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    Preamp noise

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    Sprags

    Posts : 123
    Join date : 2013-02-27

    Preamp noise

    Post by Sprags on Fri May 10, 2013 8:56 pm

    Is a slight amount of noise coming directly from a tube preamp 'normal'?

    skriefal

    Posts : 118
    Join date : 2011-09-20
    Location : Utah, USA

    Re: Preamp noise

    Post by skriefal on Fri May 10, 2013 10:09 pm

    I've owned several tubed pieces -- Fisher 800-C (receiver), Audio Research LS-7 (pre), Juicy Music Blueberry Extreme (pre), Modwright SWL 9.0SE (pre). And I'd say that yes, a slight amount of low-level tube hiss is normal. Some tubes are quieter than others, though.

    Sprags

    Posts : 123
    Join date : 2013-02-27

    Re: Preamp noise

    Post by Sprags on Fri May 10, 2013 10:19 pm

    The sound is more like a hum than a hissing sound.

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: Preamp noise

    Post by j beede on Fri May 10, 2013 10:46 pm

    It sounds like you are talking about mechanical hum conducted through the air. If so, is your power transformer mechanically decoupled from the chassis? This is usually done with rubber grommets. You could try loosening the transformers mounting hardware to see if this changes the hum at all.

    skriefal

    Posts : 118
    Join date : 2011-09-20
    Location : Utah, USA

    Re: Preamp noise

    Post by skriefal on Fri May 10, 2013 11:14 pm

    I missed the "coming directly from" in your initial post. That does suggest that you're hearing this directly from the pre-amp rather than through your speakers.

    I've also found that a light hum from the power transformer isn't unusual -- even with solid state equipment. Sometimes this is caused by loose transformer mounting hardware, as stated by jbeede. Or it might be caused by dirty AC power -- e.g. light dimmers adding DC voltage on your AC power line. It could even originate from one of your neighbor's homes. And there's probably many other causes that I'm not aware of!

    Sprags

    Posts : 123
    Join date : 2013-02-27

    Re: Preamp noise

    Post by Sprags on Fri May 10, 2013 11:29 pm

    Well...I know the wiring in this condo sucks. I have the amp and preamp both plugged into an industrial line conditioner. The hum directly from the preamp isn't getting louder. I currently have the cover removed and I bet when I put the cover installed the hum volume will be less noticeable if not completely inaudible.

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: Preamp noise

    Post by j beede on Fri May 10, 2013 11:59 pm

    The dc bias supplies in my electrostatic speakers hummed--the sound actually kept me awake at night on occasion. I removed the mounting hardware, slightly enlarged the mounting holes, inserted rubber grommets and reinstalled the transformers with slightly smaller hardware. Dead silence. End of story. Do it.

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Preamp noise

    Post by peterh on Sat May 11, 2013 2:23 am

    Sprags wrote:Well...I know the wiring in this condo sucks. I have the amp and preamp both plugged into an industrial line conditioner. The hum directly from the preamp isn't getting louder. I currently have the cover removed and I bet when I put the cover installed the hum volume will be less noticeable if not completely inaudible.
    You have the solution - install the cover.

    Bob Latino
    Admin

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

    Re: Preamp noise

    Post by Bob Latino on Sat May 11, 2013 8:12 am

    Hi Blake,

    Yes - as others have said "put the cover on". Any preamp especially one with a PHONO STAGE will nearly always have more hum with the cover OFF. Another trick is to buy small RCA shorting plugs for any UNUSED RCA input on your preamp. On SOME preamps, an RCA input jack if left open can act like an antenna and pick up hum/noise from the preamp's circuitry. Check the link below for one example of an "RCA shorting plug".

    RCA shorting plug

    Bob

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