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McIntosh MC 427 Amplifier Review

Rate this car amplifier:

  • 1. Poor (headless panther)

    Votes: 1 0.6%
  • 2. Not terrible (postman panther)

    Votes: 1 0.6%
  • 3. Fine (happy panther)

    Votes: 36 23.2%
  • 4. Great (golfing panther)

    Votes: 117 75.5%

  • Total voters
    155
They did make one car amp with the autotransformers... the mighty MCC602TM. 300w x2 at 8,4, or 2 ohms

Damn.. I really really find their design ugly. I'm like surprised every time I look at it.
It's like the Dr. Pepper I buy once every two years because I think to myself "nah, It can't be as bad as I remember". :facepalm:
 
Looks like a positive and negative rail generating SMPS with a class B linear amplifier. Very nice performance. Definitely a 'blameless' design.

The only way I can think of improving this would be to use a class-G topology to improve the efficiency at normal listening levels (easy to generate rails at 30% or so of the normal ones with such an SMPS by tapping the transformer).

Enter Hifonics/Zed audio "Varipower" amps.
The ones with more than 50WPC were class G. Similar vintage as the Mac measured here. Gen 7 and 8 where the GOAT.
 

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Obviously a fine bit of kit. But to me there is something about a McIntosh car amp that just seems….off. Unless the car is a vintage Mercedes gull wing perhaps.
 
Impressive!

But expected. :). McIntosh in the Clarion era was SOTA. Clarion was so focused on precision that they introduced the water jet glass front panels of the home unit.

Yes, McIntosh sells expensive coffee table books and wall clocks, but I still think that the McIntosh is a reliable choice for performance when cost is taken out of the equation.

They sold a MPM4000 which is a power meter which fits in a single DIN slot.
I recall seeing a McIntosh unit that had a face plate showing the iconic logo, nice and back-lighted. There was absolutely nothing audio in the box itself, and the unit cost an arm & a leg to boot. Wonder how many of those they sold.
 
Great review!! I've used and still use the MC427 and the MC443 mono block since 1995, both units are still in my old, retired competition vehicle. They still play after all these years, the 443 has ran a 1ohm load since day one. Never cracked open but I probably need to check the caps. Great equipment though and they were built in the same build house by the same hands as the home gear thanks to Clarion of Japan's insistence. If it wasn't for the Clarion's purchase, we would have never seen McIntosh car audio line. Their customer service from the factory to the local reps were impeccable during my competition years. They always told me to send them in at the 20yr mark and they would guarantee they would meet the original test specifications are they would repair it free of charge till they did.

Car audio technology has changed so much over the last decade, with class D technology and the DSP offerings with high end DAC's of today and the ability to have active crossovers, time align and tune each individual driver almost infinitely is beyond miraculous in an automotive environment. I thought the best car audio years were behind us but after building another SQ competition level vehicle, the best is really right now, even with using high end class D amplification. The hard part is getting around the built in head units of today's vehicles and catching a clean signal. Many of these DSP's though, you can run a digital source straight to the unit with whatever quality music file you can find from whatever device you like. Even stream from phone or whatever. Luckily my factory head has Toslink out which runs into a converter box to maintain all my factory controls and functions. Unfortunately, not all OEM's offer such.

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Especially those who participate in SPL competitions: I remember seeing some amps able to go as low as 1 ohm.
Way back in the 1990s Orion was already making amps that were ½Ω stable. As it is now 1Ω is a typical mono amp rating. They do this because you can buy subwoofers in any load you want, even dual 1Ω voice coils. Getting the most from each piece is not limited to how well it tests on a bench. Matching pieces needs to consider size, power draw, mounting style and how well parts work together on top of the final overall look. There's much more to car stereo than you think. With so many choices available, you're only real limitation is how much room you have available and how much room you want to have after the installation is complete.

The way they make cars now has severely dampened the aftermarket sales and installation business, climate control is now integrated with the audio system which makes for a complicated installation. It's no longer a matter of tossing a new head unit in the dash and running wires to your trunk, then building a box that fits in that area.

Once you're ready to upgrade a system in any car made after 2010 you'll be shocked at the price of installing and not the cost of the parts that make up the system. To see big gains it takes a bunch of knowledge in the field. A constant changing field that requires more experience with each part than any home system. The time spent planning a system is not even comparable to the time spent planning any other audio set up (except live or maybe movie theaters) since you're limited on space and custom changes will cost a fortune.

Read up in some car audio forums and see if you can even decipher what they're even talking about. Knowledge of current parts has never been more important, the problem is, where to start, that's something only the best installers can answer and it comes at a price. You'd be foolish to consider tackling it yourself, you'll just end up spending more money and time after you find it doesn't work.

Never expect an upgraded system to be a competition winner even when the cost far exceeds what a typical competition system installed in an old Blazer or Jimmy will cost. You just can't make those changes to new vehicles, read more about how each car maker has put safeguards in place to stop you from even trying to upgrade a speaker.

Car audio has surpassed the McIntosh amp tested here, there are so many amps available that can exceed what you can do with that amp, for its day, it was fantastic for SQ but for competition there were amps capable of much more for the same price, sadly those amps have little value in today's market. Power output is no longer a problem, amps now can put out over 10KW for about $500, problem is powering them. That piece is one for a collector and has no real world use, you can get the same output from new amps at half the price and half the size and half the current draw. Upgrading charging systems on a new car is just as costly as the install of a system in a new car, you simply cannot consider skipping the upgrade of both unless you aren't expecting any improvements over the stock system.

Electric cars have removed audio upgrades completely, as I said, car audio is dying, it's for the wealthy and the poor., no longer a middle income investment. I say that because poor people own older cars that can be easily upgraded by the owners. Although it's nice to see old pieces like this tested by a true professional but the limited interest in that amp makes it barely worth the time. Due to the high output and huge power needed to power car amps, dyno testing can only be done by a few reputable sources and I doubt Amire is going to invest in a 14.4vdc / 1KA PSU or a bank of 10 batteries plus ½Ω loads that can handle 10kw!

You'd be surprised at how many people would be interested in seeing much cheaper items tested rather than testing pieces that everyone expects to have good results. Popular stuff is rarely costing over $500, regardless of what it is, I guarantee you'll find much more interest in results concerning cheap stuff or even older very popular items over higher priced new gear, I'm not saying don't test new stuff, but it's to a point now where the differences in results are minimal and don't really reflect the pieces usefulness when in use in an audio SYSTEM rather than numbers on a graph.

Speakers tend to make the largest changes to sound in any system. Even though they all are totally different and some work better than others on certain other pieces, we all know that tests can't give a real description of their sound or how well they'll match up with other pieces. I guess what I'm saying is testing numbers can only be used to see if manufacturers are truthful, other than that, tests are kinda useless in this hobby.

Nothing against this site, I see the minimal differences in results coming down to cost of each piece. Kind of like an overall value per performance rating would need to be used to get a buyers stand point rating on new gear. I think price is a major factor in choosing pieces with specs being a distant second place when buying new gear. Then maybe looks, longevity seems to be forgotten in these times, something that was one of the main concerns in years past. Just shows you how priorities change and tests used to be important since there was a huge difference between pieces, not so much now, unless price is factored in.

As always thanks for the professional tests posted here, few are capable of doing this correctly and even fewer ever join in the conversation after the results are posted. Responding with facts and a level head has always been what makes this site appealing.
 
Hi guys,

I sent this amp in to Amir. I've been refurbishing McIntosh car amps for a bit, validating my work on a QA401 analyzer, but these amps were measuring (for me) such low overall THD+N I knew I had to send it in to Amir for a spin on the mighty AP! Anyway, here's some further info about this amp:

- It was a restoration project for me. It had acid-based damage on multiple traces due to leaking electrolytics. I replaced all of the electrolytics with appropriate Nichicon capacitors (FG, ES, and UPW) you can see in the photos.
- From studying the design (and some rogue service manuals I found the web) the pre-amp stages have dedicated, highspeed non-signal path op-amps to detect and cancel noise and harmonics in the pre-amp stages, a McIntosh special I think. (Maybe the reason for the higher cross-talk?)
- The output power stage has 4 stages that I can identify - a differential stage, followed by two voltage stages, and the final current stage. It is class AB with a heavy A-bias (for a car amp). That's why it draws 7A for 5 watts output. There is no global negative feedback - the negative feedback for the output stage of each channel is only reflected back to the first differential stage in the power section.
- It does have a special protection feature called PowerGuard. Which is maybe what Amir saw in his 200Hz sweep. It activates the yellow LED. What happens is, when the onset of clipping is detected on the output of any channel, the signal in the pre-amp stage is proportionally reduced to maintain less that 2% THD and prevent clipping at the output. It looks super weird on a scope - you can increase the input, see the output grow, until the output just stops, holding a steady sine wave just before clipping. With my QA401 analyzer, I had to set my THD+N sweep stop setting to -60dB (0.1%) instead of 1%, as the amp itself would pull back the clipping, causing the analyzer to just keep increasing input level until it reached the top step of the signal to the amp.

Attached are the measurements I was able to take on this amp before sending it to Amir. It looks close but the QA401 is definitely no AP test rig :) Plus I wouldn't necessarily consider myself a professional in the measurement department like Amir :)

Hi all so I finally got to reading "Designing Audio Power Amplifiers" by Bob Cordell, and I am now able to properly parse what's going on in this amp. Most of the above I actually got wrong. Here's a quick run-down of what's in this amp:

- The output stage is a triple emitter-follower (specifically, a Locanthi-T triple, where the driver and pre-driver are in continuous conduction). Each channel has two pairs of output BJT devices.
- The IPS (input power stage) and transimpedance amplifier stage (or VAS, votlage amplifier stage) for each channel are provided by an STK Pack (STK350-000). This pack provides a single-differential IPS and single-ended VAS for each channel.
- There is a negative feedback loop around the output triple and IPS-VAS. There appears to be a phase-lead capacitor in parallel with the attenuating resistor in the feedback path. There are zobel networks at the IPS inputs.
- There are two pre-amp stages. The first stage is where overall gain is set. In addition, the first pre-amp stage uses some kind of feed-forward active noise cancelling scheme. The 'quiet ground' resistor in the first stage is monitored by an op-amp which feeds forward and mixes a signal into the output of the 1st stage. (The RCA shields are grounded through this 'quiet ground' sense resistor). Active common-mode noise rejection maybe?
- The second pre-amp stage is where optional filtering and bass boost can be done. It does not appear to use the same 'quiet ground' as the 1st pre-amp stage.
- This amp has the 'McIntosh power guard' dynamic attenuator circuit for each channel. The output of each channel is compared to it's respective IPS input, and any output distortion triggers a JFET attenuator in the 1st pre-amp stage, in proportion to the detected distortion. The effect is the 1st pre-amp stage output is 'lowered' dynamically to limit clpping. Neat!
- For the output devices there appears to be some kind of "flying" or floating SOA protection that clamps the bias if the current through the emiiter resistors becomes too high.
- Bias is achieved via a typical Vbe multiplier connected across the output of the STK IPS-VAS.
- The power supply appears very tightly regulated, with a BJT providing set-point feedback from the power rails directly to the power supply IC. No optical isolation.

Thanks again for reviewing this amp @amirm. For those interested in seeing how class-AB amps are designed in depth, and the ways they can be non-ideal (and how you deal with that), I can't recommend Bob's book enough. His sections on Class-D are also really insightful, and helped me get into Bruno Putzeys self-oscillating class-D papers and his articles about grounding and feedback (known as "The F-word" and "The G-word" articles).

Cheers
 
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