Home Donate New Search Gallery Reviews How-To Books Links Workshops About Contact McIntosh MC240 Intro Specifications Measurements Sound Compared Usage Recommendations McIntosh MC240 (rated 40 watts continuous output per channel with less than ½% THD at any frequency from 20 ~ 20,000 Hz, measured output 47W at 1 kHz at <½% THD, 56 pounds/25.4 kg, measured 145 watts idle power draw, about $3,500 used). enlarge. I got mine at this link directly to them at eBay (see How to Win at eBay). This all-content, junk-free website's biggest source of support is when you use those or any of these links to approved sources when you get anything, regardless of the country in which you live. Buy only from the approved sources I use myself for the best prices, service, return policies and selection. Thanks for your support! Ken.
Rear, McIntosh MC240. enlarge.
August 2023 audio reviews tube reviews all reviews Why Tubes Sound Better 27 May 2015 The amazing thing is that the old one I got with tubes that dated as far back as 1961 certainly didn't meet its specifications, but for actually enjoying music sounded absolutely spectacular. My renovation brought this amplifier back to its original laboratory specifications. As it arrived, it already sounded out of this world. It turns out that all it needed to meet factory specifications was a new set of inexpensive Russian Tung-Sol tubes from TubeDepot (only about $200 total) and one bad resistor replaced, a 17¢ part. While I had it open, I replaced all four 12 kΩ 2W resistors, as well as replaced all four major power supply capacitors. I also rebuilt the bias supply with a 1N4007 diode and a 220 Ω resistor to bring bias to exactly -150.0V when the B+ was 435 V, and replaced the original 12 µF electrolytic with a 12 µF 250V film capacitor. The lower voltage supply is 135V (not 140V), which will needing slight adjustment later. I replaced the thermistor per McIntosh's suggestion, and the new one loses less voltage when hot (1.13V versus 2.5V), giving me a few more watts maximum power. I raised the drooping 140V line from 135V to 140V by adding a 2 MΩ shunt across the 150 kΩ dropping resistor. The original resistor actually measured about 156 kΩ and the 100 kΩ resistor measures about 104.7 kΩ. The best tube amps, like McIntosh, have separate feedback windings, so the output taps are completely isolated from everything else, with zero DC offset and no need for fuses to protect speakers against failed tubes. This lets us series or parallel the outputs to our heart's content for mono or increased power outputs, or driving just about any imaginable load impedance. For instance, the McIntosh MC240, MC275 and MC225 in mono can be wired happily to drive any of 2Ω, 4Ω, 8Ω, 16Ω or 32Ω loads as their optimum impedance. These amplifiers also have higher impedance outputs (typically 150Ω, 600Ω in stereo and many more in mono) for driving very long lines at high powers. This also means that there is no worry about interference or ground loops, no matter how you wire the outputs. Contrast this to solid state amplifiers, whose outputs are almost never isolated from anything, and may never be connected in series or in parallel. The mono options on stereo solid-state amplifiers are bridge connections, which tend to lower performance.
Introduction top Intro Specifications Measurements Sound Compared Usage Recommendations
My McIntosh MC240 is the best power amplifier I've ever heard. While most great amps do nothing to alter the sound (what comes in is what goes out), and when transistor amplifiers alter the sound it's almost always for the worst, the subtle things tube amplifiers and especially the MC240 do to the music make it sound absolutely fantastic. Of course the MC240 sounds completely lucid, fluid and detailed as you expect from top-end tube amps, but what you probably don't expect is that it has bottomless bass thanks to its huge transformers and special patented design that offers low distortion and flat response down to 3 cycles! My MC240 has better bass response than many of today's preamplifiers and power amps, along with the astonishingly lifelike midrange and a high end that simply shimmers. No other amplifiers have McIntosh's Unity Coupled circuit, that along with its huge Bifilar-wound output transformers, lower the distortion to the lowest levels found in tube amplifiers. This unique circuit cross-couples the two 6L6GC output tubes into such a tight a lover's embrace that they operate as class-A at the most important audio levels. The circuit is so linear and independent of tube characteristics that it plays at low levels even with one output tube missing! The McIntosh MC240 is McIntosh's first stereo amplifier, and their last new tube amplifier design before they gave up on tubes in favor of transistor amplifiers. The MC240 was first, and it was followed the next year by the more powerful MC275 (75 WPC) and simpler MC225 (25 WPC). This review applies just as well to the MC275 and MC225.
History There are at least four versions. There are three versions known to most people, and the last version as seen and reviewed here.
Specifications top Intro Specifications Measurements Sound Compared Usage Recommendations
Rated Output Power 40 WPC (16.02 dBW) stereo into 4Ω, 8 Ω, 16Ω, 125Ω (70.7V) or 600Ω (140V) from 20-20,000 Hz with less than 0.5% THD. 80 watts mono into 2Ω, 4Ω, 8Ω, 16Ω or 32Ω depending on output connections. This is a very serious output power specification, I know of no tube amplifier today that specifies output power at such a low level of distortion over such a huge frequency band. Modern high-end tube amplifiers lack the low-end guts of the McIntosh and can't put out much power at all at these low distortion figures at low frequencies.
Inputs Three switch-selectable inputs, all 250 kΩ:
MONO One mono input jack with a level control. 500mV minimum to 30V maximum input for rated output.
TWIN Two input jacks, each with its own level control. 500mV minimum to 30V maximum input for rated output.
STEREO Two input jacks. No level control, but there is a balance control to fine-tune the system's balance between the two channels. Level fixed at 2.0V input sensitivity for rated output.
Frequency Response 16 ~ 40,000 cycles +0, -0.1 dB at rated power. 16 ~ 60,000 cycles +0, -0.5 dB at rated power. 10 ~ 100,000 cycles +0, -1.0 dB at half rated power. These again are serious specifications not matched today, even by McIntosh. Today's tube amps cover a similar range, but at -3 dB. All other tube amplifiers, including new ones from McIntosh, have much sloppier specifications, and they certainly aren't rated at full power.
Phase Shift ± 6º 20 ~ 20,000 cycles.
Signal to Noise Ratio ≥ 90 dB below rated output. This is ≤ -73.98 dBW or ≤ 566 µV or ≤ -64.95 dBV.
THD < 0.5% from 20 ~ 20,000 cycles at rated power. ≤ 0.3% typical.
IMD < 0.5% for peak powers up to twice rated power for any two frequencies from 20 ~ 20,000 cycles.
Damping Factor > 10, regardless of selected output impedance. Output Connectors Barrier block, 3/8" spacing for low impedance loads. Octal socket for high impedance loads.
Tube Complement (4) 6L6GC or 7027A for output. (3) 12AX7. (2) 12BH7. (2) 12AU7.
Quality Made in Binghamton, New York, USA.
Construction Sheet steel chassis.
Power Input 105-130 VAC 50 ~ 60 cycles. Negative temperature coefficient thermistor for inrush current limiting for soft turn-on.
Rated Power Consumption 145W at idle. 270W at full power. 3.2A slo-blo fuse. 3A internal B+ fuse. See also Actual Power Consumption.
Size 8 x 10-3/4 x 17-1/2" HWD.
Weight 56 pounds. 65 pounds as shipped.
Price, USA August 2023About $3,500 used if you know How to Win at eBay.
October 2021About $2,400 used if you know How to Win at eBay.
May 2015About $2,400 used if you know How to Win at eBay. A really shiny one will sell for about $2,800, and a rusty one will sell for about $1,800.
1960$288 list price.
Measurements top Intro Specifications Measurements Sound Compared Usage Recommendations These measurements are made with an exotic Rohde & Schwarz UPL laboratory analyzer. The traces from the Rohde & Schwarz UPL laboratory analyzer are color coded for the Left Channel and for the Right Channel. When they don't lie on top of each other, it's due to channel imbalance. When they do lie on top of each other, the trace turns blue. Unless otherwise specified, all measurements are RMS at 1 kHz at 1 W into 8Ω from the 8Ω tap, both channels driven. I used a 117 VAC supply voltage.
Frequency Response Distortion Noise
Input Levels measurements top
STEREO Input 475 mV for 1 WPC output. 1.502 V for 10 WPC output. 3.030 V for 40 WPC output.
MONO or TWIN Inputs 83 mV for 1 WPC output. 524 mV for 40W output.
Gain measurements top
STEREO Input 15.533 dB when set to level both inputs. 16.123/15.156 dB at center.
16.182/15.107 dB at center. 16.543/15.543 dB when set to either side.
old tubes: 15.500 dB left, 15.500 dB right, at balance S. 15.714 dB left, 15.272 dB right, at balance center. 16.092 dB left, -50.22 dB right, at balance left. -53.71 dB left, 15.979 dB right, at balance right.
Twin Inputs 30.845 dB left, 30.708 dB right, at maximum.
30.86 dB left, 30.62 dB right, at maximum. Old tubes: 30.453 dB left, 30.825 dB right, at maximum.
MONO Input 30.823/30.68 dB.
30.824/30.580 dB at maximum.
Old tubes: 30.432 dB left, 30.855 dB right, at maximum. 16.734 dB left, 17.17 dB right at 12 o'clock.
Frequency Response measurements top Frequency response.
Frequency response, expanded scale.
Phase response.
Infrasonic response, cursor at 18 cycles at -0.1 dB.
Infrasonic response, cursor at 8.5 cycles at -0.5 dB.
Infrasonic response, cursor at 6 cycles at -1 dB.
Infrasonic response, cursor at 3 cycles at -3 dB.
Infrasonic response, cursor at 3 cycles at -3 dB.
Infrasonic response to 1 cycle.
Infrasonic response to 100 millicycles.
Ultrasonic response, Stereo input.
Ultrasonic response, DUAL input at full level.
Distortion measurements top Waveform at 25 watts per channel, old tubes.
Waveform after new tubes from TubeDepot.
Waveform after new tubes from TubeDepot; a 12k Ω 2W resistor is still out-of-spec and causing the clipping on the negative part of the right channel.
Waveform at 20 cycles after new tubes from TubeDepot; a 12k Ω 2W resistor is still out-of-spec.
Waveform at 40 watts per channel after new tubes from TubeDepot and replacing a 12k Ω 2W resistor that was out-of-spec.
Waveform at 20 cycles at 40 watts per channel after new tubes from TubeDepot and replacing a 12k Ω 2W resistor that was out-of-spec.
Waveform at 20 cycles at 44 watts per channel after new tubes from TubeDepot and replacing a 12k Ω 2W resistor that was out-of-spec.
Waveform at clipping after new tubes from TubeDepot and replacing all four 12k Ω 2W resistors. Finally, symmetrical clipping!
10 cycles burst at 1kc after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.
5 cycle burst at 50 cycles after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.
2 cycle burst at 20 cycles after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.
THD versus power, old tubes, April 2015.
THD versus power after testing tubes, putting good ones on Right Channel and then replacing two weak 6L6WGH with two new 6L6GH on Left Channel, 29 April 2015.
THD versus power after testing tubes, putting good ones on Right Channel and then replacing two weak 6L6WGH with two new 6L6GH on Left Channel and swapping an old USA 12AX7 and 12U7 for new Chinese tubes, 29 April 2015. Left channel finally meets 40 W at < 0.5% THD; it does it at 0.2%!
THD versus power after new tubes from TubeDepot, 08 May 2015. Right channel still out-of-spec due to what turns out to be a bad 12k Ω 2W resistor.
THD versus power after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor, 29 May 2015.
THD versus power at 20 cycles after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor, 29 May 2015. 42 watts at 0.5% THD at 20 cycles, not bad! What looks like higher THD below 1 watt is actually 60 cycle power supply hum looking like harmonics; ignore it.
THD versus power after new tubes from TubeDepot and replacing all four 12kΩ 2W resistor, 29 May 2015. Note matching distortion signatures. Moving the cursor also shows 0.15% THD at 38W, 0.3% THD at 43W, 0.5% THD (spec) at 45.5W, 1% THD at 48W and 4.5% THD at 51W. Of course the maximum power varies as the square of the applied power supply voltage. Run this amplifier from 125V instead of 117V as tested and the clipping part of the curve would move to the right by 6 watts.
THD versus power after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies and replacing the thermistor, 12 June 2015. Now I get over 45W at less than 0.5% distortion.
THD versus power after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies, replacing the thermistor and then calibrating all the power supply voltages, 12 June 2015. Now I get 42W at 0.2% THD.
THD versus power after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies, replacing the thermistor and then calibrating all the power supply voltages, 12 June 2015. Now I get 44W at 0.25% THD. THD versus power after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies, replacing the thermistor and then calibrating all the power supply voltages, 12 June 2015. Now I get over 47W at less than 0.5% THD. HA!
THD at 10 mW after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.
THD at 10mW after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor. THD at 100mW after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor.
THD at 1 W, old tubes.
THD at 1W after testing tubes, putting good ones on Right Channel and then replacing two weak 6L6WGH with two new 6L6GH on Left Channel, 29 April 2015.
THD at 1W after new tubes from TubeDepot, bad 12kΩ 2W resistor in right channel.
THD at 1W after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.
THD at 1W after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies and replacing the thermistor, 12 June 2015. Now the channels match.
THD at 10W after new tubes from TubeDepot, bad 12kΩ 2W resistor in right channel.
THD at 10 W after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.
THD at 20 W after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.
THD at 40 W after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel. THD went up to 20% at 10 cycles at 40W.
Same plot as above (THD at 40 W), except plotted on same scales as McIntosh Audio Clinics used, also after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.
Same thing (THD at 40W) after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies and replacing the thermistor, 12 June 2015. THD at 40W after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies and replacing the thermistor, 12 June 2015.
Harmonic distortion content at 1 mW, new tubes and replaced resistor.
Harmonic distortion content at 10 mW, new tubes and replaced resistor.
Harmonic distortion content at 100 mW, new tubes and replaced resistor.
Harmonic distortion content at 1 W, old tubes. Looks same at 10W, just different overall level.
Harmonic distortion content at 1 W, new tubes and replaced resistor.
Harmonic distortion content at 10 W, new tubes and replaced resistor.
Harmonic distortion content at 40 W, new tubes and replaced resistor.
19 + 20 kHz difference-frequency distortion at 100 mW after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.
19 + 20 kHz difference-frequency distortion at 1 W, old tubes.
19 + 20 kHz difference-frequency distortion at 1W, new tubes and new resistor.
19 + 20 kHz difference-frequency distortion at 10 W, old tubes.
19 + 20 kHz difference-frequency distortion at 10 W, new tubes and new resistor.
Output Noise measurements top Output noise spectrum.
Damping Factors measurements top Frequency response driving an 8Ω resistor (green trace) and driving a real B&W CDM2 bookshelf loudspeaker (yellow trace), whose impedance varies between 6 and 45 Ω, a typical loudspeaker load. First try with old tubes.
Frequency response driving an 8Ω resistor (green trace) and driving a real B&W CDM2 bookshelf loudspeaker (yellow trace), whose impedance varies between 6 and 45 Ω, a typical loudspeaker load. Now with new tubes and four new 12 kΩ resistors. Note lower output impedance. Same as above, expanded scale.
Actual Power Consumption measurements top 60Ω thermistor 24 June 2015 Big 60 Ohm thermistor: 12V drop, then 6V after 1.5M. 4.4V after 3.5M. 4.11V after 5M. 5V drop at 40 WPC, then 2.83V drop at idle. Power line reads 3.6Ω DC after running. 11Ω after a minute blowing on it.
Turn on at 120V 75W 15s 116W ar 30s 124W at 45S 127W at 60s 129W at 90s
117V 128W at idle. 286W at 40 WPC
120V: 135W idle. 144W at 1 WPC 195W (2.01A) at 10 WPC 294W at 40 WPC (3.0 A) B+: 500 mA idle, 623 mA 1 WPC, 1.04A at 10 WPC,1.7A at 40 WPC
------- Cool Down 15s 3Ω 30s 3.8Ω 1M 4.7Ω 2m 6.2Ω 3m 8.3Ω 4m 10.7Ω 5m 13.25Ω 7m 18.75Ω 10m 26.45Ω 11.5m 30Ω 14:10 35Ω 15m 36.25Ω 20m 41.9 Ω 30m; 47.5Ω 45m 51.3 Ω 57m: 52.9Ω 68.5 Ω 12 hrs 23º C 65.9 Ω 16 hrs 24.3º C
75W at turn-on, then 69W, then 135 at 30 seconds 145W (1.52 A) idle at 117 VAC after 90s. 149W (1.54A) at 1WPC 194W (2.01A) at 10 WPC. 286W (2.95A) at 40 WPC.
Old tubes 135 watts at idle. It draws 141 W at 1 WPC. It draws --- W at 40 WPC.
Sound top Intro Specifications Measurements Sound Compared Usage Recommendations Sound is fantastic. Unlike lesser amplifiers, the huge transformers make this amplifier completely unflappable with low frequencies and subsonics.
Compared top Intro Specifications Measurements Sound Compared Usage Recommendations
Versus the new McIntosh MC275 The new digitally controlled (lights and power switching) MC275 is nice, but a very different design. It uses different tubes and different transformers. The tube complement was changed to use tube types that are easy to find today. Its design is optimized to work perfectly with crummy modern tubes, even if they only have 25% of the emission that they ought to. The new MC275 uses RoHS lead-free solder so that it can be sold overseas. I haven't tested this new amplifier yet. Exciting is how its design has been updated to perform flawlessly with the tubes we easily can buy new today, not harder-to-find tubes used in the original design, which expected great tubes.
Usage top Intro Specifications Measurements Sound Compared Usage Recommendations Every so often before you turn on your MC240, check that all the tubes are seated properly and check that the speaker wires are all firmly connected at both the amplifier and at the speakers.
Feet
Connections
Level Controls Leave that level all the way up and control it from the amp's own control.
Recommendations top Intro Specifications Measurements Sound Compared Usage Recommendations Get one. It should be the last amplifier you ever need. Enjoy!
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02 Aug 2023 price update, 03 October 2021, 04 Feb 2019, October 2017, August 2015, 15 April 2015