Test Amp

Project completed February, 2009

I use SoundEasy (v14) to test drivers and speaker systems. I started out using a retired Sony home theater receiver. This receiver does something most home theater receivers do, and most users will never be concerned with; It processes the line level inputs from analog sources completely in the digital domain (this includes tone control, as well as the other signal processing chores). It has no analog pass-through path, all signals entering the receiver are converted to digital signals using a 44.1 kHz sampling rate. This means that there is no audio signal content above 22 kHz coming out of the speaker terminals. This sudden cutoff causes some problems in SoundEasy.

An experienced user would have a good idea of how to tail the measured responses of tweeters, but I sure didn't. After a year acclimating myself to a complicated CAD program, written by a gentleman from somewhere in central Europe, now living in Australia, I finally understood enough to see the value in actually capturing my measurements out to 50 kHz.

This isn't required, nor is it "the answer" to higher accuracy. It's just a (relatively) easy way improve the accuracy of the phase information derived from MLS based SPL measurements. I also noted improvements in my impedance testing. While the conclusion at the end of the write-up concentrates on improvements in impedance plots generated using MLS techniques, I think these were mostly due to improvements in my measuring techniques and equipment. The real benefit for me was the ability to see the high frequency response and tail it more accurately.

The search for a cheap, easy to build power amp, with low distortion, capable of flat, extended, frequency response led me to National Semiconductor's LM3886 chip, Gain Clone amps, and Chip Amps in general. The resulting amp has worked wonderfully for many months now, and is still just as clear and noise free as it was the day I first fired it up.

The attached PDF is a detailed description of the amp I built and the results...

Jay R. Taylor

02/21/2010

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