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The Dynaco Tube Audio Forum

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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    New member with PAS 3

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    keego38


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    Join date : 2012-10-14

    New member with PAS 3 Empty New member with PAS 3

    Post by keego38 Sun Oct 14, 2012 10:32 am

    Hello All,
    I have just joined the forum, and I'm looking for advice regarding a PAS 3 that I recently re-discovered in the attic. I used it back in the late '70s - early '80s as the preamp in a turntable based system based around a Thorens 125, Grace arm, Supex 901, into the PAS3, then a Luxkit power amp, and finally early Rogers ls3/5a speakers. When properly set up, this system sounded fabulous, but then the PAS transformer shorted out, and a few other problems caused the whole system to be sidelined for many years..
    Now I want to put it all together again based around a modified Rega planar 3 with a Denon dl110 high output MC. ( I have sourced a replacement trans former for the PAS and still have the LUX and ls3/5a's) I have replaced the two power supply caps in the filament circuit, and will probably replace the quad can as well, but my main question at the moment is would it be possible to just use the phono stage of the PAS to drive the power amp directly from the volume control, bypassing the line stage, eq etc., using it as a minimalist phono only preamp as such. Any advice would be appreciated.
    Thanks,
    keego38
    Bob Latino
    Bob Latino
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    New member with PAS 3 Empty Re: New member with PAS 3

    Post by Bob Latino Sun Oct 14, 2012 11:59 am

    Hi,

    I would check out the Tubes4hifi PAS-3 upgrade page. Many good PAS-3 replacement and upgrade parts. Link below ..

    Tubes4hifi PAS preamp page

    Bob
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    New member with PAS 3 Empty Re: New member with PAS 3

    Post by Guest Mon Oct 15, 2012 12:18 am

    .


    Last edited by PeterCapo on Wed Dec 02, 2020 3:03 pm; edited 1 time in total
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    keego38


    Posts : 4
    Join date : 2012-10-14

    New member with PAS 3 Empty Re: New member with PAS 3

    Post by keego38 Mon Oct 15, 2012 8:55 am

    PeterCapo wrote:You might be able to do it if the input impedance of your amplifier is high enough.

    How high is enough? Check me on this, but I believe the output of the phono section directly drives the tape out jack in the original design. How high was the input impedance of tape recorders in those days? If you can research this online by finding the manuals for some old tape decks from, say, the 1960s, it might give you an idea of how high the amplifier’s input impedance would need to be.

    On the other hand, if the input impedance of those tape decks was higher than what you'd actually need, then this information might not be of any real help. But, if it turned out to be similar to the input impedance of your amplifier, then maybe it might work.

    Then there is also the problem of level control. You'd either need an amplifier with its own gain control or you'd still have to go through the PAS volume pot, which might need to be replaced.

    Your cartridge would also need enough output to drive the phono section, as well - this would be even more important since there would be no active line stage after it.

    Do you know what the specification is for the input impedance of your amplifier?

    I would also question what the implications might be for the line stage card with its input open but power still applied – I’d want to know that this would not stress the two tubes in the line stage, especially if they are older, rarer tubes.

    So, there would be a number of considerations. It might not be as easy as just rerouting a couple of wires.

    - Peter
    Hi Peter,
    I've been thinking about level and indeed impedance matching, and as far as I can make out, typical input impedance for most tape decks is about 20K, with input sensitivity of 175ma. My moving coil cartridge only puts out 2mv so I don't think I would have sufficient gain to drive the power amp sufficiently..
    What I have decided to do is bypass the selector/balance/filter/tone controls, and replace the volume control with a good quality ALPS 250k stereo pot, and continue using the line driver stage. When I get everything up and running I may experiment with the original idea, bearing your advice in mind- will post again when I get that far, should be within the next month or so. I'm definitely going to upgrade the power supply though, Bob sent me a link to Tubes4hifi and their supply looks like just the job.
    Thanks Guys,
    Paul

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