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    PAS-3 hiss in phono section.

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    RMG

    Posts : 16
    Join date : 2010-06-13

    PAS-3 hiss in phono section.

    Post by RMG on Thu Sep 01, 2011 7:59 pm

    What would be the cause or causes of hiss in the phono section of my PAS-3. With the phono inputs shorted I hear a pronounced hiss starting around 11:00 o'clock on the volume control getting progressively louder. The other inputs are relatively quiet. Thanks.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: PAS-3 hiss in phono section.

    Post by Bob Latino on Thu Sep 01, 2011 9:54 pm

    Hi,

    Of course the phono stage on a PAS-3 has much more gain than any line stage input. Extra gain tends to make any noise more noticeable. If your speakers are a little more sensitive than normal, that will make the situation worse also. If you have the original boards with the older carbon composition resistors > these resistors sometimes tend to get "hissy". You could try replacing all the original carbon composition resistors on the phono board with metal film resistors. Metal film resistors are very quiet and rarely do you get one that is noisy.

    Bob

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: PAS-3 hiss in phono section.

    Post by tubes4hifi on Sat Sep 03, 2011 3:14 pm

    Bob is right, a phono preamp has an extra 40db of gain which is 10,000x the voltage gain.
    It's possible you've got a bad resistor or capacitor, but just as likely you've got some noisy tubes.
    You can buy tubes (for an extra price) that have been pre-tested to have lower noise. Tube noise is "hiss".

    RMG

    Posts : 16
    Join date : 2010-06-13

    Re: PAS-3 hiss in phono section.

    Post by RMG on Sun Sep 04, 2011 9:23 am

    Thanks for the replies. Where is a good place to buy metal film resistors and capacitors? I'd like to solder in some new ones and see if it improves the noise. Thanks.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: PAS-3 hiss in phono section.

    Post by Bob Latino on Sun Sep 04, 2011 10:31 am

    For smaller amounts of metal film resistors you can just go to Radio Shack. The only catch is that they don't have every resistor value available although they do have the more common values there. They sell capacitors there also but may not have the exact values available. For larger quantities of resistors try Mouser Electronics or Digikey.

    Bob

    j4570

    Posts : 141
    Join date : 2010-08-30

    Re: PAS-3 hiss in phono section.

    Post by j4570 on Sun Sep 04, 2011 5:41 pm

    Mouser and Digikey are good, as is Parts Express. I use Mouser and DIgikey, but Parts Express a good bit too. If I place one order a year, it's a miracle, as I keep the projects to a minimum these days.

    Another place I use for old radios (and amps too) is Justradios.com

    He's in Canada but ships quick, and you can get all those odd values others don't always carry (at the same place), and shipping is under $10. He has a good deal on spaghetti heat shrink, get a pack, I use it to isolate all my leads. Just a satisfied customer.

    Jason

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