Audio Emotion 
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  1. #1
    Wammer sentinel one's Avatar
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     Dec 2010
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     Lenco 75
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     A C Magnum CD 2020
     Lumley M2 MK II
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     Lumley M120
    In the Hi-Fi industry?

    Problem with electrical feedback on valve phono stage? Any suggestions?

    Problem with electrical feedback on valve phono stage?

    I have recently purchased my first valve phono stage (second hand) which sounds absolutely great. However, at switch off the amp makes a thump – if I am not mistaken only on the left channel - there is some sort of electrical discharge that perhaps shouldn’t be there? The thump can be heard irrelevant of whether the volume has been turned up or down.

    What is this problem most likely to be and is this an easy problem to rectify?

    Thanks for any hints or relevant info. in advance. Much appreciated.

  2. #2
    It's quite common to get a "thump" at the speakers when powering down, and one school of thought is to always turn the amp's volume down and power the amp off before the source to avoid any possible damage to the speakers. Switch on is the opposite (ie switch the source on first, keep the amp volume low or zero, and then turn the amp on). If the phono stage sounds good and you have no problems with it during use, don't worry too much. This thump can be caused by the amps power supply capacitors discharging at switch off. Some people prefer the opposite route, ie power amp on first then source and vice versa. It depends largely on whether your amp has a soft start/stop function to prevent capacitor surge. DC offset issues with amps can also cause a thump which might point to an issue with the amp rather than the source.
    Last edited by Pac67; 22-12-2011 at 09:15 AM.

  3. #3
    Super Wammer
    Join Date
     Sep 2006
    In the Hi-Fi industry?
    Simple. Turn off the power amp first. Which is the correct thing to do anyway, because of things like this.
    CD players - computer audio for dummies.

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