Comments for page: MXR Preamp

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Posted by Steve L. October 22, 2014 - 08:09 am
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: [see homepage ABOUT].

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
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

Posted by Steve L. March 25, 2011 - 08:32 pm
Fascinating, Mark. Thanks for checking-in. I will contact you by email.

Posted by Mark C March 25, 2011 - 08:11 pm
They where from MXR...never completed. I was trying to find out what they where and came across your site

Cameron_amps "at" yahoo com

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