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.

Posted by Steve L.

June 17, 2014 - 02:18 pm

Hi DCinDC, It's good to hear a contrary point of view. I respect your opinion but I'm puzzled by the reaction you had to the MXR preamp. It's true that the jacks weren't gold plated but otherwise they have held up well for 30-plus years. The pots don't have the resin damping which the Japanese components did back then. But then, I didn't know of any American-made pots which did. While the resin gave pots a smooth feel, it was purely an aesthetic feature and didn't affect the electrical quality. Otherwise, the pots seem of good quality to me and have also held up well. As far as the cabinet is concerned, the thick, extruded aluminum front panel and oiled walnut sides gave me the impression of it being built like a tank. Moreover, it's built with a fiberglass PCB, which was unusual for consumer products. To me, its simple but attractive physical design eschews fancy frills and favors convenient functionality. As far as performance is concerned, the specifications, backed-up by test reports and 30-years of happy listening, speak for themselves. The lack of a loudness switch (and tone controls) is by design. If the rest of your system is flat, such frequency response adjustments shouldn't be necessary. If the rest of your system isn't flat, those adjustments would probably not be sufficient for accurate sound. The issue should be addressed at the offending element. If I had to characterize the sound of the MXR preamp, the word that springs to mind is: transparent. It does nothing to alter the sound and I've never found fault with it in any way. Having said all that, I would like to thank you for posting and sharing your thoughts.

Posted by DCinDC

June 17, 2014 - 11:49 am

I bought one of these new from the AAFES catalog (service member BX) back when new for about $150. I bought it for the features, like most people. I thought it was poorly constructed and performed poorly. The cheap rca jacks, pots and cabinet really put me off. I also really missed a loudness control. I returned it almost immediately. Still I always liked it's functionality. Thanks so much for your efforts at the schematics and collecting this info. Maybe I will try to recreate it with better components. I am trained electronics tech and have been inside many pieces of good stereo components. I consider the PL 4000 poorly made also. I had one of those around the same time and dumped it quick.