McIntosh C28 Preamp Complete Collector Set - Box & Literature - Serviced in 2020

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General:

Incredibly Rare Complete Set C28!

The C28 Preamp is a true McIntosh classic. It features the McIntosh vintage look and feel including a backlit glass faceplate, large polished knobs, and satisfying pushbuttons with LED's.

Controls are just what you need with an input selector, volume, balance, and tone controls. The individual left and right tone controls are a nice plus.  

Cosmetically it presents very nicely. The glass faceplate is clean and the paintwork is without any deep scratches, rust, or wear.

The C28 is a crowd favorite from this era. It's easy to service, has a low noise floor, great vintage looks, and every feature we would want.

This particular C28 was purchased from its original owner and has all its original paperwork and also its box! Super rare for a preamp of its age, and perfect for an ardent McIntosh collector or someone looking for an exemplary sample.  

SkyFi Cosmetic Notes:
Light wear is present on the back of the top cover. The rear RCA jacks were polished to improve connectivity, but some of the plating has flaked off which reveals bare copper. Light scratching is present near the low level input section on the rear panel

SkyFi Technical Notes:
This unit received a basic service in 2020. It also has an official DOB McIntosh clinic sticker from 1990. It looks like one capacitor was replaced at that time in the line stage.

We swapped out the replaced 100uF capacitors along with its original counterpart in the other channel for good measure. This C28 passed out bench testing with better than expected results. The volume control in these units usually exhibit significant channel imbalance, but the control in this unit tracks very well throughout its whole range. This is likely due to the unit's late production year and care over the years by the original owner.

Brand Background:
McIntosh Laboratory is an American manufacturer of handcrafted high-end audio equipment based in Binghamton, New York. The company was founded in 1949 by Frank McIntosh. The company designs and produces audio amplifiers, stereo tuners and other consumer electronics products.

Ownership:
Single Owner

Connections:
RCA High and Low Level Inputs, RCA Line Output, 7 Power outlets

General Sound:
Smooth, uncolored, undistorted natural and clean

Cosmetic Condition:
8/10 = Very Good. Excellent front faceplate, one minor flaw on chassis side or top. See our detailed rating description here.

Working Condition:
Working perfectly and tested in our lab and listening room.

Included:
Complete Kit as it Left the Factory

Packing:
Original Manufacturers Packing

Specs:
Frequency response: 20Hz to 20kHz
Total harmonic distortion: 0.1%
Gain: 62 dB (MM), 20 dB (line)
Input sensitivity: 2.5mV (mic), 2mV (MM), 250mV (line)
Signal to noise ratio: 78 dB (MM), 90 dB (line)
Output: 250mV (line), 2.5V (Pre out)

Dimensions:
16" x 5.4" x 15"

Weight:
25 lbs.

Approximate Age:
1970

Link to Manual:
Click Here

Recommended Cables:
Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - Phono Interconnects

Testing Process:
We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect the PCBs for discoloration from resistors or transistors that may have been running hot. On vintage units we often spot check select capacitors for value and ESR.

When we first power on a preamplifier we connect its RCA output to a Sencore PA81 Power Analyzer which simulates real world loading conditions and gives us an oscilloscope interface.

The first order of business is checking that the volume control works smoothly throughout its entire range with acceptable channel balance. This is accomplished by feeding a 1 KHz sine wave into one of the preamp’s line level inputs while monitoring the preamp’s output on an oscilloscope.

We then switch to a 1 KHz square wave to test the tone controls, loudness function, and filters where applicable. During this step we are watching for equal alteration of the test signal by both channels. This also helps us identify dirty controls that will need treatment.

Once the basic line stage functions are verified, we test each input individually. This is especially important for devices that use relays to select their sources. If the preamp is equipped with a phono stage we test that as well. We use an inverse RIAA filter which allows us to feed a reference test signal into the phono input with the proper RIAA equalization and level. A square wave or sine sweep is used to verify that the device’s phono stage is faithfully reproducing the RIAA curve. If the preamp under test has balanced inputs and/or outputs these are tested as well.

We finish up our bench testing with a listening test with our bench amplifier and reference speakers. During this test we check for hum or hiss that may not have shown up in earlier testing. We also check that all of the tone controls and filters perform as expected.

If the preamplifier has remote control functions these are also tested. Preamps with tube circuits or complicated power supply topologies are connected at our long term test rig for extended stress testing under real world conditions.