Measurement and Design Tools
For measurements, I use the Liberty instruments Praxis measurement system. This system is fairly old, but does everything I need quite well. Being an older system, Praxis necessitates that I maintain a Windows XP computer. This is still going okay, but may not be forever... Praxis does two channel measurements, so phase can be directly measured. A calibrated mic is included, so absolute SPL levels can be reliably measured. Praxis does impedance sweeps. Measurement parameters are all readily adjustable and there are plug-ins for many additional tests. To do speaker measurements, a testing amplifier is also required. I typically look for a small class AB amplifier that is likely to be well behaved with respect to frequency and load impedance. The first amplifier I used was the original Parasound Zamp, which was rated at 30 watts per channel (WPC) into 8 ohms and 45 WPC into 4 ohms. It did eventually let out the magic smoke, so it was replaced by a Sherbourn (parent company for Emotiva) 2/100 amplifier, which has a compact form factor and is rated for 50 WPC into 4 ohms.
I've had no trouble with this amp, but would be interested in how it compares to other competing projects. Actually, now that I have some amplifier testing equipment, this question can be answered... Here are a couple of pictures of the amp and the dashboard
SINAD is better than 80 dB and seems somewhat limited by power supply (60 Hz and harmonics) noise. Gain is also pretty high, so backing off on that would likely make these numbers a little better.
Frequency response (below) is flat and predictable. It is down about 0.4 dB at 20 Hz and 0.3 dB at 20 kHz and is independent of load. There is a 1 dB difference between left and right channel output, which isn't really an issue for measurements as I am only using one amp channel. The issue might be that the right channel never gets used...
Power output is as advertised. The 8 ohm curve crosses 30W at about 0.3% THD+N, at which point we have 50W into 4 ohms. At 20W into 8 ohms and 40W into 4 ohms, THD+N is less than 0.01%, which is quite good.
Unlike most Class D amplifiers, distortion is nearly independent of frequency. There is slightly elevated distortion at 5 kHz and above. but only above 3W. Overall, this is a well-behaved amp and shouldn't produce any surprises when providing test signals.
