Musical Fidelity A3.2 integrated amplifier Measurements

Sidebar 3: Measurements

Before I performed any measurements, I preconditioned the Musical Fidelity A3.2 with both channels driven at one-third power into 8 ohms for an hour. Though the top cover above the internal heatsinks was only just too hot to keep my hand on, the amplifier otherwise handled this test with aplomb.

With the volume control at its maximum, the voltage gain into 8 ohms from the speaker terminals was 41.8dB, 12dB from the preamp outputs, and all the inputs preserved absolute signal polarity. The line input impedance at 1kHz was to specification at a usefully high 220k ohms. The phono input impedance was somewhat lower than specified at 11.1k ohms, though this will not be a problem.

The phono stage's 1kHz gain, measured at the preamp output jacks, was 53dB (MM), which is a little on the high side, and 68dB (MC). (These figures include the 12dB line-preamp gain, of course.) Phono frequency response (fig.1), again measured at the preamp outputs, was characterized by a slightly shelved-down treble, with the ultrasonic output rolling off above 20kHz. Given the wide frequency range covered, the treble shelf will be audible as a slight warming of the sound. The MC phono overload margins were good, at 24dB at 20Hz, 18.1dB at 1kHz, and 17.1dB at 20kHz, though slightly less good via the MM setting at 15.5dB, 14.8dB, and 15.1dB, respectively, due to the higher-than-usual gain. The phono stage's A-weighted signal/noise ratios were excellent, at 84.4dB (MM) and 68.7dB (MC), though the unweighted, wideband figures were less good: 68.7dB and 56.4dB, respectively.

Fig.1 Musical Fidelity A3.2, phono-input RIAA error (0.5dB/vertical div., right channel dashed).

The preamp output impedance was 48 ohms over most of the audioband, though this rose to 414 ohms at 20Hz, suggesting that the partnering power amplifier should not have an input impedance below 10k ohms or so if the sound is not to become a bit lean. The power amplifier's output impedance was below 0.1 ohm across the audioband, which means that modification of a speaker's response by the voltage-divider action of its impedance and the amplifier source impedance will be minimal (fig.2, top dotted trace). This graph also shows that the line-input's ultrasonic bandwidth does decrease slightly with lower load impedances, but even into 2 ohms the response is down by just 0.7dB at 20kHz, which will not be very significant, subjectively.

Fig.2 Musical Fidelity A3.2, line-input frequency response at (from top to bottom at 2kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (0.5dB/vertical div., right channel dashed).

This ultrasonic rolloff results in a slight rounding of a 10kHz squarewave's leading edges (fig.3), but the waveform is free from overshoot or ringing. Channel separation via the line input (fig.4) was excellent at low frequencies, but marred at higher frequencies by capacitive coupling. Given the A3.2's dual-mono construction, this coupling presumably occurs at the shared volume control. However, as the crosstalk at 20kHz is still at -52dB (L-R) and -62dB (R-L), there should be no subjective consequences. The Musical Fidelity's A-weighted noise floor via its line input was acceptably low, at 79.1dB ref. 1W into 8 ohms, though this worsened slightly to 69.5dB with an unweighted, wideband measurement.

Fig.3 Musical Fidelity A3.2, small-signal 10kHz squarewave into 8 ohms.

Fig.4 Musical Fidelity A3.2, line-input channel separation (10dB/vertical div., R-L dashed).

Measured via its line input, the Musical Fidelity A3.2 was extraordinarily linear in the midrange and bass. Fig.5 shows how the measured THD+noise percentage changes with frequency at a reasonably high output level (12.67V, equivalent to 20W into 8 ohms). Below 2kHz, the measured figure is actually dominated by noise, but distortion harmonics make an appearance above that frequency and continue to rise above 30kHz. At high powers, the dominant harmonic is the third (fig.6), though at low output levels the second harmonic dominates, with then the third and fourth present at successively lower levels (fig.7). These harmonics are all too low in level to be audible, but if they were, this reduction in level with increasing order is something that is subjectively benign. Some AC supply components can be seen in this spectrum, but as they are all below -100dB, they should be inaudible.

Fig.5 Musical Fidelity A3.2, THD+N (%) vs frequency (from bottom to top at 4kHz): 12.67V into 8 ohms, 4 ohms, and 2 ohms, and at 2.83V into simulated loudspeaker load.

Fig.6 Musical Fidelity A3.2, 1kHz waveform at 83W into 4 ohms (top), 0.0058% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.7 Musical Fidelity A3.2, spectrum of 1kHz sinewave, DC-10kHz, at 1W into 8 ohms (linear frequency scale).

Intermodulation distortion was also low in level, even just below visible clipping on the oscilloscope screen (fig.8). However, clipping with this demanding high-frequency signal was violent, with the positive halves of the waveform being clamped to 0V. Not a pretty sight, but it's fair to note that clipping at lower frequencies was normal, with the waveform tops squaring off.

Fig.8 Musical Fidelity A3.2, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 90W into 4 ohms (linear frequency scale).

Finally, even with both channels continuously driven, the A3.2 more than met its specified output power into 8 and 4 ohms. Fig.9 reveals that 140Wpc was available into 8 ohms, at our usual 1% THD clipping point (21.5dBW), with 204W into 4 ohms (20.1dBW). The amplifier turned off at 129.4W into 2 ohms (15.1dBW), however, suggesting that very demanding speaker loads are best avoided. There was a little more DC offset present on the outputs, at 26mV (left) and 21mV (right). This will be harmless, but I did note that rapidly rotating the volume control caused temporary changes in the offset voltage.

Fig.9 Musical Fidelity A3.2, distortion (%) vs 1kHz continuous output power into (from bottom to top): 8 ohms, 4 ohms, and 2 ohms.

Other than these caveats, the Musical Fidelity A3.2 offers good audio engineering and high power at a relatively affordable price.—John Atkinson

Musical Fidelity
US distributor: Kevro International
902 McKay Rd., Suite 4
Pickering, Ontario L1W 3X8, Canada
(905) 428-2800