AudioPrism Mana Reference monoblock power amplifier Measurements

Sidebar 2: Measurements

Because of the Mana Reference's wide variety of configurations—four output transformer taps, ultralinear or triode output tube/transformer operation, balanced and unbalanced inputs—I carried out a complete set of measurements on the amplifier only with balanced input and ultralinear output. I did check some of the performance parameters in the other modes, however. But if no input or output modes are mentioned, assume it's balanced and ultralinear, respectively.

Before I did anything, I warmed up the amplifier at one-third power into 8 ohms for 60 minutes, after which I checked that the bias current for the output tubes was correctly set to 45mA each in ultralinear mode. The input impedance measured 95k ohms single-ended and around 400k balanced (the exact figure is hard to specify, as the voltage-drop method I use gave a change of only a couple of millivolts with this high an impedance). The A-weighted noise level was low, the S/N ratio (ref. 1W into 8 ohms) measuring 72dB. However, unweighted with a wide, 10Hz–500kHz bandwidth, the S/N worsened to 54dB due to out-of-band noise. Voltage gain into 8 ohms with balanced drive was 23.7dB (ultralinear) and 22.4dB (triode). With an unbalanced input signal, the gain increased by around 5.5dB, presumably due to the phase-splitter action. (While each output half of the splitter is the same level as the input, the two halves produce two signals in antiphase, giving a theoretical gain of 6dB.)

The output impedance from the 8 ohm tap was very high, at 3.3 ohms (triode) and 3.7 ohms (ultralinear) over most of the band, rising to 4.75 ohms and 5.5 ohms (respectively) at 20kHz. Using the 4 and 2 ohm taps, the output impedance dropped to 1.7/0.9 ohms (T) and 1.9/1.1 ohms (U) at 1kHz, but these are all high enough figures to give significant frequency-response modification with real loudspeaker loads.

This can be seen in fig.1, which shows the response into 8 ohms, 4 ohms, and Stereophile's simulated speaker load from the 8 ohm tap. Into pure resistances, there is some extra HF rolloff into the lower loads, with the response 1dB down at 20kHz. But note the large variations into the simulated speaker—these will be audible. Note also the vestigial peak at 55kHz.

Fig.1 AudioPrism Mana Reference, 8 ohm tap, ultralinear mode, frequency response at (from top to bottom): 2.828V into simulated loudspeaker load, 8 ohms, and 4 ohms (1dB/vertical div.).

Even from the 2 ohm tap (fig.2), the response variation will still be audibly significant. But from this tap some peaking can be seen in and above the top two octaves of the audioband, reaching +0.75dB at 30kHz. However, there is now no sign of the 55kHz peak. In triode mode (fig.3) there is no peaking at 30kHz or 55kHz, but there is at 87kHz, more so with balanced drive than with unbalanced. This damped but still apparent ultrasonic behavior can also be seen in the Mana's squarewave responses (figs.4 & 5).

Fig.2 AudioPrism Mana Reference, 2 ohm tap, ultralinear mode, frequency response at (from top to bottom): 2.828V into simulated loudspeaker load, 8 ohms, and 4 ohms (1dB/vertical div.).

Fig.3 AudioPrism Mana Reference, 8 ohm tap, triode mode, frequency response at 1W into 8 ohms, balanced (top) and unbalanced (bottom). (1dB/vertical div.)

Fig.4 AudioPrism Mana Reference, small-signal 1kHz squarewave into 8 ohms, 4 ohm tap, triode mode.

Fig.5 AudioPrism Mana Reference, small-signal 10kHz squarewave into 8 ohms, 4 ohm tap, triode mode.

Even with just 6dB of loop negative feedback, the Mana is very linear from the bass through the mid-treble (fig.6), but less so in triode mode (fig.7). The harmonic content of the distortion is heavily second and third harmonic (fig.8), which will tend to be subjectively benign. The third harmonic is a little stronger in triode mode (fig.9), but the ultralinear mode has more higher harmonics (fig.10)—which, all things being equal, will make it sound less sweet. Intermodulation performance in either mode was only fair (fig.11), as expected from the low feedback and less good HF linearity. (This graph was plotted at 21.6W, just below the point at which visible clipping was visible with this very demanding signal.)

Fig.6 AudioPrism Mana Reference, 4 ohm tap, ultralinear mode, THD+noise (%) vs frequency at (from top to bottom at 2kHz): 4W into 2 ohms, 2W into 4 ohms, 1W into 8 ohms, and 2.83V into simulated loudspeaker load.

Fig.7 AudioPrism Mana Reference, 4 ohm tap, triode mode, THD+noise (%) vs frequency at (from top to bottom at 2kHz): 4W into 2 ohms, 2W into 4 ohms, and 1W into 8 ohms.

Fig.8 AudioPrism Mana Reference, 4 ohm tap, triode mode, 1kHz waveform at 3.4W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.9 AudioPrism Mana Reference, 4 ohm tap, triode mode, spectrum of 50Hz sinewave, DC–1kHz, at 67W into 4 ohms (linear frequency scale).

Fig.10 AudioPrism Mana Reference, 4 ohm tap, ultralinear mode, spectrum of 50Hz sinewave, DC–1kHz, at 67W into 4 ohms (linear frequency scale).

Fig.11 AudioPrism Mana Reference, 4 ohm tap, ultralinear mode, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 21.6W into 4 ohms (linear frequency scale).

With continuous drive, the Mana more than met its output power specification when the output transformer was matched to the load, with about 115W available at 1% THD+noise in ultralinear mode (20.6dBW), 70W in triode (18.5dBW). I explored the Mana's clipping behavior in some detail using the Miller Audio Research Amplifier Profiler, which drives the amplifier with a low-duty-cycle toneburst that more closely resembles the dynamic demands of a real music signal. Fig.12 shows how the amplifier's distortion changes as the output voltage increases into 8, 4, 2, and 1 ohm (2 ohm tap, triode mode). The same pattern can be seen into each load: a linear increase in THD+N as the voltage increases, followed by a critical output level at which the amplifier suddenly becomes considerably more linear. This is then followed by clipping proper, which is slower into the higher impedances. The magenta line in this graph shows the 1% distortion point, which we normally define as "clipping."

Fig.12 AudioPrism Mana Reference, 2 ohm tap, triode mode, distortion (%) vs burst output voltage into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).

The actual maximum power available under these dynamic conditions was not significantly greater than with continuous drive, presumably due to the highly regulated nature of the Mana's power supplies. But as can be seen from fig.13, the amplifier is not comfortable driving loads below the nominal tap value, and is particularly unhappy in ultralinear mode (fig.14).

Fig.13 AudioPrism Mana Reference, 4 ohm tap, triode mode, distortion (%) vs burst output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).

Fig.14 AudioPrism Mana Reference, 4 ohm tap, ultralinear mode, distortion (%) vs burst output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).

The maximum current delivery was quite high for a tube amp: 5.2A, 7.9A, and 12A from the 8 ohm, 4 ohm, and 2 ohm taps, respectively (3% THD+N). Nevertheless, Mana owners should use the lowest-impedance tap that will give them the sound pressure levels they wish from their speakers, trading off ultimate loudness against improved linearity and a more neutral frequency response.—John Atkinson

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