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    Powered Subwoofer With Pas2.......Driving Me Crazy!

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    Daveinthedesert

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    Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by Daveinthedesert on Wed Dec 06, 2017 1:23 pm

    Trying to hook up a powered sub with my Pas2 . Using a "y" connector out of main audio outs. Sometimes it works but usually I get a fuzzy- distorted sound out of the subwoofer. Not the sub because I hooked it up in my other "bedroom" system ( non tube) and works just fine. It's almost like by splitting the output on the Pas2 the sub isn't receiving enough of a line level signal. Anybody have subs with their Pas2's or3's? Anybody with thoughts or ideas please chime in.
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    PeterCapo

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by PeterCapo on Wed Dec 06, 2017 4:28 pm

    I once tried running my Stereo 35 into a set of Martin Logan ESL that had a built-in amp for the woofer, and the result was awful.  Generally speaking, I don't think old gear was intended for this kind of thing.  The PAS was designed before solid state gear came into widespread use.

    What kind of load does an internally amplified, crossover-ized speaker present?  With some hand-holding, an original PAS might be able to be tweaked so that it works for something like this, but it will need some thought.  Paralleling the PAS output into two devices probably dropped the impedance the PAS sees to too low a number.  There may have been other issues with the load the PAS saw, I don't know.  Audio Regenesis has a good paper on PAS loading issues that might provide some insight: http://www.audioregenesis.com/documents/PAS%20Compatibility%20Issues.pdf


    Last edited by PeterCapo on Wed Dec 06, 2017 4:29 pm; edited 1 time in total
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    peterh

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by peterh on Wed Dec 06, 2017 4:29 pm

    Daveinthedesert wrote:Trying to hook up a powered sub with my Pas2 . Using a "y" connector out of main audio outs. Sometimes it works but usually I get a fuzzy- distorted sound out of the subwoofer. Not the sub because I hooked it up in my other "bedroom" system ( non tube) and works just fine. It's almost like by splitting the output on the Pas2 the sub isn't receiving enough of a line level signal. Anybody have subs with their Pas2's or3's? Anybody with thoughts or ideas please chime in.

    The PAS2 has a limited driving capability.
    Is your sub transistorized ? And what s it's input impedance ?
    When you connect to 2 amps via a 'Y' the load impedancs is further reduced as
    the 2 amps are paralleled.
    You might need a buffer between the PAS and your load(s).
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    Peter W.

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by Peter W. on Thu Dec 07, 2017 8:28 am

    Daveinthedesert wrote:Trying to hook up a powered sub with my Pas2 . Using a "y" connector out of main audio outs. Sometimes it works but usually I get a fuzzy- distorted sound out of the subwoofer. Not the sub because I hooked it up in my other "bedroom" system ( non tube) and works just fine. It's almost like by splitting the output on the Pas2 the sub isn't receiving enough of a line level signal. Anybody have subs with their Pas2's or3's? Anybody with thoughts or ideas please chime in.

    a) You will need to do the impedance mod on the PAS, if you have not done so already.
    b) Have tried driving the sub directly - no Y-connector? What happens then?
    c) That pre-amp has two outputs. Why the Y-connector?

    On the Impedance Mod:

    To do the impedance mod - per the Dynaco literature:

    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.

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    PeterCapo

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by PeterCapo on Thu Dec 07, 2017 10:40 am

    That procedure is fine if you are running a non-X PAS into an amp with a 100K Z-In, like the original solid state Dynaco Stereo 120.  But, depending on what the actual load is that his PAS sees with the subwoofer in parallel with his amp, it still might not solve the problem.  Or, then again it might just provide the little he needs to get past the distortion he hears, but we cannot really say from here.  Yet again, if he has an X-model PAS, then I am not sure what this procedure would end up doing as this procedure was not intended for the X-models.

    Ideally, it would have to be established exactly which version of PAS he has and then also the make and model of the powered subwoofer so someone can look up its input impedance spec, if it is even available.

    Then, per the Audio Regenesis study, if he has a non-X PAS, and if he is keeping the tone controls, those resistors at the output have to be tweaked so the PAS sees a total load of about 50K (resultant value of the resistor(s) at the PAS output || with the amps's Z-In).  

    But, if he has an X-model PAS and wants to keep the tone controls, it would need further study of the Audio Regenesis paper.

    I think we first need to know how his PAS is presently configured and the Z-in of the powered subwoofer.


    Last edited by PeterCapo on Thu Dec 07, 2017 10:47 am; edited 1 time in total
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    erhard-audio

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by erhard-audio on Thu Dec 07, 2017 10:45 am

    I think the bottom line here is that the original PAS was really not designed for this style of operation, I guess kinda like trying to cram a V8 engine into a 4 cylinder engine bay Wink
    Even with the suggested mod, still not a perfect solution for the poor ole PAS!
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    PeterCapo

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by PeterCapo on Thu Dec 07, 2017 10:52 am

    The PAS was designed before solid state consumer equipment was generally in use.  It can be made to work with some solid state gear today, more so than generally acknowledged.  For someone with basic electronics knowledge, the tweaks are actually extremely simple, and the reward is a fine sounding preamp replete with the virtues of tube sonics that so many audiophiles cherish.

    However, for someone who has no inclination to get under the hood - even for such a simple tweak - then other accommodations are in order.

    But, after all, this is a DIY venue, making the original "poor ole" PAS quite viable for most.
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    erhard-audio

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by erhard-audio on Thu Dec 07, 2017 11:05 am

    indeed! Nobody disputes the upgradeability of any PAS model, after all, that's why we sell those upgrade kits Wink
    But it is still important to know the PAS' limitations.
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    PeterCapo

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by PeterCapo on Thu Dec 07, 2017 11:20 am

    erhard-audio wrote:Even with the suggested mod, still not a perfect solution for the poor ole PAS!

    We can't say that until we know more about the OPs PAS and his subwoofer.  Peter W's suggestion still might do the trick, but if it is not quite adequate, then some other simple tweak might be.

    erhard-audio wrote:indeed! Nobody disputes the upgradeability of any PAS model, after all, that's why we sell those upgrade kits Wink
    But it is still important to know the PAS' limitations.

    It is also important for the folks to be aware that "upgradeability" does not necessitate a departure away from the original circuit topology.  Understanding the limitations of the original PAS must also be understood in correct proportion – they are not nearly as bad as some people seem to think.

    Daveinthedesert wrote:Trying to hook up a powered sub with my Pas2 . Using a "y" connector out of main audio outs. Sometimes it works but usually I get a fuzzy- distorted sound out of the subwoofer. Not the sub because I hooked it up in my other "bedroom" system ( non tube) and works just fine. It's almost like by splitting the output on the Pas2 the sub isn't receiving enough of a line level signal. Anybody have subs with their Pas2's or3's? Anybody with thoughts or ideas please chime in.

    Daveinthedesert, how is your powered subwoofer configured?  Can you quote the make and exact model?  I thought these powered subwoofers have a set of RCA inputs but also have RCA outputs to then route the signal to the power amp.  Is this the case with your setup?

    If you have these kinds of connections, you might try ditching the Y-connector, run the PAS directly into the powered subwoofer only, and then run RCA from the powered subwoofer to the power amp.  Depending on how your powered subwoofer is internally configured, this might or might not help.

    Of course, if you do not have all of these kinds of connections, or if the subwoofer just ends up paralleling the output again, then this idea will not go anywhere.  But, it might be possible that the input to the subwoofer might go to an active crossover that can send the high pass part of the signal to the power amp.  This might, for the better, change the load the PAS sees.  Worth looking into.
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    Peter W.

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by Peter W. on Thu Dec 07, 2017 12:13 pm

    One observation from 30,000 feet and many years of Dynaco:

    The OP calls what he has a PAS2. Whereas it is possible that prior to his ownership someone did the impedance mod, and it is *JUST* infinitesimally possible that it had that mod from the 'factory', the real odds of an OEM PAS2 being modded are slim at best. But, all he has to do is pop-the-top and look.
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    PeterCapo

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by PeterCapo on Thu Dec 07, 2017 12:15 pm

    Agreed, we need more information.  We need to know exactly how his PAS is configured and we need to know about his subwoofer.  

    It might possibly be something other than a big impedance mismatch, I suppose.  His PAS is likely very old and might need parts refreshed 1:1 in the power supply, for instance.  Maybe this is why he is reporting distortion, but I can't say for sure.

    The original Dynaco manual included a table of reference voltage values measured at different nodes of the circuit.  It is a way to tell if the PAS is functioning as it should.  If it were possible for the OP to do safely, they would be very worth checking.

    Of course, at the end of the day, it might very well need some kind of buffer as peterh mentioned.

    GP49

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by GP49 on Fri Dec 08, 2017 3:15 am

    With alteration of the in-circuit load resistors, a stock PAS can be modified to drive impedances down to about 25KΩ. When making this modification, I found the best way was a frequency sweep, while viewing a low-power output of the actual power amplifier being driven, on an oscilloscope. All the load resistors would be removed and a 100KΩ potentiometer, connected as a rheostat across the PAS's output, would be varied to provide the flattest output in the bass. The resistance would then be measured and duplicated using precision resistors in parallel.

    This procedure took into account the variations from manufacturer specifications, in the input impedance of the power amplifier, and variations in that impedance with frequency. If the user wanted to use a different power amplifier, the procedure would have to be repeated with that amp.
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    PeterCapo

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by PeterCapo on Fri Dec 08, 2017 11:00 am

    That's the best way, on the bench.  But, lacking this, and if we knew the supposed input impedance of whatever is being driven, we should be able to provide the OP with the results of a simple calculation that will cause the PAS to see a total load of 50KΩ (referenced to the AR study), and then recommend simply removing the two existing resistors at the output and replace with the new calculated value.  I would think this should work reasonably well?

    In situations like the OP's, I usually hear of rolled-off bass.  But, he is reporting distortion, most but not all of the time.  The causes me to wonder what else might be afoot..?


    Last edited by PeterCapo on Fri Dec 08, 2017 11:34 am; edited 1 time in total (Reason for editing : Clarification.)
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    peterh

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by peterh on Fri Dec 08, 2017 11:09 am

    The one and only solution that does not involve soldering gun and lots of work is to add an external buffer. A few of the buffers on ebay is ok ( are pure cathode followers), a lot of them are not ok.

    One could wish for such a device , either as a circuit board for diy or mounted and ready or
    mounted in a box with power supply. If done simple it could be quite cheap.

    Any takers ?
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    PeterCapo

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by PeterCapo on Fri Dec 08, 2017 11:12 am

    peterh, can you recommend a specific external buffer as you mention?
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    peterh

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by peterh on Fri Dec 08, 2017 12:24 pm

    PeterCapo wrote:peterh, can you recommend a specific external buffer as you mention?
    No. I build my own in an old FM-1 as i wanted a remote volume also.
    http://n.manet.nu/fm1/index.html

    But i do remember a thread here when one ebay device was found and used with
    good results.
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    PeterCapo

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by PeterCapo on Fri Dec 08, 2017 2:24 pm

    Okay, hold the phone for a minute, folks...

    The OP's main concern is the distortion he is getting from his subwoofer.  Could it be caused by DC offset at the PAS outputs?  A resistor might be able to be tweaked for a possible improvement in the impedance match.  But, if there is DC offset from the PAS and the powered subwoofer is having difficulty with it, then impedance matching is not the most immediate problem here.  Is it certain the input to the subwoofer is capacitor coupled?
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    Peter W.

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by Peter W. on Fri Dec 08, 2017 2:41 pm

    PeterCapo wrote:Okay, hold the phone for a minute, folks...

    The OP's main concern is the distortion he is getting from his subwoofer.  Could it be caused by DC offset at the PAS outputs?  A resistor might be able to be tweaked for a possible improvement in the impedance match.  But, if there is DC offset from the PAS and the powered subwoofer is having difficulty with it, then impedance matching is not the most immediate problem here.  Is it certain the input to the subwoofer is capacitor coupled?

    A small 1:1 transformer would isolate any DC offset. A number of equipment maker contemporaneous with Dynaco used interstage transformers for this purpose - AKA converting an unbalanced input to a balanced output. They are not expensive. Actually IS transformers survived right into the solid-state era and solved some inconvenient problems for those just starting to compete with tubes in power and resilience - AR for instance.

    Good catch!
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    peterh

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by peterh on Fri Dec 08, 2017 2:43 pm

    Peter W. wrote:
    PeterCapo wrote:Okay, hold the phone for a minute, folks...

    The OP's main concern is the distortion he is getting from his subwoofer.  Could it be caused by DC offset at the PAS outputs?  A resistor might be able to be tweaked for a possible improvement in the impedance match.  But, if there is DC offset from the PAS and the powered subwoofer is having difficulty with it, then impedance matching is not the most immediate problem here.  Is it certain the input to the subwoofer is capacitor coupled?

    A small 1:1 transformer would isolate any DC offset. A number of equipment maker contemporaneous with Dynaco used interstage transformers for this purpose - AKA converting an unbalanced input to a balanced output. They are not expensive. Actually IS transformers survived right into the solid-state era and solved some inconvenient problems for those just starting to compete with tubes in power and resilience - AR for instance.

    Good catch!

    good point !
    Even better , a small 5:1 transformer could take care of some of the misalignment
    Could even be mounted inside the PAS ( far from the mains transformer)
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    Dave_in_Va

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by Dave_in_Va on Fri Dec 08, 2017 2:55 pm

    In case the OP needs a buffer, I bought this one and it still works perfectly (on the advice of peterh).

    https://www.ebay.com/itm/YAQIN-SD-CD2-Stereo-Audio-Processor-Tube-Buffer/132415150123?epid=1029041475&hash=item1ed48ed82b:g:uA8AAOSwrklVc26d

    ....and here's the old thread where we discussed how to get my old SP-10/PH-10 (since upgraded) to work with a CD burner.

    http://dynacotubeaudio.forumotion.com/t2378-connecting-a-cd-recorder-to-my-sp-ph10

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    PeterCapo

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by PeterCapo on Fri Dec 08, 2017 3:04 pm

    Dave_in_Va wrote:In case the OP needs a buffer, I bought this one and it still works perfectly (on the advice of peterh).

    https://www.ebay.com/itm/YAQIN-SD-CD2-Stereo-Audio-Processor-Tube-Buffer/132415150123?epid=1029041475&hash=item1ed48ed82b:g:uA8AAOSwrklVc26d

    It looks like this Yaqin buffer has capacitors in series with its inputs - unless I am seeing that wrong?  If so, it should solve any DC offset and also impedance matching issue (cathode follower).  Worth looking into and much easier than surgery, depending on how the OP prefers to address things.  Good recommendation.
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    Pillo69

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by Pillo69 on Sat Dec 16, 2017 1:43 pm

    It is clear that an external buffer (as Peterh has commented), or a new PCB with a cathode follower (model TCLA, Roy, for example), solve the problem of connection to computers or transistor equipment.
    But this new output influences the loss of quality with its connection to the ST70 ?.
    A greeting.
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    peterh

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by peterh on Sat Dec 16, 2017 3:17 pm

    Pillo69 wrote:It is clear that an external buffer (as Peterh has commented), or a new PCB with a cathode follower (model TCLA, Roy, for example), solve the problem of connection to computers or transistor equipment.
    But this new output influences the loss of quality with its connection to the ST70 ?.
    A greeting.
    No. A proper cathode buffer won't affect the sound in any negative way.

    In fact i'd suggest connecting the buffer at the PAS output and then the
    power amp and other stuff on the output of the buffer.
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    deepee99

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by deepee99 on Sat Dec 16, 2017 3:42 pm

    May I posit that a *good* match between speakers and amp(s) does not need a subwoof.?
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    Peter W.

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    Re: Powered Subwoofer With Pas2.......Driving Me Crazy!

    Post by Peter W. on Sat Dec 16, 2017 4:23 pm

    deepee99 wrote:May I posit that a *good* match between speakers and amp(s) does not need a subwoof.?


    That works only if the main speakers are full range. More-or-less a rarity these days.

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