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The Dynaco Tube Audio Forum

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    PAS Signal Chain Weirdness... What should a square wave look like as it goes through the gain stages?

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    kenmwallace


    Posts : 2
    Join date : 2021-12-18

    PAS Signal Chain Weirdness... What should a square wave look like as it goes through the gain stages? Empty PAS Signal Chain Weirdness... What should a square wave look like as it goes through the gain stages?

    Post by kenmwallace Sun Dec 19, 2021 1:02 am

    I want to use my PAS-2 primarily for phonograph. So, I need the PC6 to work well.
    I fed a 1kHz square wave in at a few hundred mV. The waveform looked pretty good after the first stage of the 12AX7. But, after the second stage it was a triangle.
    Is that normal? Could the tube be bad? I don't have a tube tester.
    peterh
    peterh


    Posts : 1689
    Join date : 2012-12-25
    Location : gothenburg, sweden

    PAS Signal Chain Weirdness... What should a square wave look like as it goes through the gain stages? Empty Re: PAS Signal Chain Weirdness... What should a square wave look like as it goes through the gain stages?

    Post by peterh Sun Dec 19, 2021 3:22 am

    kenmwallace wrote:I want to use my PAS-2 primarily for phonograph. So, I need the PC6 to work well.
    I fed a 1kHz square wave in at a few hundred mV. The waveform looked pretty good after the first stage of the 12AX7. But, after the second stage it was a triangle.
    Is that normal? Could the tube be bad? I don't have a tube tester.
    The manual shows a good square wave.

    Where are you measuring ? Correct place is the output post. The manual shows the square wave.
    In Your case i assume treble is turned down.
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    nmchiefsfan


    Posts : 71
    Join date : 2012-03-21

    PAS Signal Chain Weirdness... What should a square wave look like as it goes through the gain stages? Empty Re: PAS Signal Chain Weirdness... What should a square wave look like as it goes through the gain stages?

    Post by nmchiefsfan Fri Dec 24, 2021 9:49 pm

    My guess would be there is some filtering going on. I am not familiar with the Pre-Amp or where the RIAA and tone control stuff is done but could one of those be the culprit?
    audioregenesis
    audioregenesis


    Posts : 30
    Join date : 2008-12-30
    Location : Canada

    PAS Signal Chain Weirdness... What should a square wave look like as it goes through the gain stages? Empty Re: PAS Signal Chain Weirdness... What should a square wave look like as it goes through the gain stages?

    Post by audioregenesis Thu Jan 06, 2022 1:57 pm

    If you feed a square wave directly into the phono input, then yes, you would xpect to see something akin to a triangle wave at the output. The phono stage is RIAA equalized, thus the square wave effectively sees a low pass filter. You would need a reverse RIAA filter at the phono input to perform square wave testing.
    tubes4hifi
    tubes4hifi
    Admin


    Posts : 1779
    Join date : 2008-11-30

    PAS Signal Chain Weirdness... What should a square wave look like as it goes through the gain stages? Empty Re: PAS Signal Chain Weirdness... What should a square wave look like as it goes through the gain stages?

    Post by tubes4hifi Fri Jan 07, 2022 1:39 am

    AudioRegenisis,
    great to see you still contributing here !! Keep up the great work !!

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