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    VTA PAS 3 preamp

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    tubenutr

    Posts : 61
    Join date : 2015-01-25
    Age : 59
    Location : Northern Ontario

    VTA PAS 3 preamp

    Post by tubenutr on Sat Dec 19, 2015 4:53 pm

    I have a VTA PAS that I would like to temporarily use with a SS power amp (ya I know, evil).
    If the PAS has a 1k output impedance and the power amp has a 22k input (in parallel with 62 pF cap), why does it sound
    thin, bright and just plain terrible.

    Any help would be greatly appreciated.

    Mike

    peterh

    Posts : 641
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: VTA PAS 3 preamp

    Post by peterh on Sat Dec 19, 2015 5:18 pm

    The PAS don't have 1k output impedance, rather it needs > 100k to be even reasonable good.
    You need a buffer / cathode follower, and as PAS don't have enough power for an extra tube
    it will have to be an external unit.

    A very primitive workaround might be a series resistance of 100k on the outputs,
    that would reduce the signal to 1/10 but it might be enough to your setup.

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: VTA PAS 3 preamp

    Post by PeterCapo on Sat Dec 19, 2015 6:52 pm

    http://www.audioregenesis.com/documents/PAS_Line.pdf

    peterh

    Posts : 641
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: VTA PAS 3 preamp

    Post by peterh on Sat Dec 19, 2015 8:19 pm

    PeterCapo wrote:http://www.audioregenesis.com/documents/PAS_Line.pdf

    Good reading. And we are discussing a PAS ( not -X ) and "eskimo" experiences
    "thin, bright" sound. Just to be expected on 22k loading.

    Bob Latino
    Admin

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    Location : Massachusetts

    Re: VTA PAS 3 preamp

    Post by Bob Latino on Sat Dec 19, 2015 9:16 pm

    The PAS-3 was made to work with the 470K loading of the Dynaco tube amps of the day. The PAS-3 does not work well with amps that have input impedances of less than 250K or so. You can see from the chart that the low end is really depressed even at a 100K loading. With a 22K load from your SS amp, the low end has undoubtedly dropped out of sight - probably 3 to 5 dB down for any frequency below 100 Hz.

    Bob



    PeterCapo

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    Join date : 2008-12-05

    Re: VTA PAS 3 preamp

    Post by PeterCapo on Sat Dec 19, 2015 9:38 pm

    A PAS with Dynaco’s original X-mod can drive amplifier loads down to 100K just fine. With a simple tweak, the X-mod version can successfully drive some loads lower than 100K. For example, in the original manual for Dynaco’s solid state Stereo 150 amp (35K Z-in), this point is addressed along with very simple instructions for the tweak.

    The Audioregenesis paper discusses this in more detail, so that it should be possible to “tune” the output of the PAS for whichever solid state amp you want to drive it with. However, there might be an ultimate limit to how low an input impedance the PAS can drive, even with resistor tweaking at the outputs. IIRC, GP49 has done work in this area.

    GP49

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    Re: VTA PAS 3 preamp

    Post by GP49 on Sun Dec 20, 2015 12:38 am

    My "work:" I would put the owner's PAS preamp on the bench along with his power amplifier, and remove all the load resistors.  I substituted a ten-turn variable resistor in place of the load resistors, while sweeping the line input through the low-to-midrange, and watching the synchronized output on a scope as I adjusted the resistor.  I'd do this at several different output voltages but generally that didn't make much difference as long as I kept to "reasonable" levels.  When satisfied that the response was as flat as I could get it (generally I weighted my judgment more heavily to 40Hz and above, while making sure the 20-40 Hz range didn't go too far off), I'd measure the potentiometer and use 1% resistors to make up that value load resistor and install it.  Same was done on both channels.  The specific tubes used in each channel were noted and the owner advised that if he rolled tubes, he might change the response.  Of course the owner would also have to use only that particular power amp or another identical one.

    I generally found that in a stock PAS, the lowest I could go was around a 25K load.  Below that, irregularities appeared, not all of them in the bass range.

    peterh

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

    Re: VTA PAS 3 preamp

    Post by peterh on Sun Dec 20, 2015 3:25 am

    We seem to agree that 22k is to low for a PAS3
    ( eskimo named it a "VTA PAS" but i assume he was referring to a "Dynaco PAS3" )

    The fact that nu suitable stand-alone buffer / cathode follower seems available on the market
    opens up a niche for such a device. ( yes there is lots of "bear-like" chinese things on ebay, but
    none of them is a cathode follower as far as i know)

    Any takers for designing such a thing complete with 110/230V AC power supply ?

    tubenutr

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    VTA PAS 3 aka tubes4hifi PAS zmod

    Post by tubenutr on Sun Dec 20, 2015 3:54 pm

    Just to clarify what preamp I referring to.

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