McIntosh C-28 Preamplifier with wood case

In very nice condition! Original box and packing (we promise to pack carefully) are not included. Manual can be found on line.

A classic Mac piece - circa 1970-1975. Last image is the full specs

Highlights:

>Built-in headphone amplifier : listen to your favorite music - privately

>You can use 3 tape machines 2 with their own electronics and 1 tape playback deck with complete easy front panel switching

>Main and remote loudspeaker switching:  turn the main speakers on or off without affecting the remote speakers and vice versa (operates with accessory relay)

>New low noise phono input: listen to your records with a new quietness

>Individual channel phono level controls: match levels from different phono cartridges without degrading signal to noise ratio

>Individual channel output level controls: perfect balance from your stereo always

>Individual channel tone control switches: complete, repeatable flexibility

>New compensation control: one position for loudness compensation, one position is flat and a third position that adds presence compensation!

The techie stuff:

Each channel of the C 28 consists of four main parts: low level amplifier, high level amplifier, active filters and headphone amplifier. Common regulated power supplies furnish power to both channels.

LOW LEVEL AMPLIFIER: Each channel of the low level amplifier is a four transistor differential amplifier input circuit. Precise equalization is maintained by the use of negative feedback and appropriate components specially designed for low noise. Particular attention has been exercised in the use of transistors having exceptional low noise characteristics. The differential amplifier stage is the basis for the high open-loop gain of this portion of the circuit. With a higher open-loop gain there is ample negative feedback available under all equalization conditions to ensure an extremely low level of harmonic distortion. Because equalization has been designed for minimum error the response from both tape and records is accurate. One of the important features of this section is the use of set level potentiometers in the phono circuit. The gam of the phono preamplifier stages can be adjusted from 32 dB to 42 dB to correspond to the sensitivity of your phono cartridge. The phono set level controls in your C 28 are designed into the negative feedback loop. They are not potentiometers at the input or output of the low level amplifier. This important design consideration allows the gam to be reduced from 42 dB to 32 dB without adversely affecting the noise level. Because the con- trol is in the feedback loop the level of noise and distortion is reduced 10 dB but the maximum input voltage that this section can handle is increased by 10 dB. With 42 dB of gain in the phono section the maximum input voltage that the low level amplifier can accept without overloading is 150 mV at 1000 Hz. When the gam is reduced to 32 dB this section can accept over 0.5 volt without overloading at 1000 Hz.

HIGH LEVEL AMPLIFIER: The linear high level amplifier has a gain of 20 dB. The same careful design considerations apply with respect to noise and the reduction in non-linear distortion. Each channel of the high level amplifier consists of 3 transistors selected for low noise characteristics. In the feedback loop, two 11 position switches allow up to 20 dB of boost or cut at 20 Hz and up to 18 dB of boost or cut at 20,000 Hz. At the input of the high level amplifier are the bass trim control and the switch which selects loudness or presence compensation.

FILTER AMPLIFIER: The filter amplifier serves three functions: (1) a high frequency filter, (2) a low freqency filter, and (3) a 20.000 Hz low pass filter. Each channel of the filter amplifier has two transistors connected in compound emitter follower configuration. The resistive and capacative elements form a 12 dB per octave active RC filter. The high frequency filter is a 7000 Hz active, low pass, filter. The low frequency filter is a 50 Hz active, high pass, filter. The filter amplifier also forms a 20,000 Hz active low pass filter to reduce noise outside the useful sound spectrum while leaving the preamplifier response flat to 20 000 Hz. A 20,000 Hz active, low pass, filter is connected at all times to reduce wideband noise yet maintaining flat frequency response up to 20,000 Hz. The non-linear distortion of these active filters is very low even at cut off frequency. This is not the case in passive fiters where a very low impedance at the cut off frequency is presented to the driving source. This low impedance will cause high non-linear distortion at and near the cut off frequency. The design of the C 28 eliminates this problem.

HEADPHONE AMPLIFIER: The headphone amplifier is capable of producing 0.75 volts of output into 8 ohm headphones with less than 0.1% harmonic distortion. This is adequate power to drive any dynamic type headphone. The maximum output of the headphone amplifier is 2.5 volts into 50 ohms or greater at the LINE OUT-PUT terminals on the rear panel. The internal  impedance of the headphone amplifier is less than 0.2 ohms. Long shielded cables can be used without adversely affecting the frequency response. The headphone level controls adjust the output level and balance of the headphone amplifier.