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McIntosh MC275

MC275 - Front View MC275 - Front Side View MC275 Front Side View with Tube Shield

MC275 - Side View MC275 - Top View MC275 - Top View with Tube Shield

This Amplifier was produced by McIntosh from May, 1961 through July 1973. The original Unity Coupled Circuit and McIntosh bifilar wound output transformer set a standard for low distortion performance back in 1947. The output transformer had two primary windings which are wound in a bifilar pattern. In this technique, both primaries are wound side by side. Each turn of primary number one is next to the same turn of primary number two. There is near complete magnetic coupling between the two wires. The magnetic coupling is reinforced by the capacitance between the two wires.

In the McIntosh Unity Coupled Circuit, one of the bifilar primary windings is connected through the power supply to the plate and cathode of one of the output tubes. The other bifilar winding is connected to the other tube in the same manner. This unity coupled output virtually eliminates notch distortion.

The MC275 used an advanced design of this technology by incorporating a trifilar wound primary to improve the power bandwidth. The third winding is connected to the plates of the cathode follower driver. Groups of interleaved primary and secondary windings improve coupling and bandwidth of over 100,000 cycles is achieved.

  • Tube Complement:
      Voltage Amplifier - One 12AX7 (drives both amps)
      Phase Inverter - Two 12AU7
      Drive Amplifier - Two 12BH7
      Cathode Follower Drive - Two 12AT7 or 12AZ7
      Output - Four KT88 or 6550
  • Selectable source input for Stereo, Twin-Amp or Mono operation.
  • Ouput is rated at 75 watts continuous per channel in Stereo or Twin configuration. 150 watts continuous mono.
  • Harmonic Distortion is less than 0.5% at rated output or less for 20Hz through 20kHz. Typical performance is 0.3% or less at 20Hz and 20kHz full power. Typical performance in the mid frequencies is 0.1% or less at full power.
  • Output barrier strips provide taps for speaker impedances of 4, 8 and 16 ohms stereo or 2, 4 and 8 ohms mono. The terminal strips may also be connected for a constant voltage output of 25 volts in either stereo or mono, or for 70.7 volts in mono. The secondary of the McIntosh output transformer may be operated either grounded or not grounded. If grounding is desirable, a wire may be connected between chassis and common of either or both output termnal strips.
  • An octal socket marked line output has connections for 70.7volts, 115 volts, 230 volts or 600 ohms for stereo 300 ohms for mono operation. The 600 ohm winding and the 300 ohm winding are balanced to ground. Pin 1 of the octal socket is the center tap for these windings and is grounded.

McIntosh MC275 Gordon Gow Commemorative Edition

MC275 Commemorative - Front View MC275 Commemorative - Front Side View MC275 Commemorative - Front Side View with Tube Shield

MC275 Commemorative - Side View MC275 Commemorative - Top View MC275 Commemorative - Emblem

This Amplifier was produced by McIntosh from 1993 until 1996 to commemorate the late Gordon J. Gow, McIntosh President from 1997 until 1989 and Vice President since the inception of the company in 1949. Mr. Gow was the co-inventor of the original McIntosh Unity Coupled Amplifier circuit. He was also the person most responsible for the unparalled success and reputation of McIntosh Laboratory as the manufacturer of the world's finest audio electronics.

The overall appearance of the commemorative MC275 amplifier is very close to the original. It has the same dimensions, chrome chassis and black transformer housings. Several changes were made to the original design to take advantage of newer technology and accommodate the needs of contemporary sound systems.

  • Gold plated XLR balanced input connectors were added on the sloping section of the chrome chassis. This allows the use of balanced interconnect cables for lower noise and improved signal transfer. The amplifier sensitivity at the balanced input jacks is fixed at 2.0V for rated output.
  • An increase in gain was necessary to accomodate the balanced inputs. Circuit changes were made for this purpose, resulting in the change of four vacuum tubes. Each 12BH7 was replaced by a 12AZ7 and each 12AU7 was replaced by a 12AX7.
  • Two gold plated RCA unbalanced input jacks were placed on the front of the chrome chassis. Above these input jacks on the sloping section of the chassis are correspoding left and right volume controls. The sensitivity at the unbalanced inputs, with controls set at maximum, is 1.0V for rated output.
  • Two slide switches were placed below the balanced input connectors. One switch selects balanced or unbalanced inputs. The other switch selects stereo or mono parallel configuration.
  • Gold plated high current output barrier strips were placed on the sloping section of the chrome chassis. Output taps were provided for speaker impedances of 4, 8 and 16 ohms stereo or 2, 4 and 8 ohms mono.
  • The octal output soket that originally connected to the high impedance (500 ohm) transformer tap was removed. The 3 position lever mode switch, stereo balance control, twin amp volume controls, mono volume control and the twin amp input jacks were also removed.
  • Close tolerance film resistors, polypropylene coupling capacitors and fiberglass printed circuit boards were used.
  • A replica of the original Owner's manual, including the listed changes, was supplied with each unit. The standard McIntosh 3 year factory service contract was made available.

McIntosh MC275 Stainless Steel

MC275 Stainless Steel - Front View MC275 Stainless Steel - Front Side View MC275 Stainless Steel - Front Side View with Tube Shield

MC275 Stainless Steel - Side View MC275 Stainless Steel - Top View MC275 Stainless Steel - Top View with Tube Shield