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    Ultimate PAS upgrade project, I need a little advice on hum and balance pots.

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    Elrick

    Posts : 71
    Join date : 2012-09-02
    Location : Bellingham WA

    Ultimate PAS upgrade project, I need a little advice on hum and balance pots.

    Post by Elrick on Sat May 25, 2013 2:27 pm

    I'm pretty much finished with the project and it does sound terrific but have a couple of concerns to finish the job right. I've put together the Regulated Power Supply, Aikido line stage board, PH10 phono board and a new 6 position switch. My PAS already had a replacement blue volume control that works well. Unfortunately, I'm still using the original balance pots that need to be eventually replaced since they are noisy and the right channel is slightly quieter than the other with a little less high end.

    I am still getting a bit of a 60 cycle hum that can be heard between tracks even when the volume is all the way down, when it's turned up more than 10 oclock or so it starts to get louder and is annoying. I guess I must have a ground loop or something but not sure where and so far I can't get rid of it. I still have to finish up the wiring for the TAPE INPUT slider and the SPARE input.

    Do you think the old balance pot could be responsible for the hum?

    I did replace the 12X4 rectifier tube with one from another PAS3 that seemed to reduce the hum, but as you can see on the pictures it's still there. As expected hum coming through the PHONO is louder than when using the other inputs. When the selector is turned to an unused input it's dead quiet.

    Could someone point me to a good suitable replacement for the balance pot?

    Any help or ideas would be appreciated.

    Here are some pictures of the wiring and a couple showing the hum as seen on my scope. The trace showing the more complex wave is from the line stage board while the trace with the more saw tooth wave is from the PHONO. It measures at 60 Hz.











    Bob Latino
    Admin

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

    Re: Ultimate PAS upgrade project, I need a little advice on hum and balance pots.

    Post by Bob Latino on Sat May 25, 2013 3:53 pm

    Try putting the metal cover on your preamp and compare the amount of hum. I am sure that the hum level on your PAS preamp will be lower with the original metal cover back on the preamp.

    Bob

    Elrick

    Posts : 71
    Join date : 2012-09-02
    Location : Bellingham WA

    Re: Ultimate PAS upgrade project, I need a little advice on hum and balance pots.

    Post by Elrick on Sat May 25, 2013 4:05 pm

    Hi Bob, I get the hum even with the cover tightly screwed on but it is an improvement.

    Bob Latino
    Admin

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

    Re: Ultimate PAS upgrade project, I need a little advice on hum and balance pots.

    Post by Bob Latino on Sat May 25, 2013 5:32 pm

    Another thing that you can try is to use an "RCA shorting plug" on any UNUSED input source especially phono inputs or any other low level (tape head input etc.) input. When you have any input "open" on SOME preamps they act like antennas and sometimes pick up hum/noise from the preamp itself. You can make them up yourself or buy them already premade. All it is is a regular RCA male plug that has a connection (short) between the center pin and the outer shield. See photo below. Google "RCA shorting plug" for more examples.

    Bob



    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Ultimate PAS upgrade project, I need a little advice on hum and balance pots.

    Post by tubes4hifi on Sun May 26, 2013 1:10 pm

    Elrick,
    send me an email or PM and I will send you a filament mod that needs to be done for this combination of boards.
    Nice job and nice photos! I also have the ALPs balance pots if you need one.
    Roy

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