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Thread: 6J1 Pre-amp modifications (hum removal)

  1. #51
    Join Date: May 2008

    Location: Surrey

    Posts: 5,893
    I'm Rob.

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    I got mine today. Have put a couple of hours on it and it sounds pretty decent.......far more decent than the paltry price tag (and fear of counterfeit parts) would suggest. I need to do the "hum" mod (although mine is making a sort of low level fizz) than hum.

    Question for Alan : Am I right to assume that this board uses a voltage doubler?
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  2. #52
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 3,802
    I'm Alan.

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    Yes Rob, it uses two voltage doublers to generate plus and minus 30V, so any hum component on the output is going to be at 50Hz, not the 100Hz from a full wave rectifier.


  3. #53
    Join Date: May 2008

    Location: Surrey

    Posts: 5,893
    I'm Rob.

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    Quote Originally Posted by Firebottle View Post
    Yes Rob, it uses two voltage doublers to generate plus and minus 30V, so any hum component on the output is going to be at 50Hz, not the 100Hz from a full wave rectifier.

    Aha! I like my Saturday nights to have some FIZZZZZZ. Thanks Alan. Will do the "Hummmm/Fizzzzzz" mod soon.
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  4. #54
    Join Date: May 2008

    Location: Surrey

    Posts: 5,893
    I'm Rob.

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    I did the "hum" mod, but the fizz has been replaced with a..........hummmm hm? 50/60hz probably. I have disconnected one side of the 470uf cap (as I hot glued the cap to the board and might need to re-connect it - ?) Oddly the fizz has all but gone and there is only a very feint hum left. The cap has been left connected by it's NEG leg to the join of the 47r resistors only.
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  5. #55
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 3,802
    I'm Alan.

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    Sounds a bit odd. You should have plus and minus about 30V from the voltage doublers if everything is working properly.

    Is it pretty close to 12V AC you are putting in? If it is 13V or more then the caps could be overrated.

  6. #56
    Join Date: May 2008

    Location: Surrey

    Posts: 5,893
    I'm Rob.

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    Quote Originally Posted by Firebottle View Post
    Sounds a bit odd. You should have plus and minus about 30V from the voltage doublers if everything is working properly.

    Is it pretty close to 12V AC you are putting in? If it is 13V or more then the caps could be overrated.
    I measured the PS (a musical fidelity wall wart for an X-10D) and it was just over 13vac which I thought would be ok?

    Would it be worth changing the cap? It had been used before, but I am not aware that it was duff.
    Last edited by Puffin; 21-03-2017 at 08:12.
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  7. #57
    Join Date: Nov 2010

    Location: Sheffield/Peak District. UK

    Posts: 375
    I'm Richard.

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    Sorry to ask a daft question, but what is the difference, in this case, between the preamp version and the buffer version?
    Thanks,

  8. #58
    Join Date: May 2008

    Location: Surrey

    Posts: 5,893
    I'm Rob.

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    The Preamp has gain, the buffer has no gain.
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  9. #59
    Join Date: Nov 2010

    Location: Sheffield/Peak District. UK

    Posts: 375
    I'm Richard.

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    Hi, thanks; so what's the point of a buffer?
    R

  10. #60
    Join Date: Feb 2013

    Location: W Lothian

    Posts: 38,553
    I'm Grant.

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    Quote Originally Posted by Bonky View Post
    Hi, thanks; so what's the point of a buffer?
    R
    Impedence mismatching
    Regards,
    Grant ....

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