For the measurements below, the Modulus-86 was operating from a ±30 V power supply. Click on any of the graphs below for a larger view. The graph below shows the THD+N vs output power for 8 Ω load. The amplifier delivers 40 W at the onset of clipping. Note that the sharp jumps (aside from when the amplifier clips) are caused by range switching in the APx525. The THD+N vs output power plots mostly show the THD+N floor of the measurement system.

Repeating the measurement with a 4 Ω load reveals:

The onset of clipping occurs at 70 W. The THD+N vs frequency plots for 40 W into 8 Ω and 65 W into 4 Ω, respectively, are shown below. Note that the measurement bandwidth was changed to 60 kHz to capture at least three harmonics of 20 kHz. This also increases the noise bandwidth, hence the THD+N, of the measurement.

The Modulus-86 operates in Class AB, so the plot below may appear a bit out of place as it shows the THD+N vs output power and frequency measurement commonly found in data sheets for Class D amplifiers. I am including it here to showcase that the Modulus-86 performs 10-100× better than most Class D amplifiers.

Siegfried Linkwitz argues that the 1 kHz + 5.5 kHz intermodulation distortion (IMD) measurement is one of the measurements which is more indicative of the perceived sound quality. He bases this argument on the fact that IMD products in this measurement fall in the frequency range where the ear is the most sensitive (see the Fletcher-Munson curves for more detail). I think this argument carries a good amount of weight, so I measured the Modulus-86 accordingly. The measurement is shown below. Note that due to a limitation in the DFD IMD source of the APx525, the frequencies used must be an integer multiple of each other. Thus, I measured at 917 Hz (5500/6) + 5.5 kHz. I performed this measurement at 1.0 W. The result is shown below. Note that the performance of the Modulus-86 is over 20 dB better than the performance of any of the amps shown on Linkwitz's site.

The more conventional IMD measurements are shown below. The two plots show the SMPTE (60 Hz + 7 kHz @ 4:1) IMD and DFD (18 kHz + 19 kHz @ 1:1) IMD, respectively. Poor SMPTE IMD is often indicative of thermal issues or power supply issues in the amp. The 18k+19k IMD is indicative of the loop gain available in the amp near the end of the audible spectrum, which can be telling of an amplifier's sound quality. The Modulus-86 provides excellent performance on both of these measurements.

Audio Precision has developed a multi-tone test signal, which contains 32 tones from 15 Hz to 20 kHz, logarithmically spaced in frequency. This test signal sounds a bit like an out-of-tune pipe organ. It is basically the closest I can get to music with a deterministic test signal. Thus, I argue that this multi-tone signal should be used in an IMD test for the best correlation between measurements and perceived sound quality. I run this test at levels just below clipping (40 W, 8 Ω, which is also the 0 dB reference in the plot). Note that even the tallest IMD components are 132 dB below clipping level! This is likely why the Modulus-86 sounds transparent. This measurement shows that it does not add anything (or at least extremely little) to the source signal, even at levels just below clipping where the amplifier is working the hardest. Also note that the amplifier output is completely free of mains-related hum or noise.

The Modulus-86 shows only a tiny amount of residual mains hum. Note that this measurement was taken with the amplifier board sitting unshielded on a lab bench, thus, actual performance once enclosed in a metal chassis is likely to be better. The plot below shows the noise floor of the amplifier when powered by a pair of well-regulated laboratory power supplies (HP 6643A).

For completeness, I measured the amplitude response and gain flatness as shown below.

As mentioned in the Key Features, the Modulus-86 features a differential input. The common-mode rejection of this input is shown below.

Finally, the output impedance and resulting damping factor for 8 Ω load are shown below.