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    PAS-3 humming phono

    ViperZ
    ViperZ

    Posts : 100
    Join date : 2010-03-26

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    Post by ViperZ on Thu Mar 19, 2020 7:59 pm

    Hopefully this is the last one for a long time...

    I have always had hum in my PAS-3 phono stage.
    It is only on phono.
    It is volume dependent - starts around 12 o'clock and gets progressively worse.

    So far I did the following:
    * Replaced mismatched no-name tubes with reissue Mullard 12AX7 a couple of years ago.
    * Replaced volume control with Alps and eliminated loudness control.
    * Replaced rear panel with dynakitparts panel, eliminated tape output and tape monitor input.
    * I removed 10-ohm resistors between output and input grounds since new rear panel has continuous ground plane
    * Phono grounds are isolated from other input grounds, each connected to matching black wire coming off phono board.
    * Line level inputs are silent, sweeping with volume all the way from 0 to max

    PAS-3 humming phono Img_2019
    PAS-3 humming phono Img_2018
    ViperZ
    ViperZ

    Posts : 100
    Join date : 2010-03-26

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    Post by ViperZ on Thu Mar 19, 2020 8:01 pm

    PAS-3 humming phono Img_2020
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    boeingpilot

    Posts : 22
    Join date : 2015-06-20
    Location : Central PA

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    Post by boeingpilot on Mon Mar 23, 2020 10:53 pm

    Have you tried 1 - shorting plugs into phone to make sure you're not picking up external hum
    2 - if your turntable has a ground lead for the input, attaching to back of PAS?
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    rjpjnk

    Posts : 220
    Join date : 2018-07-18

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    Post by rjpjnk on Tue Mar 24, 2020 11:11 pm

    How much noise? Are you sure it is not normal? The phono stage adds 40+ dB of gain and in my experience always causes hum if turned up to near halfway. It's not just the PAS, but other phono preamps as well (I assume all of them). Ive spent a lot of time hum-shooting my PAS 3x and also my SP12/PH12, but with that much gain it is difficult to be quiet. The hum is usually due to 120Hz power supply noise.

    The suggestion above to short the phono inputs and see if it gets quieter is a good one.
    tubes4hifi
    tubes4hifi
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    Post by tubes4hifi on Thu Mar 26, 2020 8:15 pm

    suggest to replace the rat's nest of wiring on the original selector switch, or at least move the two RIAA caps directly to the phono PCB,
    then wire the phono inputs directly to the phono board rather than the selector switch. Outputs of phono board can remain as usual on the original selector.
    Make sure the phono grounds are not connected to any of the line input grounds, only directly to the phono board ground.
    Use either shielded cable or very tightly twisted pairs to input of phono board.
    ViperZ
    ViperZ

    Posts : 100
    Join date : 2010-03-26

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    Post by ViperZ on Fri Mar 27, 2020 2:26 pm

    To answer some questions above.

    boeingpilot (supercool nick!), attaching shorting plugs did not help and obviously my turntable ground is attached to PAS chassis. It is definitely picking up noise inside the preamp.

    rjpjnk, I do understand what you're saying, and I experience it with many phonos in my house. At full volume, I am measuring 20-25mV peak to peak noise on phono background.

    tubes4hifi, I did remove two phono wires (L/R) from the rat's nest, tightly twisted them in a separate run. It didn't help. I did disconnect phono grounds jumpers from other grounds on the back panel, so like you said, phono input grounds were only connected to dedicated black wires going to phono board.
    I also followed my Regenesis power supply manual suggestion, and rerouted two bottom ground wires from phono board, which ran to the stacked capacitor, closer to the new power supply board - to the bolt that holds that board. This actually helped a little bit.

    I used to hear this noise at 1 o'clock on the original volume control pot, or at about the same level on new Alps pot. After I removed loudness contour completely, volume control is now much more linear. It is definitely louder at around 11-12 o'clock, and that's where I like to listen with my system. Background noise starts at around 12 o'clock now.


    Oh, I never understood why phono inputs go to the selector first. Next time I'll be opening this preamp, I will follow your suggestion and will move the input wires and RIAA capacitors to the phono board. There is only so much I can squeeze out of this amp with original boards/circuits.
    ViperZ
    ViperZ

    Posts : 100
    Join date : 2010-03-26

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    Post by ViperZ on Fri Mar 27, 2020 2:33 pm

    PAS-3 humming phono Img_2021
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    setu

    Posts : 7
    Join date : 2013-03-11
    Location : California

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    Post by setu on Tue Mar 31, 2020 6:15 pm

    from another forum(2009):
    https://www.audioasylum.com/cgi/vt.mpl?f=dynaco&m=14621

    I tried to be extreme in star grounding strategies----esp phono input.
    grounded metal 'fence' around the PS needs vent holes.
    I foolishly did not remove those old 120V rear sockets---more helpful as vent holes.

    I have elsewhere used Triplett model 9601 to sniff 5---600VAC.
    ****obviously sniffing live circuits is dangerous

    good luck,
    setu

    ps: I found it difficult to make the wiring look good while simultaneously avoiding unwanted induction(s)
    avoid the induced AC....

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