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    DYNACO ST70 "MOTORBOATING "

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    tookie00

    Posts : 2
    Join date : 2010-11-26

    DYNACO ST70 "MOTORBOATING "

    Post by tookie00 on Wed Oct 03, 2012 1:47 pm

    HI ALL, I FINALLY FIGURED OUT WHAT WAS CAUSING THE MOTORBOATING IN
    MY ST 70. I HAVEN"T USED IT FOR THIS REASON SO I HOOKED IT UP THE OTHER
    DAY WITH A DIFFERENT SET OF SPEAKERS AND THE MOTORBOATING STOPPED.
    SOUNDS GREAT WITH BOZAK SPEAKERS AND A MARANTZ MODEL 7C PREAMP. THE
    OTHER SPEAKERS THAT WERE CAUSING THE PROBLEM WERE KEF 101"S.

    wbain

    Posts : 8
    Join date : 2012-01-17
    Age : 64
    Location : Near Manassas Va.

    Re: DYNACO ST70 "MOTORBOATING "

    Post by wbain on Wed Oct 03, 2012 3:26 pm

    Please turn the caps lock off.

    Motorboating is usually a power supply problem.

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: DYNACO ST70 "MOTORBOATING "

    Post by Bob Latino on Fri Oct 19, 2012 9:09 pm


    Your problem could be a power supply problem as wbain has mentioned but the fact that the amp is stable with your Bozak speakers leads me to believe that you could have a speaker load issue.

    Sometimes a speaker will present a load across the secondaries of the output transformer that an amp doesn't like. An improper load can send the amp into mild oscillation (motorboating). Those KEF 101's are from the early 1980's and it is possible after 30+ years that some component in the crossover of each 101 is breaking down and the crossover needs to be rebuilt. Your Bozak speakers are presenting a load on the output transformer that "stabilizes" the amp as most speakers should. The ST-70 is a very stable amp as long as it gets a proper load across the speaker binding posts.

    Bob

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