MXR’s System Preamp II (M-150)
-- Reverse Engineering a Schematic
and Newly Uncovered MXR Schematics of  M-140!

This is the preamp which I have used for about 30-years in my main stereo system. It has always been a flawless performer. Unfortunately, no schematic was provided with it. When I scoured the Web trying to find one, nothing turned-up. The rights to the MXR name and designs were carried-on by Dunlop Manufacturing, Inc., but they did not pick up MXR’s “Desktop” line. I guess that MXR wasn’t very big in the home stereo market.

Admittedly, the M-150 is a weird bird, but that’s why I like it so much. I found out about it by reading a Julian Hirsch review (at right) in Stereo Review, published March, 1981. He gave it very high marks, also mentioning that that it was about to be upgraded to the Model 150. (That added a second phono preamp but left the unit otherwise, almost identical–more about that below.) Highlights of the review include:

• Phono RIAA accuracy ±0.25dB.
• Phono response is affected less than 0.5dB by cartridge inductance.
• Phono noise is below their measurement floor. (Specified 88dB below 10mV.)
• Switchable three-pole phono filter is only -1.5dB at 20Hz but -31dB at 5Hz.
• Distortion 0.001% at 1Vrms.
• Frequency response 6Hz to 170kHz at -3dB.
• Free of transient noises, due to FET muting circuits at key points.
• (MXR specified the SNR at 90dB below 1Vrms, which is 107dB below max.)

The only criticism in the whole review was the lack of a switched AC outlet. That was something I remedied with a simple wiring change. The switch they provided has had no trouble lasting all these years switching my Hafler-200 amps.

What makes the unit so unique are its mixer functions and ability to insert external processors, which are very unusual for a stereo preamp. I had always wanted to be able to fade between sources and this preamp can do that. The options to insert external loops are handy for doing equalization, noise filtering and patching-in TV audio. The clincher for me was the fact that there are no tone controls on the preamp. This was before bypassing tone controls was cool. All those years, I had been carefully setting tone controls at center and worrying whether they were really flat at that exact point. Finally, I didn’t have to worry about that anymore. Join me now for a little tour of this unique component.

If you doubleclick the pic at the top of the page, you will see it in high resolution, which makes the functions clearer. (Widening the browser window may be needed.) Let’s start with the “MAIN” group of controls. The Volume of course sets the level going to the power amplifier. The Select switch there lets you listen to either the A-source, the B-source, the output of the Mixer control or either of two tape decks.

At the left, the two Source switches choose the A-source and B-source. Either can select one of six inputs, including two separate RIAA phono preamps. So, for example, you can have Phono-1 selected on A and Phono-2 selected on B. The Mixer control fades between them. The Main Select switch can choose the Mixer signal or A or B directly, as mentioned above.

What really makes the preamp handy for light DJ work though, is the fact that it has the separate monitor Select switch for the headphone output. It can choose A, B, Mix or either tape, just like the Select for the Main channel. The Main channel can be playing Phono-1 on A, while you are cueing-up Phono-2 on B, using the headphones. Pretty cool, eh?

This feature is absolutely necessary if you are going to create mixes in real time. Not that I have ever been able to do that! But if I could, it would be so much faster that way. Back in college, I worked for an educational radio network. There was a jazz program producer named Lee Nance, who had a weekly hour-long show there. He would arrive and in one night, produce a whole month of shows. It takes me all evening to produce a 45-minute mix! (Of course, I obsess over every detail :) The next morning, the studio would be a mess, though, with an overflowing ashtray and litter all over. I didn’t mind tidying-up though, because Lee produced such great shows and was a really nice and interesting guy.

But I digress. Let’s get back to the MXR: Other unique features of the preamp include the front-panel guitar/mic input and the two send/receive loops. The loops let you insert an external processor in the audio path. You can choose where in the path you want it. One can be in A or Main, while the other can be in B or Mix. Believe it or not, over the years, I have used almost every one of the M-150’s interesting features. The support for two tape decks was handy when I added a minidisc recorder to complement the Nakamichi cassette deck.

Anyway, nine years ago, I needed a schematic for the M-150. I forget now what the reason was. (I have made a couple mods to it.) Since I love the critter and couldn’t find one, I decided to trace out the circuits and create one from scratch. That turned out to be a major task. But I hung in there with it and am happy to be able to offer it to you now:

MXR System Preamp II (M-150) Schematic

Below is a scan of the 21-page Operation Manual for the M-150. On pages 13-17, they demonstrate some of the advanced things you can do with the preamp, using pictorial aids. The impressive specifications are on the last page:

MXR System Preamp II (M-150) Operation Manual (5MB)

I would be interested to hear from any other owners of this unit. Except for the ones reported below in Reader Comments, mine is the only one I have ever heard of.

Newly Uncovered Original MXR Schematics of the M-140!

Thanks to a kind contribution from Nate Mudge, we can now present copies of the original MXR schematics and assembly drawing for the M-140 System Preamp. Since the M-140 appears to be very similar to the M-150, we can now generally infer the original reference designators (R1, R2 etc.) for the parts on the M-150. It is unlikely that they would have changed much between units.

The story behind this remarkable find is interesting. Let’s let Nate tell it, in his own words:

From 1975 to 1980, I was the MXR manufacturer’s rep for the state of Florida. At the end of my days as a rep, I had a rep sample of the M-140 System Preamp, which made its way into my home stereo system. It has been with me to this day. I am using it in my audio duplicating setup, where I am digitizing my entire LP collection. The only problem I have had with it was dirty contacts on the switches, which I cleaned with Caig Labs contact cleaner. It still performs perfectly. As I recall, when this unit was first introduced, the factory bragged about using military grade PC boards and components. I also have a poorly Xeroxed copy of the original schematic and pc board layout. This is a very special unit, unlike anything else.

We have restored the documents as much as possible and they are quite useable:

MXR System Preamp (M-140) Schematic Page-1

MXR System Preamp (M-140) Schematic Page-2

MXR System Preamp (M-140) PCB Assembly Drawing

Many thanks to Nate!

Curious about how closely the M-140 and M-150 match, I did an overlay (in Red) of the M-140 PCB Assembly Drawing and a B&W photo of the M-150 PCB:

Overlay of M-140 in red and M-150 PCB B&W photo

The scanner photo (from the old days) is a bit blurry but a casual perusal of the board shows a very close resemblance. My M-150 originally had an add-on board providing a second phono preamp. I believe that it was the main difference between the two models.

Below: Half-page ad for the MXR-140, which appeared in Stereo Review, September, 1980.

Reader Comments

Posted by Steve L.

October 09, 2011 - 07:44 pm

Hi RT, It is very interesting that you think yours might be different from the one pictured here. The models 140 and 150 were virtually identical. I am not aware of any other preamp models that MXR made. If there is another, I would very much like to find out about it. Could you send a pic? You can contact me at: steve (AT) tronola.com. Thank you for posting.

Posted by R. T. Barr

October 09, 2011 - 05:22 pm

I have a MXR preamp and graphic equalizer that I bought new from the old Stereo Cost Cutters (SCC) mail order store in approximately 1985. At the same time I bought two Marantz MA-5 monoblock amplifiers and a Marantz 2130 tuner. I believe SCC could not get any Marantz preamps as closeouts, and so they sold the MXR for those who wanted to buy a whole system. My unit is smaller and simpler looking than the one pictured here. It has no tone controls, but does have external processing loops where the graphic equalizer is easily added. I no longer use this unit on a daily basis, but it is in my garage in like new condition. I am sure I will find a use for it someday.

Posted by Steve L.

July 12, 2011 - 08:10 pm

Hi Nate, I really appreciate you sharing your story here. The M-140 will be an excellent way to transcribe your LP collection, I'm sure. The MXR preamps did indeed use epoxy-glass PC boards, which is unusual in consumer products. I also noticed that caps used in critical places (e.g. RIAA compensation) looked like high-class parts. I would love to add scans of the schematic and layout with the article. If you are able to share those, you can email me at: steve(AT)tronola.com. Any other information about the M-140 and 150 would be most appreciated. I heartily agree that they are very special and unique. Best Regards, Steve

Posted by Nate Mudge

July 12, 2011 - 05:26 pm

Hi, I just ran across this site, and would like to add my 2 cents worth. From 1975 to 1980, I was the MXR manufactures rep for the state of Florida. At the end of my days as a rep, I had a rep sample of the M-140 System Preamp, which made its way into my home stereo system. It has been with me to this day. I am using it in my audio duplicating setup, where I am digitizing my entire LP collection. The only problem I have had with it was dirty contacts on the switches, which I cleaned with Caig Labs contact cleaner. It still performs perfectly. As I recall, when this unit was first introduced, the factory bragged about using military grade PC boards and components. I also have a poorly xeroxed copy of the original schematic and pc board layout. This is a very special unit, unlike anything else.

Posted by Steve L.

May 17, 2011 - 11:08 am

Hi Matt, It's so nice to hear from another MXR Preamp owner! Sounds like you have found a great application for its unique features. I take it that the M-140 was titled as "System Preamp" instead of "System Preamp II" ? The mods I made weren't much. The basic performance of the preamp is excellent as-is. I removed one of the two phono preamps, making that input line-level for a CD player. I also made the power outlet switched, so that I can control the whole stereo system with it. Very glad that the schematic and manual are helpful. Thank you for the kind message. Sincerely, Steve

Posted by Matt

May 17, 2011 - 10:26 am

Just wanted to say thanks for the schematic and manual! now I understand what the low pass filter button does... I must have one of the rare M-140s, but more or less looks exactly the same (minus the stereo reverse function I think). I bought it on HarmonyCentral 5+ years ago and have been using it as my guitar rig preamp since. I used to run the loops into my eq and fx, which let me balance wet/dry easily... Works great, sounds great, looks vintage cool of course too! What mods did you do to yours? anything easy you would recommend? Thanks, Matt

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