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    Running two amps off Dynaco PAS-3?

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    ViperZ

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    Running two amps off Dynaco PAS-3?

    Post by ViperZ on Sun Jun 17, 2018 9:43 pm

    Did anyone ever run two amplifiers off a single Dynaco PAS-3 preamplifier?
    I used to connect Y-adapters, to PAS-3 output, and run ST-70 and Velodyne ULD-15 subwoofer amp in parallel with no problems years ago.
    I want to connect my Velodyne back to the system again. Is PAS-3 going to be ok running Velodyne amp in parallel with a pair of MK-III amps? Thank you.

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    j beede

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    Re: Running two amps off Dynaco PAS-3?

    Post by j beede on Sun Jun 17, 2018 11:49 pm

    A stock PAS is a curious thing... I do not know of a preamp whose output characteristic is as sensitive to amplifier input impedance as the PAS. In the 1980s it was very common to add a cathode follower to the PAS output to reduce its output impedance and reduce this sensitivity. Having said that, a stock PAS can drive the high input impedance of a pair of Mk III.

    Does the Velodyne amplifier include a mixer to blend the left and right outputs from the preamp? What is the input impedance of the Velodyne amplifier?
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    ViperZ

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    Re: Running two amps off Dynaco PAS-3?

    Post by ViperZ on Sun Jun 17, 2018 11:52 pm

    Yes, Velodyne has RCA inputs for left and right channels.
    Velodyne input impedance is 20k. MK-III input impedance is 500k. So paralleling them would give 19.2k impedance that PAS-3 will see.
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    peterh

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    Re: Running two amps off Dynaco PAS-3?

    Post by peterh on Mon Jun 18, 2018 4:18 am

    ViperZ wrote:Yes, Velodyne has RCA inputs for left and right channels.
    Velodyne input impedance is 20k. MK-III input impedance is 500k. So paralleling them would give 19.2k impedance that PAS-3 will see.
    That is too little impedance. You will need an external buffer between the PAS and the
    poweramps.
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    ViperZ

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    Re: Running two amps off Dynaco PAS-3?

    Post by ViperZ on Mon Jun 18, 2018 7:56 am

    So what if I put a tube buffer in series with subwoofer amplifier after the Y-splitter? Buffers generally have about 500k input impedance, and 100-500 ohms output impedance. This way PAS will see 250k, and sub amp will see 100-500 ohms. Both should be happy, right?
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    peterh

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    Re: Running two amps off Dynaco PAS-3?

    Post by peterh on Mon Jun 18, 2018 8:35 am

    ViperZ wrote:So what if I put a tube buffer in series with subwoofer amplifier after the Y-splitter? Buffers generally have about 500k input impedance, and 100-500 ohms output impedance. This way PAS will see 250k, and sub amp will see 100-500 ohms. Both should be happy, right?
    Yes, that's perfect.
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    ViperZ

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    Re: Running two amps off Dynaco PAS-3?

    Post by ViperZ on Mon Jun 18, 2018 9:41 am

    Thank you. I've never done this before. Hopefully I can find a tube buffer that doesn't roll off low frequencies.
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    PeterCapo

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    Re: Running two amps off Dynaco PAS-3?

    Post by PeterCapo on Mon Jun 18, 2018 10:46 am

    I've never tried these, but I think there has been some discussion about these in the forum - hope this link works: https://www.ebay.com/str/Canadian-HiFi-Online/Tube-Buffers/_i.html?_storecat=896690011

    ... although looking at it again the input impedance might still be low for the PAS.  BTW, bypassing the tone controls should allow driving just about anything without the need for the buffer.
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    Peter W.

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    Re: Running two amps off Dynaco PAS-3?

    Post by Peter W. on Mon Jun 18, 2018 11:24 am

    PeterCapo wrote:I've never tried these, but I think there has been some discussion about these in the forum - hope this link works: https://www.ebay.com/str/Canadian-HiFi-Online/Tube-Buffers/_i.html?_storecat=896690011

    ... although looking at it again the input impedance might still be low for the PAS.  BTW, bypassing the tone controls should allow driving just about anything without the need for the buffer.

    Three things:

    a) There are several iterations of the PAS. The PAS-3 version is referred to by the OP.  The 2 and the pre-mod 3 *will* be challenged driving the two amps together. At low volumes, this might not be an issue.
    b) The PAS-3X will not be challenged by this option. The tone-controls are 100% out-of-circuit when in the null position, and the output impedance has been changed from the older versions.
    c) It is possible to to the X-mod to a 2 or 3 as it applies to output impedance, but not to the tone controls. This will help in most cases and possibly be all that you need.

    Per Dynaco, as follows:

    1. Obtain two 100,000 ohm, 1/2-watt 10% tolerance resistors.
    2. Unsolder and discard the two 510,000 ohm (green-brown-yellow)
    resistors connected to the audio-output sockets on the back panel of
    the preamplifier and replace them with the 100,000 ohm resistors.
    Solder all connections.
    3. Snip out and discard the two 62,000 ohm (blue-red-orange) resistors
    on the PC-5 circuit board adjacent to eyelets 8 & 18. This completes
    the modification.

    Given the relative difference in cost, I would use 5% resistors and
    screen them as well.


    As to how you can tell if you have the X tone-controls:
    1. with your VOM go to either element terminal and the wiper.
    2. Operate the control through its range. At/near the center-point
    there should be a signficant drop or a significant increase in
    resistance. If that happens, you have the X controls. If the
    resistance change is approximately linear to the control and with no
    change at the center-point you have the pre-X controls.

    But, for the record, I very much dislike mutilating this pre-amp for dubious results easily corrected otherwise. I look at it in almost the same light a declawing a cat. Or docking a dog's tail.
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    ViperZ

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    Re: Running two amps off Dynaco PAS-3?

    Post by ViperZ on Mon Jun 18, 2018 11:48 am

    I'm honestly getting sick of the tone controls, or specifically not tone controls, but the stereo/mono switch. The fact that it makes the volume control uneven drives me nuts.

    Peter, I know that mine is not 3X for sure, just a "regular" PAS-3. I'm looking at the schematic - what is this resistor modification trying to achieve? Lowered output impedance of the preamp? Thank you


    Last edited by ViperZ on Mon Jun 18, 2018 11:53 am; edited 1 time in total
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    PeterCapo

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    Re: Running two amps off Dynaco PAS-3?

    Post by PeterCapo on Mon Jun 18, 2018 11:48 am

    Peter W. wrote:
    PeterCapo wrote:I've never tried these, but I think there has been some discussion about these in the forum - hope this link works: https://www.ebay.com/str/Canadian-HiFi-Online/Tube-Buffers/_i.html?_storecat=896690011

    ... although looking at it again the input impedance might still be low for the PAS.  BTW, bypassing the tone controls should allow driving just about anything without the need for the buffer.

    b) The PAS-3X will not be challenged by this option. The tone-controls are 100% out-of-circuit when in the null position, and the output impedance has been changed from the older versions.

    Right, but if you nudge them off center, they won't work right if it is looking into a load less than 100K. The tone control bypass procedure can be done carefully so as not to "mutilate" the PAS. If done carefully, it should be able to be reversed easily enough.
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    ViperZ

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    Re: Running two amps off Dynaco PAS-3?

    Post by ViperZ on Mon Jun 18, 2018 11:59 am

    Maybe I'm overcomplicating the whole thing and should just get full-range speakers. I just love that ULD-15 subwoofer though Sad
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    PeterCapo

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    Re: Running two amps off Dynaco PAS-3?

    Post by PeterCapo on Mon Jun 18, 2018 12:02 pm

    Which version of PAS do you have? Is it a 3 or 3X? http://www.audioregenesis.com/documents/Dynaco%20PAS%203%20or%203X.pdf
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    ViperZ

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    Re: Running two amps off Dynaco PAS-3?

    Post by ViperZ on Mon Jun 18, 2018 12:09 pm

    PeterCapo wrote:Which version of PAS do you have?  Is it a 3 or 3X?  http://www.audioregenesis.com/documents/Dynaco%20PAS%203%20or%203X.pdf

    Definitely 3, not 3X. I checked that a few years ago.
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    corndog71

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    Re: Running two amps off Dynaco PAS-3?

    Post by corndog71 on Mon Jun 18, 2018 12:19 pm

    ViperZ wrote:Maybe I'm overcomplicating the whole thing and should just get full-range speakers. I just love that ULD-15 subwoofer though Sad

    Or get a better preamp like the VTA SP-13 or 14.

    My SP-13 has no trouble driving 2 VTA M125's and 2 Rythmik A370PEQ subwoofer amps.
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    Peter W.

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    Re: Running two amps off Dynaco PAS-3?

    Post by Peter W. on Mon Jun 18, 2018 12:49 pm

    ViperZ wrote:I'm honestly getting sick of the tone controls, or specifically not tone controls, but the stereo/mono switch. The fact that it makes the volume control uneven drives me nuts.

    Peter, I know that mine is not 3X for sure, just a "regular" PAS-3. I'm looking at the schematic - what is this resistor modification trying to achieve? Lowered output impedance of the preamp? Thank you

    I will answer later at some length, God help me, but for now:

    Yes, lower the output impedance to be tolerant down to a nominal 50K or so.
    And, removes part of the R/C network around the bass control to tighten it up considerably.

    And, in your case, you might lower the 100K resistor to perhaps 75K or so (matched channel-to-channel) to handle the exceptionally low combined impedance. But change back if you cease that practice.

    As to the VC walking when the mono-stereo switch is engaged - make sure that _ALL_ the components within that system are matched-by-test from channel-to-channel. Dynaco used parts from the lowest bidders assembled by jobbers who also were low-bid and would not bother to match anything at all from board-to-board. Meaning that with 10% parts (at best) variations could be as much as 20% from side-to-side.
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    PeterCapo

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    Re: Running two amps off Dynaco PAS-3?

    Post by PeterCapo on Mon Jun 18, 2018 12:49 pm

    Well, if it is an original 3 and not a 3X, and if it has not had the original Dynaco-recommended resistor removal to accommodate a 250K load, then you will need a buffer with an input impedance greater than 250K.

    On the other hand, if your subwoofer has a level control, it might possibly be able to compensate for the low frequency rolloff, perhaps in conjunction with simply turning up the bass controls on the PAS.

    The PAS is a fine preamp. It's just that it was designed in an era when it it was not expected to drive lower impedance loads.
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    ViperZ

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    Re: Running two amps off Dynaco PAS-3?

    Post by ViperZ on Mon Jun 18, 2018 12:53 pm

    Thank you for the explanation!

    Regarding VC, it's completely equal across both channels. What I don't like is that it increases fairly slowly up to 12 o'clock position, then makes a jump to a louder spot. I read that it's fairly common and due that stereo/mono switch connection. It drives me crazy because it's a bit too quiet before 12 o'clock and a bit too loud after Smile

    It's pretty amazing to learn about shortcomings of this 60s preamp. I don't want any other preamp - this is my system from the 60s. I have another SS system, but I really like tweaking this one.
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    Peter W.

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    Re: Running two amps off Dynaco PAS-3?

    Post by Peter W. on Mon Jun 18, 2018 1:11 pm

    ViperZ wrote:Thank you for the explanation!

    Regarding VC, it's completely equal across both channels. What I don't like is that it increases fairly slowly up to 12 o'clock position, then makes a jump to a louder spot. I read that it's fairly common and due that stereo/mono switch connection. It drives me crazy because it's a bit too quiet before 12 o'clock and a bit too loud after Smile

    It's pretty amazing to learn about shortcomings of this 60s preamp. I don't want any other preamp - this is my system from the 60s. I have another SS system, but I really like tweaking this one.

    Contact me off-line. I may be able to send you a far more linear VC - Fisher NOS and similar vintage, but with the proper taps.
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    ViperZ

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    Re: Running two amps off Dynaco PAS-3?

    Post by ViperZ on Mon Jun 18, 2018 1:44 pm

    Peter W. wrote:
    ViperZ wrote:Thank you for the explanation!

    Regarding VC, it's completely equal across both channels. What I don't like is that it increases fairly slowly up to 12 o'clock position, then makes a jump to a louder spot. I read that it's fairly common and due that stereo/mono switch connection. It drives me crazy because it's a bit too quiet before 12 o'clock and a bit too loud after Smile

    It's pretty amazing to learn about shortcomings of this 60s preamp. I don't want any other preamp - this is my system from the 60s. I have another SS system, but I really like tweaking this one.

    Contact me off-line. I may be able to send you a far more linear VC - Fisher NOS and similar vintage, but with the proper taps.

    Will do. Thank you.
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    tubes4hifi
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    Re: Running two amps off Dynaco PAS-3?

    Post by tubes4hifi on Mon Jun 18, 2018 2:45 pm

    Peter's help can probably get you going, but if you decide to go the tube buffer route, I can recommend this one, it's about $110 total plus about $10 for a 12vac wall-wart power supply.
    You would just need a box to put it into.
    http://glass-ware.stores.yahoo.net/acf12stbu.html

    Sponsored content

    Re: Running two amps off Dynaco PAS-3?

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