Sidebar 3: Measurements
I examined the EMM Labs MTRS's measured behavior with my Audio Precision SYS2722 system. After connecting the amplifier to the wall supply with the supplied Kimber Ascent cable, I preconditioned the MTRS, following the CEA's recommendation of running it at one-eighth the maximum power into 8 ohms for 30 minutes. At the end of that time, the temperature of the top panel was 80.6°F (27.0°C) and that of the heatsinks 94.4°F (34.7°C). Given the relatively low heatsink temperatures and the amplifier's massive size, I subsequently ran it for an hour at one-third the maximum power into 8 ohms. (With a class-AB amplifier, one-third power results in the maximum heat dissipation in the output devices.) At the end of that time, the heatsinks were somewhat warmer, at 104.2°F (40.1°C). The MTRS has more-than-sufficient heatsink capacity for its high output power.
I performed all the testing using the EMM's balanced inputs, then repeated some tests with the single-ended inputs. (Although this wasn't mentioned in the manual at the time, I needed to short the XLR jacks' pins 1 and 3 for these tests, as otherwise the amplifier's gain changed unpredictably.) Both input types preserved absolute polarity, ie, were noninverting. The MTRS's input impedance is specified as 100k ohms for both input types. I measured 114k ohms at 20Hz, 100k ohms at 1kHz, and 90k ohms at 20kHz for the balanced inputs. The single-ended input impedance was 47k ohms at 20Hz and 1kHz, 40k ohms at 20kHz. The voltage gain is specified as 30dB for both balanced and single-ended inputs. I measured 29.6dB at 1kHz into 8 ohms for both types.
Fig.1 EMM Labs MTRS, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right channel red), 4 ohms (left cyan, right magenta), and 2 ohms (green) (1dB/vertical div.).
Fig.2 EMM Labs MTRS, small-signal 10kHz squarewave into 8 ohms.
The EMM amplifier's output impedance, including the series impedance of 6' of spaced-pair cable, was relatively low at 0.22 ohm at low and middle frequencies and 0.235 ohm at the top of the audioband. As a result, the variation in the frequency response with our standard simulated loudspeaker (fig.1, gray trace) was minimal. The response into resistive loads (blue, red, cyan, magenta, and green traces) was flat in the audioband, then started to roll off above 40kHz. The wide small-signal bandwidth correlated with short risetimes in the EMM's reproduction of a 10kHz squarewave into 8 ohms (fig.2), and commendably, there wasn't any overshoot or ringing.
Fig.3 EMM Labs MTRS, spectrum of 1kHz sinewave, DC–1kHz, at 1Wpc into 8 ohms (left channel blue, right channel red; linear frequency scale).
Channel separation was 85dB in both directions across the audioband. The unweighted, wideband signal/noise ratio, taken with the balanced input shorted to ground, was an excellent 77dB (average of both channels) ref. 1W into 8 ohms. This ratio improved to 88.7dB when the measurement bandwidth was restricted to 22Hz–22kHz, and to 91.4dB when A-weighted. This is a quiet amplifier. Spectral analysis of the low-frequency noisefloor while the EMM drove a 1kHz tone at 1Wpc into 8 ohms revealed a low random noisefloor, and while even- and odd-order harmonics of 60Hz were present, these all lay at or below –100dB (fig.3).
Fig.4 EMM Labs MTRS, THD+N (%) vs 1kHz continuous output power into 8 ohms.
Fig.5 EMM Labs MTRS, THD+N (%) vs 1kHz continuous output power into 4 ohms.
Fig.6 EMM Labs MTRS, THD+N (%) vs 1kHz continuous output power into 2 ohms.
EMM specifies the MTRS's maximum continuous power as 330W into 4 ohms (22.18dBW), but there is no mention of the 8 ohm power in the published specifications. With the clipping power defined as being when the THD+noise reaches 1%, with both channels driven the MTRS clipped at 205W into 8 ohms (23.1dBW, fig.4) and 320W into 4 ohms (22dBW). When I examined the maximum power into 2 ohms with one channel driven, the amplifier went into standby mode at 349W (19.4dBW, fig.6). As usual, I had a portable FM radio tuned to NPR during the testing; peculiarly, the radio picked up bursts of static noise at the inflexion points in the traces in figs.4–6. Perhaps this was due to the amplifier's State Control & Measurement system.
Fig.7 EMM Labs MTRS, THD+N (%) vs frequency at 12.67V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), and 2 ohms (left green, right gray).
The THD+N percentage at 12.67V, which is equivalent to 20W into 8 ohms, 40W into 4 ohms, and 80W into 2 ohms, was very low into 8 ohms (fig.7, blue and red traces), and there was only a hint of rise in the THD+N in the top octave, which implies that the MTRS has a wide open-loop bandwidth. The distortion rose across the band into 4 ohms (cyan, magenta traces) and rose higher into 2 ohms (green, gray traces).
Fig.8 EMM Labs MTRS, 1kHz waveform at 20W into 8 ohms, 0.005% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.9 EMM Labs MTRS, spectrum of 50Hz sinewave, DC–1kHz, at 50Wpc into 8 ohms (left channel blue, right red; linear frequency scale).
Fig.10 EMM Labs MTRS, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100W peak into 4 ohms ((left channel blue, right red; linear frequency scale).
The distortion waveform was predominantly third harmonic (fig.8), though this was accompanied by the second harmonic at a lower level (fig.9). Intermodulation distortion with an equal mix of 19 and 20kHz tones was low in level even into 4 ohms (fig.10). The second-order difference product lay at –104dB ref. 100W peak into 4 ohms (0.0006%). The higher-order products at 18kHz and 21kHz were higher in level, but these still lay close to –80dB (0.01%).
The EMM Labs MTRS's measurements indicate that it offers high powers with very low levels of noise and both harmonic and intermodulation distortion.—John Atkinson
