly all of the room acoustics/sound system modeling programs. It is supported by more than 70 loudspeaker manufacturers and developers.

Setting Parameters
The success of the Common Loudspeaker Format has inspired the development of the Common Amplifier Format (CAF). The CAF is a report format that gives the sound system designer the ratings they need at a glance, along with some fine details for those who are interested. We learned from the CLF that a group of motivated individuals can quickly build the tools they need to do their jobs, and the same development approach was used for the CAF.

Like the CLF, the CAF is not a standard, and it was not developed like a standard. It was designed by a small, focus group of individuals that was later expanded to a wider audience as the format neared completion. The development team included manufacturers, consultants, and contractors, each bringing a different perspective on what the CAF should include. We knew what we needed, and we built it.

The CAF must at a minimum:

1) Give the amplifier a meaningful power rating for use in design calculations.
2) Provide information on how the amplifier behaves with various test stimuli, and into various loads.
3) Define the test procedures so that they can be repeated by anyone with the appropriate instrumentation.
4) Present a number of secondary metrics of interest to sound system designers, including plots as well as one-number ratings.

This information will allow proper deployment of the amplifier, and meaningful comparisons of amplifiers from various manufacturers. Given the nearly infinite number of variables in the testing of electronic devices, some guidance is needed. Since amplifiers are always used to drive loudspeakers, it makes sense to allow the amplifier measurement process to be influenced by some key loudspeaker attributes provided by the Common Loudspeaker Format.

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About Patrick

Pat Brown
Pat Brown

Principals, Synergetic Audio Concepts
Pat & Brenda Brown lead SynAudCon, conducting audio seminars and workshops online and around the world. For more information go to


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