Products > COLLECTOR ITEMS > MCINTOSH > MC2000 New Millennium Edition

MC2000 Tube Power Amplifier
New Millennium Edition :: Limited Production

Almost 50 years ago Frank McIntosh and Gordon Gow invented and patented the McIntosh Unity Coupled Output Circuit. Two features differentiate it from other designs. First,
the output tubes deliver power from both their plates (anodes) and their cathodes, not from their plates alone as in conventional circuits. Second, the output transformer’s two bifilar
primary windings give it one-half the turns ratio of conventional transformers, equating to one-fourth the impedance ratio. This allows a close coupling of the primary and secondary
windings, resulting in wide bandwidth, flat frequency response, and low distortion.

The MC2000 contains two identical channels with separate audio circuits, power supplies, and power transformers. Each channel has three amplification stages: input/phase inverter, driver, and output. The MC2000 uses the famous Unity Coupled Output Circuit with eight KT88 output tubes (four per channel) in a push-pull parallel configuration. Large transformers with grain-oriented silicon steel cores allow full power output down to 17Hz. As each channel is identical, the following description will address a single channel.

The Output Stage
The KT88s operate with fixed bias. Adjustments are needed only when the tubes have aged or been replaced. Because the tubes are additionally loaded in their cathodes they require a large drive signal (approximately 170V) for full output. This signal is provided by the 12AT7 driver. Boot-strapping the driver to the plate winding of the output transformer results in greater amplification than otherwise possible.

The Driver Stage
The driver stage is fed by the 12AX7A input/phase inverter. A resistance-capacitance step network couples these stages and minimizes low-frequency phase shift. The phase inverter has no coupling capacitors so no additional low-frequency phase shift is introduced. Emitter follower stages at the plates of the 12AX7A increase bandwidth.

The Input/Phase-Inverter Stage
Mode selectors at the rear of the chassis choose balanced or unbalanced inputs as well as input sensitivity (1.2V or 2.5V). The selectors operate Silent Electromagnetic Switches located adjacent to the input circuits. When the balanced input is used a second 12AX7A receives the balanced signal. One section of the driver tube is a cathode follower that passes the positive phase signal. The other section inverts the negative phase signal. The two outputs are combined and fed to the input/phaseinverter stage. Common mode rejection is greater than 60dB at middle frequencies.

The Feedback Circuit
Global negative feedback is taken from the output transformer secondary to the cathode of the input stage. Conventional wisdom cautions against using negative feedback, especially in tube amps where instability is a real concern. However, the MC2000 is designed in consideration of negative feedback, the many benefits of which are: lower distortion and noise, flatter frequency response, faster rise time, and higher damping factor. A common misconception is that the feedback signal is delayed and therefore introduces time distortion. This is not so in a properly designed amplifier. In fact, the time delay of the entire MC2000 circuit is less than 2 microseconds.

The Power Supplies
Each power transformer has two tapped primary and three secondary windings. The primary can be connected for 100V, 120V, or 230V. A thermistor cushions inrush current. A power MOS FET is used as an active filter to ensure extremely pure DC power. A medium-voltage secondary winding feeds the negative bias voltage supply and a low-voltage secondary feeds the filaments of the output tubes, the driver tube, and indicator lamps. This same winding feeds regulated DC power supplies for the meter circuit as well as the heaters of the 12AX7A input stages.
The Meters
Individual meters monitor the power output of each channel and include a hold mode that briefly indicates the highest power reading. The meters also serve as indicators when setting the bias for the output tubes.