Pic of MXR System Preamp II with link to hi-res version

MXR’s System Preamp II (M-150)
-- Reverse Engineering a Schematic
and 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.

Stereo Review MXR lowAdmittedly, 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 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.

Uncovered Original MXR Schematics of the M-140

Thanks to a kind contribution from Nate Mudge, we can present copies of the original MXR schematics and assembly drawing for the M-140 System Preamp. Since the M-140 is very similar to the M-150, we can now 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.

For the story behind this remarkable find, 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 for a second phono preamp, soldered on top of the main board. That is the only dif­fer­ence found between the two models.

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

MXR preamp Stereo Review Ad low


Reader Comments

Posted by Steve L. December 29, 2018 - 04:23 pm
Hi DC, I appreciate your kind post. So glad you could find another one! It seems that not many were made. Yeah, they're built solidly, with the fiberglass PCB and rugged construction. The switches on my unit have held up very well, even though I changed the wiring to let the power switch operate my two Hafler DH-200 amplifiers. Let us know how the loudness mod goes!

Posted by DCinDC December 29, 2018 - 02:00 pm
So 35 years later I bought another.
A model 140.
Thanks so much for all your work here.
I will add a loudness circuit in one of the loops. The things I don't like about these are also common in the Phase linear preamps, rca jacks etc. The MXRs somehow hold up better than those. Again thanks for drawing the schematic! I may replace some op amps with compatible and better ones for fun.

Posted by Steve L. December 19, 2018 - 08:06 am
Hi Bob, Thank you for your kind message. The list price of the MXR 140 System Preamp in 1981 was $460, ($1310 in 2018 dollars). Of course (as you know), list prices in those days were far higher than the typical "street price." I totally agree with your assessment that the preamp has a very clean and transparent sound. For other readers, the late Keith Barr who Bob mentioned was a founding member of MXR. Interested readers can find out more about Bob by searching for Bob Easton 360 Systems.

Posted by Bob Easton December 15, 2018 - 07:59 am
Many thanks for the OpMan, schem and your comments. I have an early unit Keith Barr gave me when they first came out, I've used it continuously since. Don't know what they sold for, but it's incredibly clean and transparent.

Posted by Steve L. January 25, 2018 - 04:01 pm
Hi Michael, That's a fascinating story! I didn't even know about the MXR Linear Preamp, which I gather from a Web search, is also the Model 139. If you still have a schematic you can share, I would love to add some coverage of the 139 to the article. Would also like to include your story, if you're up for that. I see there was a review of the 139 in Audio magazine, Nov., 1981. Will see if I can track that down. You can contact me at the email address on the About page. Thank you so much for posting!

Posted by Mike January 25, 2018 - 03:10 pm
My name is Michael and I was one of the original MXR techs. Working for Keith and Terry through the early 70s till late 76 when I made the decision to move to Florida and never looked back. In late 74 the audio interests of myself and another line tech. of which there were usually 3 or 4 lead us to come up with a design for an audio preamp based on the linear, low noise op amps that we were using in most of the effects at the time (TL0 22 and 44). The other tech drew up the schematic and I bread boarded a prototype with the parts on hand at the time. I built the proto on my own time paying for the parts out of my pocket and got it working and it worked well and I also made a few improvements. The other tech took the design to the engineering staff and they loved it; however the business guys did not. I think they feared a conflict of interest and/or ownership on a design not coming directly from their own engineering staff. I later on found that MXR had actually started producing the design a few years later as the MXR Linear Preamp the base design in all of the models later produced. I was able to obtain a schematic to the original unit and found it to be nearly identical to the unit I originally breadboarded up. Some years later I was able to obtain one of the original production models and still use it to this day to handle the audio output from my Flex 5000a SDR radio. Because of its low noise and linear operation it makes an excellent platform for the signal processing output from the SDR transceiver. I know I have the original breadboard I built so many years ago though it has been a long time since I have seen it and think it is most likely buried in a box along with many other projects both successful and not so successful.

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