My usual practice is to subject amplifiers to a one-hour preconditioning period at 1/3 power. In the case of the humongous McIntosh MC1201, that meant 400W into 8 ohms from its 8 ohm output transformer tap. (One-third power is chosen because this maximally thermally stresses an amplifier with a class-B output stage.) To my surprise, the MC1201 shut down after 40 minutes or so, its heatsinks too hot to touch. This happened again during the testing. Each time, the MC1201 could be reset once it had cooled down, but this suggests that the amplifier is not recommended…
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Note the rise in distortion in the mid-treble with our simulated speaker load, which suggests that the big McIntosh is uncomfortable with loads possessing moderately high phase angles. (Fig.4, taken from the 8 ohm tap, is the worst-case example.) However, this may well be of academic interest, given the amplifier's enormous power reserve and the fact that the distortion content is almost entirely third-harmonic (figs.5 and 6). Note, by the way, the enormous power levels at which these two graphs were taken; this was necessary to raise the harmonic distortion out of the amplifier's very low…
Figs.8, 9, and 10 show the monster McIntosh's THD+noise percentage plotted against output power for the 8, 4, and 2 ohm taps, respectively. (The discontinuities in the curves are due, I imagine, to the Power Guard circuit.) Again it can be seen that the MC1201 performs best when the load is approximately the same as the Autoformer tap, in which case the McIntosh easily exceeds its specified output power (footnote 1).
Fig.8 McIntosh MC1201, 8 ohm tap, distortion (%) vs continuous output power into (from bottom to top at 10W): 8 ohms, 4 ohms, and 2 ohms.
Fig.9…
Sidebar 4: Manufacturer's Comment
Editor: The following bullets are comments on the technical section of the review, starting at the beginning of the "Measurements" section.
• The output stage is listed as class-B. It is actually a class-A/B output stage.
• A comment about the 40-minute operation at 1/3 power: This is done to keep the top cover of the amplifier from exceeding 65 degrees C, which is a requirement for CE certification. The unit is designed using parts that are safely operated at 80 degrees C and higher. Allowing the unit to shut down at 65 degrees C is…
I've always wanted to review one of Madrigal's Mark Levinson products, and finally my prayers have been answered. The chosen victim? The No.32 Reference preamplifier. Note that "Reference" moniker. The No.32 is the first Mark Levinson preamplifier to carry such appellation. They're not kidding.
Power supply matters
When phono was the primary source and all circuits were single-ended, a separate power supply was the most obvious characteristic of a high-end preamplifier design. Even today, many low-gain line stages use an external power supply, despite the fact that, as Madrigal…
The 400Hz oscillators supply individual left- and right-channel isolation transformers, followed by rectifiers and discrete DC voltage regulators. The outputs supply clean, stable 400Hz power in the adjacent left and right "towers" capping the Controller. Each tower, machined from a solid block of aluminum, contains the power supply for its respective audio channel. An isolation transformer mounted inside the block further decouples the audio-regulation stages from the 400Hz oscillator amplifier. Soft-recovery diodes rectify and filter the transformer's output, which is then regulated with a…
Power and control connections
Each channel is connected to the Controller by a 10-conductor, heavily shielded DC cable. Four conductors deliver regulated DC power to the preamplifier's audio circuits; the other six carry DC control signals. More important, each channel's microprocessor is active only when receiving control signals—otherwise they're in "sleep mode" and entirely silent. A low-voltage, differential-drive control signal, buffered by optoisolators, issues commands. Except for the fraction of a second when the microprocessor is actually transmitting one of these commands,…
Each channel is connected to the Controller by a 10-conductor, heavily shielded DC cable. Four conductors deliver regulated DC power to the preamplifier's audio circuits; the other six carry DC control signals. More important, each channel's microprocessor is active only when receiving control signals—otherwise they're in "sleep mode" and entirely silent. A low-voltage, differential-drive control signal, buffered by optoisolators, issues commands. Except for the fraction of a second when the microprocessor is actually transmitting one of these commands,…
Volume control
The No.32 Reference introduces Mark Levinson's new active attenuator. The attenuator modules are constructed on their own four-layer Arlon boards, where local power-supply regulation and bypass capacitors make for clean power and optimum isolation. An array of precision resistors provides attenuation in 0.1dB steps down to -57.0dB, at which point the steps increase to 1.0dB. In total, this hardware provides for more than 65,000 steps, allowing the No.32's "stepped attenuator" to act—and sound—like a continuously variable control. Optimal volume was always a cinch to find…
The No.32 Reference introduces Mark Levinson's new active attenuator. The attenuator modules are constructed on their own four-layer Arlon boards, where local power-supply regulation and bypass capacitors make for clean power and optimum isolation. An array of precision resistors provides attenuation in 0.1dB steps down to -57.0dB, at which point the steps increase to 1.0dB. In total, this hardware provides for more than 65,000 steps, allowing the No.32's "stepped attenuator" to act—and sound—like a continuously variable control. Optimal volume was always a cinch to find…
Sidebar 2: Associated Equipment
Analog source: Rega Planar 3 turntable and RB300 tonearm, Grado Reference Master cartridge.
Digital source: California Audio Labs CL-20 DVD/CD player, Sony SCD-777ES SACD player.
Preamplification: Musical Fidelity Nu-Vista and A3CR preamplifiers, X-LP2 dual-mono phono stage; Blue Circle B3 Galatea preamplifier.
Power amplifiers: Musical Fidelity Nu-Vista 300 and A3CR, Mesa Baron.
Loudspeakers: Joseph RM7si, Celestion A3.
Cables: Interconnects (all Synergistic Research): Resolution Reference Mk.II, Designer's Reference with Discrete and…
Sidebar 3: Measurements
The Legend Starlet is about as basic an integrated amplifier as you can imagine: three inputs, a selector switch, and a volume control. There's no tape loop, so it wasn't possible to distinguish in the measurements between the active preamplifier section's contribution and that of the power-amp section. The significance of this will become apparent.
Maximum voltage gain into 8 ohms was lower than usual at 38.33dB; a sensible design choice, in my opinion, given that almost all preamps have too much gain for the power amps with which they'll be used. With…