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    PAS3 scratch build - all controls removed

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    FrankieKat

    Posts : 3
    Join date : 2013-06-01

    PAS3 scratch build - all controls removed

    Post by FrankieKat on Sat Jun 01, 2013 6:01 pm

    Hi there. I've mostly finished a scratch build of a "PAS3" from bare boards and a project case, and as such no factory controls at all. During the build I'd studied the schematics extensively but I cannot say for sure if I've accounted for everything in building it all this way -- meaning, no source selector, no blend switch, no balance control, no scratch filter and no bass/treble... only volume. I've found a few other very helpful threads detailing removal of bass/treble, but did not cover the others really.

    Starting at the BS I'm just a little unclear as to the role of the 47k resistor in series with the inputs to the switch would appears to be shorted out in one position (which position I'm not sure).

    Question 1 -- do I want that 47k resistor in series or not? ** I currently do NOT have this in there

    Question 2 -- if I have no interest in the blend effect (do not want the "stereo effect to be progressively eliminated") would I discard the 560k/270k (or 68k/33x if you're a 3X man) completely? ** I currently do NOT have these in the circuit at all

    Question 3 -- to remove the balance correctly, I would still want to have the resistance of the control in the center position, so I would want to put approximately 375k in series (is a standard 330k close enough) with the input, and then another 375k to ground? ** Currently do NOT have these in

    Question 4 -- to eliminate the filter switch, I had read a suggestion in another forum to simply bypass the 18 resistor between the tube sections and leave eyelet 15 completely open. Yay, nay? ** Currently have it this way

    For bass/treble I do effectively have what PeterCapo posted here, with the exception of the 1.0uf cap (my build was based on the PAS3) after the feedback loop.

    So here's the bottom line... if I use it all as-is, the output is very hot and distorted, and the volume control is super sensitive -- too much gain here, simply. Now, if I say, skip the PC5 completely and take the output of the PC6 phono stage (eyelets 1 and 7) through a 100k volume pot to my amp (a Maggie 8601/EL84 SEP) it sounds really nice -- good dynamics on the volume, clear and warm sound BUT it is a thin on the bottom end. Obviously I've got a pretty wide impedance mismatch here, but it's over my head to try to figure out how to resolve that. I can hear that the low end is thin by swapping it with a NAD solid state clinically-flat-response phono stage and it has more solid low end (and boring everything else).

    I suppose my larger question is... do I actually *need* the PC5 at all, or what use does it serve me other than to compensate for the losses in all of the various control circuits? As in, by "fixing" all of the controls in their dead center positions, would I just be creating a net-zero gain between it and the tubes? Think there's anything that could be done to match the PC6's output directly to my amp?

    Or am I just talking crazy talk?

    Thanks everyone!

    FK

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: PAS3 scratch build - all controls removed

    Post by tubes4hifi on Wed Jun 05, 2013 3:16 pm

    without tone controls you have about 20-30db of extra gain that you don't need.
    But the phono board doesn't have low enough output impedance to drive an amp properly by itself.
    Of course my recommendation would be to replace the PC5 board with my Zmod PC5 board (15db gain and no tone controls).
    You don't need the blend switch at all, and you don't need a balance control (or even the extra resistors).
    You can PM or email me for an enhanced version of the PAS schematic, MUCH easier to read and understand than the original.
    No changes needed to leave out the filters.

    FrankieKat

    Posts : 3
    Join date : 2013-06-01

    Re: PAS3 scratch build - all controls removed

    Post by FrankieKat on Mon May 05, 2014 12:42 pm

    tubes4hifi wrote:without tone controls you have about 20-30db of extra gain that you don't need.  
    But the phono board doesn't have low enough output impedance to drive an amp properly by itself.
    Of course my recommendation would be to replace the PC5 board with my Zmod PC5 board (15db gain and no tone controls).
    You don't need the blend switch at all, and you don't need a balance control (or even the extra resistors).
    You can PM or email me for an enhanced version of the PAS schematic, MUCH easier to read and understand than the original.
    No changes needed to leave out the filters.

    To clarify, what I'm doing is use this as a phono to line-level stage for an integrated amp, with its own preamp, volume control etc.  Recently I've been using just the PC6 board to drive my Fisher 500C's AUX input taking the output from the PC6's pin 1 and 7.  To my ears, this configuration sounds superior to the Fisher's stock phono stage.

    I'm more asking in an "I don't know what I don't know" way -- if this an ideal use or if there's any tips or mods that can be done to improve upon it?

    Thanks so much.

    -FK

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: PAS3 scratch build - all controls removed

    Post by PeterCapo on Mon May 05, 2014 8:35 pm

    Hi FK,

    Depending on how you changed the filter circuit, shorting the 18k might not be enough.  You might also need to disconnect the two capacitors, but, again, this depends on how you might have left the associated connections.

    Without the 1.0uf capacitor coupling the output of PC5, you’ll have some DC offset on the output, which is not desirable.

    For your volume control, you’d need to use one with loudness taps while also retaining the 4.7k resistors wired as in the original, but you can omit the 0.1uf loudness contour capacitors.  Without the loudness taps and resistors, the volume will increase too quickly.

    If you choose to retain the original Dynaco circuits, these are some things you could look at.

    Thanks.
    Peter
    PS - you didn't mention anything about how you built the power supply...

    FrankieKat

    Posts : 3
    Join date : 2013-06-01

    Re: PAS3 scratch build - all controls removed

    Post by FrankieKat on Tue May 06, 2014 10:42 am

    Hi Peter,

    What I've got is a scratch built PAS3 with two new boards and the SDS cap board as a power supply.  I'm talking about taking the output from the coupling caps from the PC6 (phono stage) connected to a normal line-level input of a different integrated amp (having its own preamp/volume control, etc).  In this case, not using the PC5 board, volume control etc at all, since the output of the PC6 normally connects to the input of the PC5 board where the other line level inputs do.  My understanding is that the normal PAS3 output is to drive a standalone power-amp, which I'm not using.  Make sense?

    I have been using it this way with my Fisher and to my ears sounds great, but I'm really looking to see of anyone has any experience doing this and if there are any tips or anything to consider to make it work to it's best.

    -FK

    corndog71

    Posts : 444
    Join date : 2013-03-19
    Location : It can get windy here

    Re: PAS3 scratch build - all controls removed

    Post by corndog71 on Tue May 06, 2014 12:47 pm

    So basically you're using it as a phono preamp.

    You should consider getting a real stand alone phono preamp. The right tool for the right job if you know what I mean.

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