Sidebar 3: Measurements
I measured the Verity Monsalvat Amp-60 using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). The Amp-60 has no heatsinks per se; instead, its massive aluminum chassis dissipates heat. Before I test an amplifier, I precondition it with both channels driving a 1kHz tone for an hour at one-third power into 8 ohms, which is the most thermally stressful condition for an amplifier with a class-AB output stage. At the end of that time, the Verity's top panel was relatively cool at 79.3°F (26.3°C); its side panels were only a little warmer, at 92.1°F (33.4°C).
The voltage gain at 1kHz from the speaker terminals into 8 ohms measured 31.7dB for both balanced and unbalanced signals, which is almost 5dB higher than the norm. Both sets of inputs preserved absolute polarity (ie, were non-inverting), the XLR jacks being wired with pin 2 hot. The unbalanced input impedance was on the low side, at 9.1k ohms at 20Hz and 1kHz, and 8.6k ohms at 20kHz. The balanced input impedance was 20k ohms from 20Hz to 20kHz.
The output impedance, including a 6'-long, spaced-pair speaker cable, was a moderate 0.18 ohm at low and middle frequencies, rising to 0.25 ohm at the top of the audioband. As a result, the modulation of the amplifier's frequency response with our standard simulated loudspeaker was ±0.2dB (fig.1, gray trace). The Monsalvat's frequency response into 8 ohms was down by just 0.3dB at 100kHz (fig.1, blue and red traces); as a result, a 10kHz squarewave was reproduced with very short risetimes (fig.2), with just a slight hint of overshoot. A 1kHz squarewave was perfectly square (not shown). Fig.1 reveals that the ultrasonic output rolls off earlier with lower impedances, with the response into 2 ohms (green trace) down by 0.9dB at 20kHz. Channel separation, measured using the balanced input, was >70dB in both directions in the midrange and below, decreasing to 62dB at the top of the audioband.
Fig.1 Verity Monsalvat Amp-60, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green) (1dB/vertical div.).
Fig.2 Verity Monsalvat Amp-60, small-signal 10kHz squarewave into 8 ohms.
The signal/noise ratios, taken with the inputs shorted to ground and ref. 1W into 8 ohms, were different for balanced and unbalanced operation. The unbalanced, unweighted, wideband ratio was an extremely high 91.5dB in the left channel and 88.3dB in the right. These ratios respectively improved to a superb 102.1 and 101.1dB with an A-weighting filter in circuit. The balanced ratios, however, were all lower, the A-weighted figures being 78.6dB for the left channel, 82.8dB for the right.
Spectral analysis of the low-frequency noise floor with unbalanced drive (fig.3) indicated that there were AC power-supply–related harmonics at 60Hz and both its odd-order harmonics (due to magnetic interference from the massive toroidal power transformer) and its even-order harmonics (due to a non-zero resistance to ground somewhere in the circuit). Other than the spectral component at 60Hz, which was at a still-very-low –94dB (0.002%), these spuriae all lay at or below –107dB (0.0005%), and the random noise-floor components were all extremely low. However, when I repeated the analysis with the balanced inputs, all of the supply-related harmonics rose by around 20dB, which is why the balanced S/N ratios weren't as good as the unbalanced ratios. This is unusual—balanced operation tends to have lower noise, due to the circuit's common-mode noise rejection.
Fig.3 Verity Monsalvat Amp-60, unbalanced drive, spectrum, DC–1kHz, ref. 1W into 8 ohms (left channel blue, right red; linear frequency scale).
The Monsalvat Amp-60's output is specified as 60Wpc into 8 ohms and 120Wpc into 4 ohms (both equivalent to 17.8dBW), and 180Wpc into 2 ohms (16.5dBW). Our review sample exceeded these specifications into 8 and 4 ohms. With clipping defined as when the THD+noise in the output reaches 1% and with both of the Amp-60's channels driven, I measured clipping powers of 95Wpc into 8 ohms (19.8dBW, fig.4) and 145Wpc into 4 ohms (18.6dBW, fig.5). I don't hold the wall voltage constant during these tests; it measured 119.8V AC at the measured clipping power. However, when I tried to measure the maximum output power into 2 ohms with only the left channel driven, the Amp-60 clipped at just 50W (11.0dBW, fig.6). I repeated the test, this time with only the right channel driven, and got the same result.
Fig.4 Verity Monsalvat Amp-60, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.5 Verity Monsalvat Amp-60, distortion (%) vs 1kHz continuous output power into 4 ohms.
Fig.6 Verity Monsalvat Amp-60, distortion (%) vs 1kHz continuous output power into 2 ohms.
I examined how the Verity amplifier's percentage of THD+N varied with frequency at 10V, which is equivalent to 12.5W into 8 ohms and 25W into 4 ohms. The percentage of THD+N was low in the bass and midrange (fig.7), with the right channel (red and magenta traces) more linear than the left (blue, cyan). While there was the usual rise in the treble due to the decrease in open-loop voltage gain as the frequency increased, this was by only a small amount.
Fig.7 Verity Monsalvat Amp-60, THD+N (%) vs frequency at 10V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta).
The left channel's THD+N waveform at 30Wpc into 8 ohms (fig.8) indicates that the distortion was low, at 0.013%, and had strong second-harmonic content, though with spuriae also occurring at the zero crossing points. With a low-frequency tone at 50Wpc into 8 ohms, the second harmonic was indeed the highest in level in the left channel, at –78dB (0.012%, fig.9, blue trace). The second harmonic was lower in the right channel, at –87dB (0.004%), and the third harmonic is now the highest in level, at –80dB (0.01%). There are some higher-order harmonics present, as well as supply-related spuriae, but these all lie below –100dB (0.001%). Tested with an equal mix of 19 and 20kHz tones, the Amp-60 produced relatively low levels of intermodulation distortion, even at high powers into low impedances (fig.10). The difference product at 1kHz lay at –77dB in the left channel (0.012%) and at –90dB in the right (0.003%). The higher-order products were all at or below –80dB (0.01%).
Fig.8 Verity Monsalvat Amp-60, 1kHz waveform at 30W into 8 ohms, 0.013% THD+N (blue); distortion and noise waveform with fundamental notched out (red, not to scale).
Fig.9 Verity Monsalvat Amp-60, spectrum of 50Hz sinewave, DC–1kHz, at 50W into 8 ohms (left channel blue, right red; linear frequency scale).
Fig.10 Verity Monsalvat Amp-60, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 50W peak into 4 ohms (left channel blue, right red; linear frequency scale).
The Verity Monsalvat Amp-60's measured performance is generally respectable. However, I was bothered by the difference in S/N ratios between the balanced and single-ended inputs, and by the shortfall in measured power into 2 ohms.—John Atkinson
