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    PAS 3X upgrade excessive hum

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    TN Allen

    Posts : 124
    Join date : 2013-01-01

    Re: PAS 3X upgrade excessive hum

    Post by TN Allen on Sun Mar 08, 2015 11:59 am

    This is an update to the hum problem with the PAS 3X preamp upgrade I started 2+ years ago.

    After trying everything everyone suggested, everything I could think of, and finally sending the preamp off to Mr. Mottram, the hum, though diminished, remained unacceptably loud. I left the project for a few years and in the interim built an SP9 and an upgrade on an ST 35, both of these were successful and I enjoy the sound they produce. Earlier this winter I skeptically decided I'd try the last option for removing the hum from the PAS 3X, the HD regulated power supply upgrade.

    Fortunately, my skepticism regarding the likelihood of a hum-less PAS 3X was unfounded. I now have a hum free, delightful preamp. I also don't have a specific reason for the hum despite careful assembly of the phono, line, tone controls and Tubes4Hifi PCBs, as well as new RCA connectors and an IEC 3 prong AC connector. In short, the only remaining parts from the original preamp were the transformer, knobs, a few screws and the chassis. The transformer is the obvious source of the hum, although, all of the voltages from it checked out within the specifications. Perhaps someone with more knowledge of transformers may have an explanation for the hum.

    The preamp is so quiet that when I first connected it, I was certain, and momentarily a bit crestfallen, that it wasn't working, it was almost dead quiet. It does have some slight hum which I can hear if the volume is at 100%, if I have my ear right up against the speaker, and if I concentrate and listen carefully in a quiet room.

    It has been an expensive project, but, given I had assembled the original kit in 1968 I had some emotional attachment to it, wanted it right and want to use and enjoy it, which I shall do.

    For anyone contemplating a PAS 3X upgrade, I'd recommend using the regulated PS rather than the less expensive PS upgrade. I'll also add that the support available via this forum is much appreciated.

    peterh

    Posts : 680
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: PAS 3X upgrade excessive hum

    Post by peterh on Sun Mar 08, 2015 5:32 pm

    TN Allen wrote:This is an update to the hum problem with the PAS 3X preamp upgrade I started 2+ years ago.

    After trying everything everyone suggested, everything I could think of, and finally sending the preamp off to Mr. Mottram, the hum, though diminished, remained unacceptably loud. I left the project for a few years and in the interim built an SP9 and an upgrade on an ST 35, both of these were successful and I enjoy the sound they produce. Earlier this winter I skeptically decided I'd try the last option for removing the hum from the PAS 3X, the HD regulated power supply upgrade.

    Fortunately, my skepticism regarding the likelihood of a hum-less PAS 3X was unfounded. I now have a hum free, delightful preamp. I also don't have a specific reason for the hum despite careful assembly of the phono, line, tone controls and Tubes4Hifi PCBs, as well as new RCA connectors and an IEC 3 prong AC connector. In short, the only remaining parts from the original preamp were the transformer, knobs, a few screws and the chassis. The transformer is the obvious source of the hum, although, all of the voltages from it checked out within the specifications. Perhaps someone with more knowledge of transformers may have an explanation for the hum.

    The preamp is so quiet that when I first connected it, I was certain, and momentarily a bit crestfallen, that it wasn't working, it was almost dead quiet. It does have some slight hum which I can hear if the volume is at 100%, if I have my ear right up against the speaker, and if I concentrate and listen carefully in a quiet room.

    It has been an expensive project, but, given I had assembled the original kit in 1968 I had some emotional attachment to it, wanted it right and want to use and enjoy it, which I shall do.

    For anyone contemplating a PAS 3X upgrade, I'd recommend using the regulated PS rather than the less expensive PS upgrade. I'll also add that the support available via this forum is much appreciated.
    How did you find place for the IEC connector ? A picture would be nice ..


    eickmewg

    Posts : 43
    Join date : 2014-08-29

    Re: PAS 3X upgrade excessive hum

    Post by eickmewg on Mon Mar 09, 2015 6:15 am

    The PAS sounds a little like the FM tuner, where the output jacks are directly grounded to the chassis. Bill Thomas recommended isolating the output jacks and running short grounding wires to the ground point iirc for V72 on the multiplex board to reduce hum. I've done this on my FM tuner and it certainly seems to work.

    TN Allen

    Posts : 124
    Join date : 2013-01-01

    Re: PAS 3X upgrade excessive hum

    Post by TN Allen on Mon Mar 09, 2015 11:08 am

    Here is a photo of the preamp with the IEC connector. I forgot to mention that I replaced the 7 pin tube socket as well. I had also located a plastic IEC connector in the center of the chassis back, but removed it to accommodate the new HD Regulated PS.

    I think I'll mill a new face plate next, I left quite a few empty holes after removing the front switches, the blend switch is also disconnected.




    peterh

    Posts : 680
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: PAS 3X upgrade excessive hum

    Post by peterh on Mon Mar 09, 2015 1:38 pm

    Thanks for the picture. But i cannot make IEC like this as i still have the dynaco transformer.
    I have a plan "B" ( a small box with IEC inlet mounted on the back)

    TN Allen

    Posts : 124
    Join date : 2013-01-01

    Re: PAS 3X upgrade excessive hum

    Post by TN Allen on Mon Mar 09, 2015 1:44 pm

    peterh wrote:Thanks for the picture. But i cannot make IEC like this as i still have the dynaco transformer.
    I have a plan "B" ( a small box with IEC inlet mounted on the back)

    The now empty odd shaped opening next to the RCA output jacks and cables had a plastic IEC connector when I still used the old transformer and the Tubes4Hifi PS upgrade. I'll look for a photo of that arrangement.

    TN Allen

    Posts : 124
    Join date : 2013-01-01

    Re: PAS 3X upgrade excessive hum

    Post by TN Allen on Mon Mar 09, 2015 2:02 pm

    Here is the best photo I can find at present:



    The IEC connector is the obscure black area near the lower left corner of the photo, directly to the left of "B+P" and the smaller capacitor top at the bottom edge of the PS PCB. I suspect that you could have the deeper metal can IEC connector if it were located closer to the top of the chassis back. I'm not certain this is electronically compatible with the rest of the preamp, others would have better advice on this.

    peterh

    Posts : 680
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: PAS 3X upgrade excessive hum

    Post by peterh on Mon Mar 09, 2015 3:06 pm

    I'm thinking of something simulat to this : http://n.manet.nu/fm1/IEC-dosa.jpg
    made in sheet metal of course.


    TN Allen

    Posts : 124
    Join date : 2013-01-01

    Re: PAS 3X upgrade excessive hum

    Post by TN Allen on Tue Mar 10, 2015 12:18 pm

    Perhaps if you fasten it to the back panel oriented differently, for example, the prongs pointing up or down, so a cord with a 90 degree connector can lay more or less flat along the back panel, you can then run the wires directly into the chassis, well protected. Then again, why not just run a 3 conductor cord into the chassis and avoid the IEC connector.

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