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    Loud Hum When Pre-Amp Off

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    bigmm79

    Posts : 23
    Join date : 2010-09-15

    Loud Hum When Pre-Amp Off

    Post by bigmm79 on Thu Mar 10, 2011 9:53 pm

    I get a loud hum through speakers when I turn on my ST-70 but my pre-amp is not turned on.
    When the pre-amp turns on it goes away and there is no hum. Does this mean I have voltage leaking into the chassis or something
    that the pre-amp is grounding out when it gets turned on? Touching the amp during humming seems to change hum and there maybe even is a little shock, very little.
    If this is the problem, where should I look? The wiring all seems kosher to me, so I'm not sure the most likely places to look.
    This is one of Bob's kits by the way.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Loud Hum When Pre-Amp Off

    Post by Bob Latino on Fri Mar 11, 2011 7:20 am

    Hi,

    From your description it might mean that your preamp does not have a direct ground to the output jacks until the amp has warmed up or until a relay kicks in after a period of time. Different preamps using different grounding schemes. The chassis itself may not be a common ground on your preamp.

    Bob

    Luddite

    Posts : 235
    Join date : 2009-02-04
    Age : 66
    Location : Texas

    Re: Loud Hum When Pre-Amp Off

    Post by Luddite on Fri Mar 11, 2011 11:28 am

    In the professional audio world, it is standard practice for power amps to be powered up last and powered down first. This is primarily to prevent any pulses caused by switching transients from damaging the loudspeakers. I have also found this to be a good practice in my home audio system. I believe if you follow this sequence you will solve your hum problem.

    Best Regards,
    Charlie

    bigmm79

    Posts : 23
    Join date : 2010-09-15

    Re: Loud Hum When Pre-Amp Off

    Post by bigmm79 on Fri Mar 11, 2011 6:49 pm

    Thanks for the input.
    I always turn on pre-amp first now.
    I'm just concerned that there may be something wrong with my wiring in the amp.
    Sounds like that probably isn't the case, but rather the hum is related to my pre-amp's own grounding scheme.
    There certainly is no hum or other problems when the pre is on.
    If I misunderstand feel free to enlighten me further.

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: Loud Hum When Pre-Amp Off

    Post by mantha3 on Fri Mar 11, 2011 8:18 pm

    Disconnect the RCAs from the AMP... Does it hum when on?

    kevco

    Posts : 70
    Join date : 2009-07-16
    Location : Aldan, PA, USA

    Re: Loud Hum When Pre-Amp Off

    Post by kevco on Mon Mar 14, 2011 1:59 pm

    mantha3 wrote:Disconnect the RCAs from the AMP... Does it hum when on?
    I discourage you from doing this without first shorting your inputs using either shorting plugs or a short single RCA cable. The cable should be as short as possible so it doesn't act as an antenna and pick up RF and transmit it into your amp. If the inputs aren't shorted, your amp could oscillate and potentially damage your speakers (and make some horrible noise!). It is absolutely the correct procedure to power up your pre before powering your amp up. Always.

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