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
- 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
- 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
- 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.