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

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

    Post by Bob Latino on Fri Dec 05, 2008 1:22 pm

    Dynaco is famous for making some of the best sounding tube equipment of the classic tube era. The PAS preamps were among the best sounding preamplifiers one could buy up until the time they went out of production. There are two circuit differences between the earlier PAS-2 and PAS-3 units and the later PAS-3X.

    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. Simple !

    Bob

    PeterCapo

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

    Post by PeterCapo on Fri Dec 05, 2008 4:31 pm

    Hi Bob,

    I believe the 1uF capacitors also served to suppress transient pops from the x-mod pots. For example, the treble pot wiper lifts near the center of its rotation, and the bass pot shorts near its center.

    Also, I believe the 1uF capacitors made it possible to use the PAS with an amplifier having as low as 100K input impedance without snipping out the 510K resistor, where the "regular" PAS could at best accommodate 250K amplifier input impedance with the 510K resistor snipped out. I can say this because I have early and late copies of the original manuals, and the late PAS-3X manual specifies compatibility with 100K Z-in but doesn't say anything about snipping out the 510K resistor.

    The X-mod 1uF capacitors could be especially helpful in the event anyone wants to use a PAS with some of the modern tube amps. I have seen a couple of examples of these where the input impedance is in the range of 100K – 150K. For example, the Audio Electronics (Cary Audio) Six-Pacs have an input impedance of 150K, and my reissue McIntosh MC275 Z-in is 100K.

    Regards,
    Peter

    Bob Latino
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    Yes - true

    Post by Bob Latino on Fri Dec 05, 2008 5:02 pm

    Hi Peter, welcome to the forum ...

    True - the wipers of the bass and treble controls do lift near the center of rotation on a PAS-3X as the tone controls are removed from the circuit. This could cause a transient pop and I am sure Dynaco was concerned about that.

    It is interesting that all the earlier Dynaco amps - the Mark II, Mark III, ST-70 and Mark IV had no DC blocking caps on their inputs but the later amps the ST-35 and the SCA-35 integrated do have DC blocking caps. I bypassed the small .1 uF @16 volt input cap on my ST-35 in hopes of "better sound" but in all honesty I can't hear any difference with this small cap in or out of the circuit.

    Bob

    Luddite

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    Dynaco PAS-3X

    Post by Luddite on Mon Aug 10, 2009 11:19 am

    The other night I was thumbing through a book by Bruce Rozenblit, "Beginner's Tube Audio Design". In one section on classic circuitry, it was mentioned that the PAS-3X, among the differences already discussed in this thread, had one other change. The two resistors on the blend control were changed from 560K/270K ohm to 68K/33K ohm. This was apparently done for greater compatibility with the Hafler three channel system. I presume that the Hafler three channel system utilized a derived center channel. I am familiar with the Dynaquad system for deriving surround information, but I don't recall a derived center channel system.

    Charlie

    GP49

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

    Post by GP49 on Mon Aug 10, 2009 7:24 pm

    Peter Capo:

    > I believe the 1uF capacitors made it possible to use the PAS with an
    > amplifier having as low as 100K input impedance without snipping out
    > the 510K resistor, where the "regular" PAS could at best accommodate
    > 250K amplifier input impedance with the 510K resistor snipped out. I
    > can say this because I have early and late copies of the original
    > manuals, and the late PAS-3X manual specifies compatibility with 100K
    > Z-in but doesn't say anything about snipping out the 510K resistor.

    There is also a 62K resistor on the linestage circuit board that can be removed if need be, to run any PAS into an amp with 100K input impedance. Even the X-models benefit from this. The bandpass of the output stage is limited by the 0.22uf coupling capacitor in the factory models; the 1uF capacitors do nothing in this regard. If you increase the 0.22uf capacitor sufficiently, generally in PAS modifications to 2uf or more, then the 1.0uf capacitor becomes the limiting factor unless the tone controls and that capacitor were removed.

    In my shop I would ask PAS-owning customers to bring in their power amplifiers. I would remove both the 62K and 510K resistors, hook up a potentiometer across the output as a rheostat, and adjust it to get flattest overall response in the bass (if you went to too high a load resistance, you'd get a midbass hump!); then measure the resistance and install a fixed resistor or paralleled fixed resistors to equal that value.

    bnc31:

    > The other night I was thumbing through a book by Bruce Rozenblit,
    > "Beginner's Tube Audio Design". In one section on classic circuitry,
    > it was mentioned that the PAS-3X, among the differences already
    > discussed in this thread, had one other change. The two resistors on
    > the blend control were changed from 560K/270K ohm to 68K/33K ohm.
    > This was apparently done for greater compatibility with the Hafler
    > three channel system. I presume that the Hafler three channel system
    > utilized a derived center channel. I am familiar with the Dynaquad
    > system for deriving surround information, but I don't recall a
    > derived center channel system.


    The Hafler three channel system used a single speaker connected between the two HOT terminals at the stereo amplifier's output, deriving a "difference" channel. That third speaker was supposed to be placed centered and BEHIND the listener. The later Dynaquad system was a derivation of the three-channel setup, essentially two speakers playing the "difference" signal.

    Luddite

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

    Post by Luddite on Tue Aug 11, 2009 10:36 am

    The Hafler three channel system used a single speaker connected between the two HOT terminals at the stereo amplifier's output, deriving a "difference" channel. That third speaker was supposed to be placed centered and BEHIND the listener. The later Dynaquad system was a derivation of the three-channel setup, essentially two speakers playing the "difference" signal.[/quote]

    Thanks GP49, that riddle is now solved. Very Happy
    Charlie

    Rowbo1

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

    Post by Rowbo1 on Fri Jan 28, 2011 3:24 am

    What are the diferences between pas2 and 3
    Dave

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

    Post by GP49 on Fri Jan 28, 2011 10:32 pm

    Rowbo1 wrote:What are the diferences between pas2 and 3

    Knobs and front panel

    Rowbo1

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    Join date : 2011-01-27

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

    Post by Rowbo1 on Sat Jan 29, 2011 3:40 am

    Thanks
    Dave

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