McIntosh MC2125 Vintage Solid State Amplifier - SkyFi Serviced

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General:
One of our favorite amplifiers from this period, rated at 125W x 2 channels.

It's not as big and heavy as some of its older brothers, but it's right at a sweet spot for powering most vintage speakers from this era.

Typical Mcintosh features such as a glass faceplate that essentially guarantees things will not age over time. Most amplifiers made with metal faceplates tend to either get scratched up or the writing rubs off over time. But due to the glass, McIntosh amplifiers and preamps still look amazing even after 30-40 years of use.

This particular example was originally serviced by Eastern Stereo Video Logic, then was serviced in house here at SkyFi and is in perfect working condition.

This listing includes the amp, box, and manual.

SkyFi Cosmetic Notes:
The amplifier has a small nick on the bottom trim piece for the glass, there is also light paint separation near the headphone jack, and misc. blemishes throughout the chassis and covers. The RCA jacks have lost the majority of their plating. The left meter's lower range text is slightly faded compared to the right meter. Overall presentable condition.

SkyFi Technical Notes:
This amplifier was tested at a McIntosh clinic in 1986 (by DOB). It was serviced at some point in the past by Eastern Stereo Video Logic where they replaced two large capacitors on the regulated power supply board. At SkyFi we replaced a broken speaker switch, performed routine service on the driver boards, and calibrated the meters.

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 input, Spare AC outlet and Speaker binding posts.

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:
Unit, Box, Manual

Packing:
Original Manufacturers Packing

Specs:
POWER OUTPUT
STEREO
120 watts minimum sine wave continuous average power output, per channel, both channels operating into 2 ohms, 4 ohms, 8 ohms, or 16 ohms load impedance, which is;
15.5 volts RMS across 2 ohms
21.9 volts RMS across 4 ohms
31.0 volts RMS across 8 ohms
43.8 volts RMS across 16 ohms

MONO
240 watts minimum sine wave continuous average power output into 1 ohm, 2 ohms, 4 ohms, or 8 ohms load impedance, which is:
15.5 volts RMS across 1 ohm
21.9 volts RMS across 2 ohms
31.0 volts RMS across 4 ohms
43.8 volts RMS across 8 ohms

OUTPUT LOAD IMPEDANCE
STEREO
2 ohms, 4 ohms, 8 ohms, and 16 ohms;
separate terminals are provided for each output

MONO
1 ohm, 2 ohms, 4 ohms, and 8 ohms; obtained by connecting together the appropriate terminals of both channels

RATED POWER BAND
20 Hz to 20,000 Hz

TOTAL HARMONIC DISTORTION
STEREO
0.1% maximum harmonic distortion at any power level from 250 milliwatts to 120 watts per channel from 20 Hz to 20,000Hz, both channel operating

MONO
0.1% maximum harmonic distortion at any power level from 250 milliwatts to 240 watts from 20 Hz to 20,000 Hz

INTERMODULATION DISTORTION
STEREO
0.1% maximum if instantaneous peak power output is 240 watts or less per channel with both channels operating for any combination of frequencies, 20 Hz to 20,000 Hz

MONO
0.1% maximum if instantaneous peak power output is 480 watts or less for any combination of frequencies, 20 Hz to 20,000 Hz

FREQUENCY RESPONSE (at one watt output)
20 Hz to 20,000 Hz, +0 -0.25 dB
10 Hz to 100,000 Hz +0 -3.0 dB

NOISE AND HUM
95 dB below rated output

SEMICONDUCTOR COMPLEMENT
45 silicon transistors
45 silicon rectifiers and diodes
8 integrated circuits

Dimensions:
Front panel measures 16 inches wide (40.64 cm) by 5-7/16 inches high (13.81 cm).
Chassis measures 15 inches wide (38.1 cm) by 5 inches high (12.7 cm) by 13 inches deep (33.02 cm), including connectors. Knob clearance required is 11½ inches (3.81 cm) in front of mounting panel

Weight:
65 lbs.

Approximate Age:
1976

Link to Manual:
Click Here

Recommended Cables:
Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - Speaker Cables - Better

Kimber Summit Series Monocle XL Speaker Cables (PAIR) - Best

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.

If the amplifier passes visual inspection, we move on to a controlled power on sequence using a Sencore safety analyzer to monitor current draw in real time. Once the amplifier is determined to be safe to operate, we connect it to full AC mains for function and power testing. We connect the speaker outputs of the amplifier to a Sencore PA81 Power Analyzer which acts as a dummy load, DC offset monitor, and oscilloscope interface. We start with a low level 1 KHz test signal at the amplifier's input and slowly increase its amplitude while monitoring the output on an oscilloscope for signs of noise, clipping, distortion, or improper channel balance. We continue increasing the signal level until the amplifier reaches clipping.

At this point we take an output power measurement and compare it to the spec sheet of the amplifier to verify proper performance. If the device under test has both balanced and single ended inputs they are both tested at this time.

We finish off the bench evaluation with a 1 KHz square wave check and a 20 Hz to 20 KHz sine sweep to assess the amplifier's frequency response characteristics.

This battery of tests will usually reveal if the amplifier has any issues that need further attention.

Before the device leaves the bench, we perform a listening test with actual music using a variety of preferred test tracks. Our benches are outfitted with familiar monitor speakers which help us identify inconsistencies that will not always show up on our test gear. The main things that we are listening for are hum or noise with no signal present, proper center image, clicks, pops, or any other obvious undesirable audio characteristics.

If the unit passes all of these tests it is moved to our long term testing rig where we simulate real word operating conditions for 6-8 hours minimum. This allows us to monitor the unit for signs of thermal runaway or intermittent issues that only crop up when the unit has fully come up to temperature.