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Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    Differences between the Dynaco PAS 2/3 and the PAS-3X preamps

    Bob Latino
    Bob Latino
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    Differences between the Dynaco PAS 2/3 and the PAS-3X preamps Empty Differences between the Dynaco PAS 2/3 and the PAS-3X preamps

    Post by Bob Latino Tue Aug 17, 2021 10:24 am

    There are two basic circuit differences between the earlier Dynaco PAS-2 and PAS-3 units and the later PAS-3X preamp.

    1) The "X" version had tone control potentiometers that were constructed to be electrically removed from the circuit when set to their flat position.

    2) A 1uF electrolytic capacitor was placed in each channel’s tone control circuit to block the DC from reaching the audio output.

    Whereas the owners of an earlier PAS-2 and PAS-3 preamps may not be able to find the specialized potentiometers that were used in the "X" version, they can and should add the 1uF capacitor to eliminate the DC Voltage in the output of the preamp. This is especially important if the amplifier that receives the signal from the preamp has a direct coupled input such as the Dynaco ST-70. The DC Voltage from the preamp will upset the bias point of the input stage of the amplifier and could cause playing issues.

    The original PAS-3X used a 1uF, 25V electrolytic capacitor but it might be better to use a polypropylene or polyester capacitor with the same voltage rating or higher for the update. These capacitors may be found at Mouser Electronics. To add the capacitors just replace the wires connected between eyelet terminals #5 and #12 on the line amplifier circuit board and the lower terminals of the bass controls with the 1uF capacitors. Just remove each wire and replace it with a capacitor.

    Bob Latino

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    Solder Slinger

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    Join date : 2009-05-24

    Differences between the Dynaco PAS 2/3 and the PAS-3X preamps Empty R.E. Differences between the Dynaco PAS 2/3 and the PAS-3X preamps

    Post by Solder Slinger Fri Aug 27, 2021 11:30 am

    I agree with Bob about the PAS differences, however, if adding a 1 mfd cap to a PAS-2 or PAS-3 or replacing it on a PAS-3X, I agree with Bob about using a polypropylene type cap but would increase the value to 1.5 or even 2 mfd. This will give you a little better bass response as well as allowing the preamp to match up with a somewhat lower impedance power amp. Bob's instructions for adding this cap are correct.

    This is covered in the PAS documentation on " www.audioregenesis.com " which has articles with an excellent analysis of the Dyna PAS series.

    Just a thought,

    -Ed
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    jwb474

    Posts : 32
    Join date : 2014-07-13

    Differences between the Dynaco PAS 2/3 and the PAS-3X preamps Empty Re: Differences between the Dynaco PAS 2/3 and the PAS-3X preamps

    Post by jwb474 Fri Aug 27, 2021 3:21 pm

    Also the 2 blend resistors were changed to provide 6 and 12db of separation.

    https://dynacotubeaudio.forumotion.com/t256-dynaco-lore-tc-3x-the-pas-3-to-pas-3x-tone-control-modification-manual.

    I cant figure out when, but the front face plate design was changed.

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    Differences between the Dynaco PAS 2/3 and the PAS-3X preamps Empty Re: Differences between the Dynaco PAS 2/3 and the PAS-3X preamps

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