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Dynaco PAS-3 tube preamp mods and parts

  • original circuit replacements

  • Z-mod upgrades

  • PAS3-AIKIDO upgrade

  • Tone Control Line Amp upgrade

  • regulated power supply upgrade

  • customer comments on our mods

  • links to hundreds of photos of PAS preamps with various modifications

  • PAS3 upgrade catalog

    As of April 2015, Tubes4HiFi and TubeNirvana have collaborated to turn the PAS3 business over to TubeNirvana
    All upgrades, modifications, and various parts will be available thru the TubeNirvana website here

    original circuit replacement PCBs

    Dynaco PAS3 preamp rebuild

    for those on a budget but wanting an updated/upgraded PAS3, we offer high-quality original circuit replacement PCBs with better than original parts.

    PC6 phono amplifier and PC5 line amplifier . . . . . . . . . . . . . . new power supply board

    click here to see our PAS3 upgrade catalog

    VTA PAS3-Z modification

    Vacuum Tube Audio Zmod upgrades for the PAS3

    Finally, a set of hi-end modifications for the Dynaco PAS3 preamp. We haven't seen anything new in the past dozen years or so,
    and the only real alternatives are the Curcio kit for $662 and the VanAlstine kit for $449, now VacuumTubeAudio at
    offers an easy plug-in 3 piece circuit upgrade for the PAS3 that is identical to it's highly reviewed SP6 preamp kit.
    This is for customers that want to KEEP their PAS3 chassis rather than building an all new preamp in a newer chassis.

    Our Vacuum Tube Audio upgrade circuits for the PAS3 replace the 50 year old circuit designs with modern high performance high fidelity drop-in replacements.
    Our Zmod-PC6 phono preamp uses a 12AX7 gain stage followed by a highly accurate passive EQ, followed by another 12AX7 gain stage, for a total gain of 45 db.
    This has higher accuracy and uses no negative feedback, for much improved sound quality and frequency accuracy, with no flabby bass and a clear natural hi-end.

    The Zmod-PC5 line stage uses a pair of 12AU7 in a cascode SRPP gain stage with excellent linearity and low output impedance.
    This is a pure hi-end design with no tone controls and no negative feedback for perfect reproduction of the original source.
    For those who want to maintain a tone-control design, see below for our Zmod-TCLA circuit as an alternative.

    The Zmod-PS power supply makes use of the original 12X4 tube rectifier for soft-start, replacing the old "quad cap" with 266uF of B+ power supply capacitance
    (compared to the original 80uF) with more reserve power for bass and complex music reproduction, and a lower power supply impedance.
    The filament supply is also improved with 6600uF of capacitance for lower noise and more reserve power (compared to original 4000uF).

    Printed Circuit Boards are mil-spec FR4 double-sided with plated-thru holes. Easy installation, with full documentation included.
    An assembly manual, schematic, layout diagrams, parts list, and installation instructions are included, along with online tech support.

    We also have available every part that is used in a PAS3 preamp, most are higher quality than the original types.
    See our parts listing of volume, balance, selector switch, slides switches, LED power indicator, and our very high quality RCA jacks for input-output.

    click here to see our PAS3 upgrade catalog

    PAS3-AIKIDO modification/upgrade

    This new board is the ultimate line amplifier for the PAS3, it takes our hi-end SP12 preamp circuit and sizes it to fit into the PAS3 chassis. The AIKIDO circuit consists of 4 - 12AU7 (or equivilent) tubes with voltage gain and noise-reduction cathode follower output. This circuit design has been optimized to be used with the standard PAS power supply, or even better, with our new regulated power supply. This board delivers the sonic goods. It offers low distortion, low output impedance, a great PSRR figure, and feedback- free amplification. The secret to its superb performance- despite not using global feedback- lies in its internal symmetry,which balances imperfections with imperfections. As a result, the circuit works at least a magnitude better than the equivalent SRPP or grounded-cathode amplifier. This is the ultimate preamp, now in your PAS3 !!

    Zmod TCLA (tone control line amplifier) modification/upgrade

    Finally, a hi-end modification for the Dynaco PAS3 preamp that INCLUDES TONE CONTROLS !!

    This modern circuit uses four 12AX7 tubes in an active Baxandall design for the extra required gain and tone control processing.
    New accurate linear tone controls are included that allow +/-12db of boost/cut for bass and treble frequencies.
    Easy installation takes about 1 hour. Full documentation included with the kit includes assembly manual, schematic, layout diagrams,
    parts list, and installation instructions.


    our new regulated power supply for the PAS preamp

    This new regulated power supply is simpler and less expensive than our previous version.
    This includes two new power transformers (one for B+ and one for filaments) and fits right where the old transformer and power supply were.
    It will require drilling two small holes. Simple installation. Fantastic performance, our ultimate power supply for the PAS preamp.

    click here to see our PAS3 upgrade catalog

    NEW input-output RCA jack panel

    Our 2nd most popular upgrade (after the power supply) is this high quality RCA jack panel.
    The old cheap quality RCA jacks that are crowded together too closely are replaced with a PCB with wider spacing and gold-plated RCA jacks.
    This has 7 pairs of jacks rather than the original 9 pair (most of those were never used anyway!).

    . .

    replace this mess . . . . . . . . . . . . . . . . . . . . with this . . .

    Our new selector switch replaces the old fragile original with a modern 2P6T rotary switch, a nice match to the new RCA jack panel.
    This will eliminate the "rats-nest" of wiring on the original.

    click here to see our PAS3 upgrade catalog

    I just wanted to thank you for looking at my Pas-3.  It arrived safely today and I had a good session with it for several hours.  
    At first I thought it sounded a bit forward, but then I realized that I had become accustomed to the sluggish/slower presentation 
    of my stock Pas-3, a backup unit.  Your circuits are faster, tighter and more revealing.  Also, I do not have to "crank" the volume knob.  
    Even better, when I do- the hum that I was experiencing with my stock Pas-3 is for the most part non-existent.  
    It was a nice revelation when I switched from my backup Pas-3.  Using your boards- I realize now what an improvement the TCLA and PH10 are. 
    Dan F. 

    PAS preamp photo collections (we've worked on several hundred of these, browse to see some photos):

    page one

    page two

    page three

    PAS3 upgrade catalog

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     tubes 4 hifi
     "the path of least resistance is through a vacuum"