McIntosh MC275 MK IV (2004-2006) Tube Amplifier - Complete

Free Shipping on *ALL* Electronics (Excluding Speakers, Contiguous 48 US States Only)

General:
Great sounding and collectible amplifier from McIntosh in its MK IV (MK 4) designation which is second next to newest. Not old enough to be considered vintage, but new enough to have all the reliability benefits of the newer generation models including balanced inputs and usable speaker terminal strips. 

The MK IV sounds as good, if not better than the current MK VI (MK 6), and foregoes what many think to be silly LED lighting on the tubes and the more complicated speaker binding posts.

This amplifier was also fitted with a full set of official Russian Genalex Gold Lion KT88 and new McIntosh Branded JJ 12AT7 and 12AX7. The original tube cover "cage" is also included as well as the manual forming a complete kit, in superb condition.

Best of all it will ship in an official McIntosh box for safe travels.

SkyFi Technical Notes:
Tube Notes:
This amplifier was fitted with a strong testing set of Russian Genalex Gold Lion KT88 and new McIntosh Branded JJ 12AT7 and 12AX7.

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 and XLR Inputs, Removable Power Cord, Screw Terminal Speaker Connectors.

General Sound:
Big beautiful musical sound with little fatigue.

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, manual, and power cord.

Manual:
Click Here

Packing:
Original Manufacturers Packing

Specs:
CONTROLS and CONNECTORS: 
• Speaker barrier strips for 4, 8 and 16 OHMS
• XLR BALANCED INPUTS (L+R) 
• RCA UNBALANCED INPUTS (L+R) 
• UNBALANCED GAIN controls: 2.5V to 1.0V
• MODE Switch: MONO or STEREO
• INPUT Switch: XLR Balanced or RCA Unbalanced

TECHNICAL SPECIFICATIONS: 
Power Output:
75 Watts Per Channel (Stereo Mode)
150 Watts Total Bridged (Mono Mode)

Frequency Response: 
20 - 20 kHz (+0 -0.2dB)

Distortion: 
0.5%

Noise & Hum: 
-100 dB (A weighted)

Output Impedance: 
4, 8, 16 ohms in Stereo Mode
2, 4, and 8 ohms in Mono Mode
(25 V line)

Damping Factor:
Greater than 10 

Input Impedance: 
100 k Unbalanced
180 k Balanced

Input Sensitivity: 
1.0 V Unbalanced
2.0 V Balanced
(Unity Coupled)

Power Requirements: 
120V at 400 Watts

Tube Complement:
4 x KT88/6550 output
3 x 12AX7A
4 x 12AZ7
(Solid state rectifier Bridge)

Dimensions:
7-1/2" H, 12" W, 16" D

Weight:
67 lbs.

Approximate Age:
2005

Recommended Cables:
Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - BALANCED XLR Connectors - Better

Kimber Kable - BALANCED XLR Connectors - Best

Kimber Kable - Speaker Cables - Better

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

Kimber Summit Series BiFocal XL Bi-Wire Speaker Cables (PAIR) - Best If Applicable

Kimber Kable - Power Cords - Better

Kimber Kable - Power Cords - 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 resistors and other passive components for signs of overheating. If tube arcing has occurred in the past we can usually spot discoloration on the output tube sockets. On vintage units we often spot check select capacitors for value and ESR.

If the amplifier passes visual inspection, we move on to a full test of all of the tubes. We use a modern Amplitrex AT-1000 Professional Tube Tester which is capable of testing both emission and Gm with a high degree of accuracy.

We document the results of each tube and replace any weak or suspect tubes before proceeding.

When we power on tube amplifiers for the first time we usually use a variac and current limited AC supply and slowly raise the voltage up to nominal mains level while monitoring plate, screen, filament, and negative bias supply voltages where applicable.

If everything is in order we feed a low level test signal into the amplifier’s input and monitor its output on an oscilloscope across an 8 ohm dummy load. At this point we are just looking to verify basic function and confirm that the output transformers are not damaged.

Once we have verified that the amplifier is safe to operate, we connected it to full mains power. For fixed bias amps we set the bias to manufacturer spec. For cathode biased amps we monitor the plate to cathode voltage to determine if the output tubes are operating in a safe range. Once the output section is verified we move onto bench evaluation.

We start by feeding the input of the amplifier with a low level 1 KHz test signal, slowly increasing its amplitude while monitoring the amplifier’s 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.

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.

For tube amps we like to run this test at least twice.

This allows us to monitor the unit for signs of thermal runaway or intermittent issues that only crop up when it has fully come up to temperature. We find this step to be essential, especially for vintage units.