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    Problem with PAS preamplifier

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    Pillo69

    Posts : 104
    Join date : 2012-04-11
    Location : Granada (España)

    Problem with PAS preamplifier

    Post by Pillo69 on Tue Apr 15, 2014 7:25 pm

    I connected the output "Tape Out" preamp to the computer input and electrical differential housing jumped.

    I have noticed that is because the computer is grounded.

    The preamplifier can not use the ground, or a defect in the preamp?.

    Thanks for attention.

    peterh

    Posts : 643
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Problem with PAS preamplifier

    Post by peterh on Wed Apr 16, 2014 1:48 am

    Pillo69 wrote:I connected the output "Tape Out" preamp to the computer input and electrical differential housing jumped.

    I have noticed that is because the computer is grounded.

    The preamplifier can not use the ground, or a defect in the preamp?.

    Thanks for attention.
    You should ground your amp too. In fact the whole stereo should have a path to ground, it will enhance safety and it will kill several sources of hum. ( it might also introduce hum if ground from several sources are interconnected. One such is coax from a cable-tv where ground is referenced elsewhere).
    You also needs a resistive damping network  to the computer as the computer has a much lower impedance then the amp is built for. A series resistor ( 270K ) + 68K to ground after the series resistor will allow the PAS3 to "see 270K" and it will also bring down the signal to a level more suitable  for the computer.
    The same can be said about driving a semiconductor poweramp from a PAS.
    Loading the PAS with too low impedance will affect sound quality.


    Last edited by peterh on Wed Apr 16, 2014 2:06 am; edited 1 time in total (Reason for editing : spelling)

    Pillo69

    Posts : 104
    Join date : 2012-04-11
    Location : Granada (España)

    Re: Problem with PAS preamplifier

    Post by Pillo69 on Wed Apr 16, 2014 3:14 pm

    Hi there
    Ok, my question is when I connect the preamp (PAS) to the electrical ground the differential housing jump. Is it normal that the preamp cannot be connected to the ground or is it an internal problem? And if it is a internal problem where is come from?
    Thank you for your help+

    peterh

    Posts : 643
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Problem with PAS preamplifier

    Post by peterh on Wed Apr 16, 2014 5:45 pm

    Pillo69 wrote:Hi there
    Ok, my question is when I connect the preamp (PAS) to the electrical ground the differential housing jump. Is it normal that the preamp cannot be connected to the ground or is it an internal problem? And if it is a internal problem where is come from?
    Thank you for your help+
    No, it's a symptom of leakage from the phase to ground.
    An isolation tester might be needed, lacking that you could start removing anything among the AC path that is not in the schematic. Then it's a damaged cable or a damaged transformer left.

    GP49

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    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Problem with PAS preamplifier

    Post by GP49 on Thu Apr 17, 2014 12:24 pm

    Or you may find that by reversing one of the two-prong power cords in the system (start with the PAS's own cord), the leakage currect goes away.  This happens more often than we like to think, due to an internal bypass capacitor being connected to one or the other prong of the power cord and coupling a very low current AC to the chassis.  This does not necessarily indicate a defect; the AC leakage, though at line voltage, could be at such a low current that it is of no harm.

    peterh

    Posts : 643
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Problem with PAS preamplifier

    Post by peterh on Fri Apr 18, 2014 5:02 am

    GP49 wrote:Or you may find that by reversing one of the two-prong power cords in the system (start with the PAS's own cord), the leakage currect goes away.  This happens more often than we like to think, due to an internal bypass capacitor being connected to one or the other prong of the power cord and coupling a very low current AC to the chassis.  This does not necessarily indicate a defect; the AC leakage, though at line voltage, could be at such a low current that it is of no harm.
    Use of caps between AC and chassies _should_ in my opinion be banned.

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