McIntosh MC901
McIntosh MC901 monoblock amplifier, combining a 300 W tube section and a 600 W transistor section within a single chassis, for bi-amplification of a speaker.
Description
The McIntosh MC901 is a monoblock amplifier designed to power a single speaker. Its distinguishing feature lies in its architecture: it brings together, under a single chassis, two amplifiers of different design, one valve-based, the other transistor-based, each dedicated to a distinct frequency range.
The tube section delivers 300 W of power and drives the midrange and treble drivers. It relies on eight KT88 power tubes, driven by four 12AT7 and two 12AX7A tubes, and is built around the Unity Coupled output transformer, a technology unique to McIntosh used since the brand's founding in 1949. The transistor section, meanwhile, delivers up to 600 W to the woofer drivers and maintains this power regardless of the speaker's impedance (2, 4 or 8 ohms) thanks to McIntosh Autoformer technology. By entrusting the low frequencies to the transistor section, McIntosh avoids having the tubes amplify a register in which they are less at ease.
Each section has its own power supply, with no power interaction between them, and the MC901 incorporates an internal adjustable filter controlled directly from the controls on the top of the unit. This filter allows the relative gain levels of each section to be adjusted between -6 dB and +3 dB according to listening preferences, and a direct input can be connected to each section to bypass the filtering. The unit also features several in-house technologies: Power Guard in the transistor section, Sentry Monitor, Quad Balanced design, Power Control, Solid Cinch terminals and McIntosh Monogrammed Heatsinks. The tube section additionally benefits from the Power Guard Screen Grid Sensor, which monitors the screen grid current of the KT88 tubes and attenuates the signal if the value is too high in order to protect the tubes.
Visually, the MC901 stands out with its DualView meter, an evolution of the brand's iconic blue meter: two overlaid mechanical needles display the power of each section in real time, one for the tube amplifier, the other for the transistor amplifier.
This monoblock is aimed at an enthusiast of high-end systems seeking an integrated bi-amplification solution, without resorting to two separate amplifiers or an external crossover. It will suit demanding listening where one wishes to combine the texture characteristic of tubes on the midrange-treble with the power handling of transistors on the bass.
What to check before buying used
On this type of hybrid bi-amplification, the condition of the eight KT88 tubes as well as the 12AT7 and 12AX7A preamplifier tubes deserves priority checking, as these valves wear with hours of operation and determine the midrange-treble section. It is also advisable to check that the two stages are properly balanced via the internal adjustable filter, whose settings may have been altered by a previous owner. An inspection of the output connections associated with the Autoformer technology, along with careful listening for hum or distortion on the bass side, will finally allow the condition of the power supply dedicated to the transistor section to be confirmed.
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