MEASUREMENTS
Iassessed the Musical Fidelity A1’s measured performance using my Audio Precision SYS2722 system.1 I initially had some problems, as while the right channel’s gain increased as expected when I pushed the frontpanel button to switch from Direct mode to Normal mode, the left channel was muted. Suspecting there was something wrong with the pushbutton, I pressed it in and out several times in quick succession, which restored correct operation. I subsequently encountered another problem: The right channel had excessive distortion and limited power if the volume control was set to its maximum. This was the case with both the Direct and Normal settings. I therefore performed all the distortion tests with the control set to –12dB; at this setting, both channels performed identically with appropriately low distortion at low to moderate powers and slightly exceeded its specified class-A power. The amplifier is specified as offering a maximum output power of 25W into 8 ohms (14dBW) in class-A. Because amplifiers with a class-A output stage run hottest when they’re not outputting a signal, I left the amplifier powered up for an hour before starting the testing. At the end of that period, the top panel was too hot to keep my hand on it; its temperature was 138.2°F (59°C).
I looked first at the A1’s line inputs. Relative to these inputs, the amplifier preserved absolute polarity at all its outputs. With the volume control set to the maximum and measuring the left channel, the voltage gain at 1kHz from the loudspeaker outputs into 8 ohms was 35.2dB in Normal mode, 23.9dB in Direct mode. It was 13.1dB and 1.75dB, respectively, from the preamplifier output. Musical Fidelity specifies the A1’s input impedance as 25k ohms; my measurements were lower than that, at 6.8k ohms at 20Hz and 1kHz, 5.6k ohms at 20kHz.
The preamplifier output impedance was 100 ohms at 1kHz and 20kHz but rose to 127 ohms at 20Hz, presumably due to the presence of a series output capacitor. The loudspeaker output impedance ranged from 0.5 ohm at 20Hz to 0.54 ohm at 20kHz. Consequently, the modulation of the A1’s frequency response due to the Ohm’s law interaction between this impedance and that of our standard simulated loudspeaker2 was ±0.4dB (fig.1, gray trace). The amplifier’s response into resistive
loads was flat in the audioband, with its output into 8 ohms (blue and red traces) down by 3dB at 60kHz. Fig.1 was taken in Direct mode with the volume control set to the maximum; when I repeated the response measurement with the control set to –12dB, the 0.5dB channel imbalance was eliminated. The frequency response was identical in Normal mode and in either mode, the A1’s reproduction of a 10kHz squarewave had short risetimes without any overshoot or ringing (fig.2).
Channel separation was okay, at 70dB in the low treble, decreasing to 60dB at the frequency extremes. The wideband, unweighted signal/noise ratio, taken with the unbalanced input shorted to ground and the volume control set to its maximum, was the same in Normal and Direct modes, at an okay 70.1dB, left, and 72.9dB, right, ref. 2.83V, equivalent to 1W into 8 ohms. These ratios improved, respectively, to 70.3dB and 73.8dB when the measurement bandwidth was restricted to the audioband and to 85.2dB and 88.4dB when A-weighted. Fig.3 shows the spectrum of the A1’s low-frequency noisefloor at 1Wpc into 8 ohms in Normal mode with the volume control set to its maximum. The spectrum was identical both in Direct mode and with the volume control set to –12dB; it is dominated by power supply–related spuriae at 120Hz and 240Hz. These spuriae will be due to a nonzero resistance to ground somewhere in the circuit; they were not eliminated by floating the Audio Precision’s output signal ground or by connecting a wire from the ground terminal on the amplifier’s rear panel to the analyzer’s chassis ground.
Fig.4 plots how the THD+noise percentage in the Musical Fidelity’s output varies with power into 8 ohms with both channels driven. At our usual definition of clipping—when THD+N reaches 1%—the A1 slightly exceeds its specified output power of 25W into 8 ohms (14dBW), clipping in both Normal and Direct modes at 30Wpc
(14.77dBW). Musical Fidelity doesn’t specify the maximum power into 4 ohms. With both channels driven into 4 ohms, the amplifier clipped at 16Wpc (9.03dBW, fig.5). Relaxing the clipping definition to 3% THD+N gave a maximum power of 31Wpc into 4 ohms (11.9dBW).
As expected from figs.4 and 5, the A1 gave significantly higher levels of distortion into 4 ohms (fig.6, green and gray traces) than 8 ohms (blue, red traces), even at a relatively low output voltage of 5V (3.125W into 8 ohms, 6.25W into 4 ohms). On the positive side, the THD+N percentage in this graph doesn’t rise in the top octaves into either load, which implies a wide open-circuit bandwidth.
The THD+N waveform, taken at 10W into 8 ohms (fig.7), is primarily the second harmonic, though the third harmonic is present at a lower level (fig.8). Even into 4 ohms, intermodulation distortion was acceptably low in level (fig.9), with the second-order difference product with an equal mix of 19kHz and 20kHz tones lying at –56dB (0.15%) ref. the peak signal level. The higher-order products at 18kHz and 21kHz were 10dB lower in level.
To examine the performance of the
A1’s phono input, I connected a wire from the grounding post on the amplifier’s rear panel to one of the Audio Precision’s ground terminals. The phono input can be switched between MM and MC operation with a pushbutton on the rear panel. It preserved absolute polarity at the speaker and preamplifier outputs in both modes. In MM mode, the input impedance is specified as 50k ohms. I measured 46k ohms at 20Hz, 53k ohms at 1kHz, and 39k ohms at 20kHz. My measured input impedances were significantly higher than the specified 1k ohm in MC mode, at 20k ohms at 20Hz, 18k ohms at 1kHz, and 17k ohms at 20kHz.
The maximum gain at 1kHz in MM mode was 70.7dB from the loudspeaker output in Normal mode and 60.3dB in Direct mode. MC gains were 22dB higher in both modes. The maximum gains from the preamplifier output were 48.5dB in MM Normal, 38.2dB in MM Direct, 69.9dB in MC Normal, and 59.5dB in MC Direct. The mute button and volume control operate on both the loudspeaker and preamplifier outputs; to avoid
overdriving the amplifier’s output stage, I performed all the subsequent phono stage measurements at the tape output, with the other outputs muted. (Gain at the tape output was 34.8dB, MM, and 56.3dB, MC.)
The phono input’s RIAA-corrected response (fig.10) was well-matched between the channels and had a slight plateau in the treble reaching +0.4dB between 10kHz and 40kHz. There was also an unusual rise in the bass that reached +2dB at 10Hz. In MM mode, the wideband, unweighted S/N ratio, ref. 1kHz at 5mV, assessed with the input shorted to ground, was a good 72.5dB in both channels. Restricting the measurement bandwidth to 22Hz–22kHz increased the ratio to 77.7dB, and inserting an A-weighting filter gave a further increase to 83.9dB. The ratios were around 12dB lower in MC mode, but this is still a relatively low level of noise.
The phono input’s overload margins were very high in both modes, at 28.5dB ref. 1kHz at 5mV from 20Hz to 20kHz (MM), and 27.6dB ref. 1kHz 500µV at 20Hz and 1kHz and a still high 19dB at 20kHz (MC). The second harmonic was the highest in level in the phono input’s distortion signature but was inconsequential at –84dB (0.006%) with a 1kHz tone 12dB higher than the nominal MM level (fig.11), and –64dB (0.06%) with a 1kHz tone 12dB higher than the nominal MC level (not shown). The levels of higher-order intermodulation products with an equal mix of 19 and 20kHz tones were very low, though the 1kHz difference product at 1kHz lay at –50dB (0.3%) ref. a peak level of 200mV in MM mode.
Assuming the anomalous behavior I noted with the Normal/Direct pushbutton and the volume control was specific to the review sample, the Musical Fidelity’s A1 offered generally good measured performance. It will work best with loudspeakers whose impedance remains above 4 ohms, though the fact that its distortion signature is dominated by the subjectively benign second harmonic will work in its favor with more demanding loads. I would be remiss if I didn’t mention the excellent phono stage, which offers low noise and distortion and high overload margins in both MM and MC modes.—John Atkinson