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    Coupling cap value- preamp, amp

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    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Coupling cap value- preamp, amp

    Post by Pooch on Sat May 21, 2011 3:44 pm

    Why does my tube preamp have 2uF coupling caps, bypassed with .15 uF, when my tube amps get by with "only" .22 uF? I understand the frequency passing thing, but why so high in the preamp? Thanks.
    Gordy

    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Coupling cap value- preamp, amp

    Post by GP49 on Sun May 22, 2011 4:34 am

    Impedance, impedance, impedance. Rule of thumb: the lower the impedance of the circuit, the larger the coupling capacitors must be to maintain response in the bass. You may run your preamp with a low-impedance input on a transistor power amp, so the manufacturer has to provide for that possibility.

    That's where the classic Dynaco PAS is often inadequate. It can't properly drive a power amp with less than 25K input impedance, and many modern transistor amplifiers fall into that category. Even that operation requires internal modification. And its phono preamp can't handle the low impedance of most transistorized tape decks, cordless headphone interfaces and computer sound cards. Of course, none of those existed when the PAS circuit was first designed as the PAM mono preamp. Heck, TRANSISTORS didn't exist, back then.

    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Re: Coupling cap value- preamp, amp

    Post by Pooch on Sun May 22, 2011 4:05 pm

    Thanks for the explanation. So much to learn! If they have to be a larger value due to lower impedance would there be any advantage to upping the 2uf in my CJ pv8? Or is a 2uf optimal? How would I figure out the optimal value? I've tried swapping brands of caps and ended up going back to the stock caps, just sounded better. Lots of questions, but maybe I can learn something. Thanks again GP49
    Gordy

    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Coupling cap value- preamp, amp

    Post by GP49 on Mon May 23, 2011 12:36 am

    Unless you know otherwise by analyzing the circuit, I'd leave the 2μF alone. You could ask conrad-johnson about it, specifying what power amplifier you are using with your PV8.

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