What is the Sentry Monitor?

Discussion in 'McIntosh Audio' started by CountD, Jan 25, 2011.

  1. CountD

    CountD Super Member

    Hello All,

    I thought I would ask you guys this question since I am writing a story on this and have talked to Roger Russell about the McIntosh Sentry Monitor in almost all Mc amplifiers (solid state). In order to get to the bottom of this, this is my conclusion on what the Sentry Monitor is:

    "Most people are under the impression that the Sentry Monitor is a limiter, but in my understanding it is not, but a circuit that allows ‘run off’ or that the SM diverts extra current away from the output transistors if the current load is outside specific safe operating parameters."

    What do you guys think of the above on what conclusion I have come to? I have read everything on this, the McIntosh book, studied diagrams, etc. I would like to reduce the explanation of the Sentry Monitor circuit to its most basic terms so everyone can understand it.


    The people that I have been asking about the Sentry Monitor, they keep referring to 'excessive current to the output.' I just want to ask you, what is considered an adverse condition or accident that would cause excessive current in the amp? Crossed speaker wires or a shorted speaker coil?
    Last edited: Jan 25, 2011


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  2. c_dk

    c_dk Addicted Member

    West Michigan
    Have you read and downloaded the patent? #3526846

    A limiter to many is always considered bad until you are walking or your music just won't play any more.

    If you are using your new (remember this is the late 60s) banks of output transistors with questionable stability as fuses, you might welcome a circuit to keep them between the lines of safe operation vs. letting them go into wild thermal runaway or oscillation and end up in the ditch so to speak.
  3. CountD

    CountD Super Member

    Once again, c_dk has come to the rescue. An interesting note: The McIntosh Sentry Monitor was patented the day before I was born, as my mother was going into labor with me - September 1st, 1970.
  4. Rob-F

    Rob-F Likes Vintage Gear and DIY

    St. Louis, Missouri
    I downloaded the circuits for the MC 2505 and MC 2105, since I'm interested in them. It looks to me like a foldback limiter, a thing I first became aware of in the 1960's when I worked for a company that used power supplies by Power Designs out of Palo Alto. The foldback limiter stays out of the way and allows full power operation unless or until the load demands current that would exceed the safe operating limits of the power supply (or amplifier). Then it "folds back" current and voltage to within the safe operating area. When the excessive load goes away, the system instantly restores normal operation. So My guess is that the Sentry Monitor is a foldback limiter.
    Last edited: Oct 1, 2016
    ron-c likes this.
  5. ron-c

    ron-c AK Subscriber Subscriber

    N. Ca.
    The latest generation tube amps, MC75 and MC275 also for the first time have added Sentry Monitor. Sentry- a guard, Monitor- to watch for threats. Every device tube or transistor has designed operation limits of current and voltage. Sentry Monitor will maintain this safe operation area, SOA under adverse conditions.
    Sentry Monitor will limit the operation of the amplifier to keep it out of the blow up area of operation.

    chuckworkb and John James like this.
  6. 4-2-7

    4-2-7 Smart Ass Sponsor Subscriber

    San Francisco Peninsula
    There you go CountD, Ron's explanation I think everyone can understand. I mean what's not to understand about "keep it out of the blow up area of operation":D
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  7. kevzep

    kevzep Its all about the Music Subscriber

    Yeah its a current limiter circuit, stops stuff blowing up........sort of...
    4-2-7 likes this.

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