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    Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

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    rjpjnk

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    Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by rjpjnk on Fri Aug 24, 2018 12:45 pm

    I was playing around with my recently refurbished PAS 3X and I noticed that when I tried driving a subwoofer with it the sound to the main speakers changed. I realize the PAS was not designed to drive low impedance loads common in modern solid state amps, and the consequences of this are likely what I was hearing. This started me thinking about what sort of loading I could reasonably expect the PAS to handle. So basically my next science experiment Smile

    I'm not 100% sure how one determines the input impedance of an amplifier, especially without a schematic. The DC component is easy, but I suspect there may be a significant contribution through the first transistor/tube stage as well. As a first test, I connected my LCR meter to the input terminals of the sub and measured the resistive component at 100Hz and 1KHz, and I got 10 KOhms. I've heard moderm SS amps are this low, so this seems reasonable.

    Next I measured the VTA ST70. It measures about 75 KOhms, which is as expected for the combination of the 270 and 10 K fixed internal resistors and a 100 K stepped attenuator.

    From what I've read about the PAS, they are expecting a 500 K external load so that the total load seen at the end of the bass pot is 50 K (comprised of 62K || 510K || 500K). It seems like the only consequence of not having a 500 K load is that the tone controls will not function properly. So my first question is is this truly the only consequence, or are there other problems with a PAS driving a low impedance load?

    Of course with the 3X model the tone controls are bypassed when centered and at this position the output impedance is not affecting the tone circuits, but I want to consider the case when the tone controls are moved off center as well.

    So the issue I am pondering is this, if I wanted to actually *use* the tone controls on a PAS 3X, what would I need to do as far as circuit modifications?

    Probably it will never be possible to drive a 10 K sub properly, but even driving the VTA70 may require modifications because the stepped attenuator drops the input impedance all the way down to 75 K and this is too low for an unmodified PAS. It is a simple matter to fix this though. We can either remover the stepped attenuator so it is not messing up the load (it is not necessary with the PAS anyway since presumably the PAS is being used to control volume), or we can change the PAS output resistors (62K and 510K) to values that maintain the 50K internal load the PAS wants to see.

    It seems that ideally, to use a PAS with a VTA driver board, one should do both. First remove the 100K stepped attenuator (if present), and then remove the 510K fixed internal resistor from the PAS. This would make remaining internal load almost exactly 50K and keep the PAS happy. Does this sound right?





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    peterh

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by peterh on Fri Aug 24, 2018 12:53 pm

    rjpjnk wrote:I was playing around with my recently refurbished PAS 3X and I noticed that when I tried driving a subwoofer with it the sound to the main speakers changed. I realize the PAS was not designed to drive low impedance loads common in modern solid state amps, and the consequences of this are likely what I was hearing. This started me thinking about what sort of loading I could reasonably expect the PAS to handle. So basically my next science experiment :)

    I'm not 100% sure how one determines the input impedance of an amplifier, especially without a schematic. The DC component is easy, but I suspect there may be a significant contribution through the first transistor/tube stage as well. As a first test, I connected my LCR meter to the input terminals of the sub and measured the resistive component at 100Hz and 1KHz, and I got 10 KOhms.  I've heard moderm SS amps are this low, so this seems reasonable.

    Next I measured the VTA ST70. It measures about 75 KOhms, which is as expected for the combination of the 270 and 10 K fixed internal resistors and a 100 K stepped attenuator.

    From what I've read about the PAS, they are expecting a 500 K external load so that the total load seen at the end of the bass pot is 50 K (comprised of 62K || 510K || 500K). It seems like the only consequence of not having a 500 K load is that the tone controls will not function properly. So my first question is is this truly the only consequence, or are there other problems with a PAS driving a low impedance load?

    Of course with the 3X model the tone controls are bypassed when centered and at this position the output impedance is not affecting the tone circuits, but I want to consider the case when the tone controls are moved off center as well.

    So the issue I am pondering is this, if I wanted to actually *use* the tone controls on a PAS 3X, what would I need to do as far as circuit modifications?

    Probably it will never be possible to drive a 10 K sub properly, but even driving the VTA70 may require modifications because the stepped attenuator drops the input impedance all the way down to 75 K and this is too low for an unmodified PAS. It is a simple matter to fix this though. We can either remover the stepped attenuator so it is not messing up the load (it is not necessary with the PAS anyway since presumably the PAS is being used to control volume), or we can change the PAS output resistors (62K and 510K) to values that maintain the 50K internal load the PAS wants to see.

    It seems that ideally, to use a PAS with a VTA driver board, one should do both. First remove the 100K stepped attenuator (if present), and then remove the 510K fixed internal resistor from the PAS. This would make remaining internal load almost exactly 50K and keep the PAS happy. Does this sound right?






    To really fix this ; add an external buffer. This will keey all features of the PAS and allow
    10k or lower power amp to be used.
    This has been discusses before here. As i remember one may locate a stereo cathode follower
    aka buffer on ebay for a reasonable sum, to be connected between the PAS en external load.

    A cathode follower _should_ not change the sound in any way. But you need to use
    one with it's own power supply as the PAS power transformer is just sufficient for the PAS itself.

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    Peter W.

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by Peter W. on Fri Aug 24, 2018 12:57 pm

    This was discussed extensively on another thread.

    If you wish to create a permanent partnership between the PAS and an amplifier, it becomes a (minor) exercise in matching the impedance such that the PAS "sees" about 50K.

    Dynaco described the process for a 100K Load (ST-120) as follows:

    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.


    With that in mind and given the 75K on the VTA 70, you will need to calculate the proper resistor in parallel with 75K to get to 50K (150K) and substitute that for the 100K listed in the procedure.

    Go for 5% screened resistors as well.

    You might find this already done if your 3X is well-and-truly a 3X. However, early versions of the 3X were hit-or-miss.

    NOTE:  This matches *THIS* PAS with *THAT* amp. If any other amp is introduced, the resistors will  have to be changed in order to get the 50K preferred match. If the amp has an input impedance too low to allow such a match, then the buffer becomes the best option.
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    peterh

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by peterh on Fri Aug 24, 2018 1:24 pm

    And the simple ones-for-all solution is to add an external buffer, preferably a cathode follower.
    That will drive any poweramp without any changes in the PAS.
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    Peter W.

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by Peter W. on Fri Aug 24, 2018 1:41 pm

    peterh wrote:And the simple ones-for-all solution is to add an external buffer, preferably a cathode follower.
    That will drive any poweramp without any changes in the PAS.

    With tongue thoroughly in cheek - adding equipment is against rjpjnk's received wisdom. Wink
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    peterh

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by peterh on Fri Aug 24, 2018 1:43 pm

    See this thread :
    http://dynacotubeaudio.forumotion.com/t2378p15-connecting-a-cd-recorder-to-my-sp-ph10

    where a named buffer is used to buffer the tape output. It could equally well be used
    to buffer the line output.

    rjpjnk

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by rjpjnk on Fri Aug 24, 2018 2:00 pm

    Definitely adding a cathode follower solves everything, and it's so simple. Too bad the original power supply is insufficient to power one more tube. I suppose I could use another transformer if I want to go that route and keep everything inside the PAS case. Don't think I would want an external box just for the buffer. If I decide I want to run the PAS long term I may end up taking this approach. Then again, it is only needed if I want to drive loads lower than 50K, right? Anything else can be accommodated by adjusting the fixed resistors in the PAS.

    Probably I will start my VTA SP12 build soon, which of course can drive any impedance with no need for an added buffer, and I may start to loose interest in the PAS if the SP12 sounds better.

    So other than the impact on the bass control calibration is there any other undesirable effect of the PAS driving low impedance loads?


    rjpjnk

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by rjpjnk on Fri Aug 24, 2018 2:03 pm

    peterh wrote:See this thread :
    http://dynacotubeaudio.forumotion.com/t2378p15-connecting-a-cd-recorder-to-my-sp-ph10

    where a named buffer is used to buffer the tape output.  It could equally well be used
    to buffer the line output.

    Thanks. I will give it a read.
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    Dave_in_Va

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by Dave_in_Va on Fri Aug 24, 2018 2:11 pm

    I had a similar problem that I brought up in a thread a couple of years ago. I followed PeterH's advice and bought one of these...

    https://www.ebay.com/itm/YAQIN-SD-CD2-Stereo-Audio-Processor-Tube-Buffer/132720795221?epid=1029041475&hash=item1ee6c69e55:g:uA8AAOSwrklVc26d

    It's worked out perfectly (and as I've mentioned before, the tubes that it uses are dirt cheap as NOS).
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    peterh

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by peterh on Fri Aug 24, 2018 2:13 pm

    You might eat your cake and keep it :-)
    See this buffer that fits inside the PAS and may be powered by the PAS ( it's FET but so what ) :
    https://www.ebay.com/itm/Dynaco-PAS-Preamp-Impedance-Matching-Module-New/302849514977?hash=item46833cdde1:g:4a8AAOSwirFa95Pq

    rjpjnk

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by rjpjnk on Fri Aug 24, 2018 2:41 pm

    Interesting. I don't have any experience with FETs. I put it in my watch list for future reference. Thanks.
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    peterh

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by peterh on Fri Aug 24, 2018 2:46 pm

    rjpjnk wrote:Interesting. I don't have any experience with FETs. I put it in my watch list for future reference. Thanks.

    I just ordered one for evaluation.

    As i understand it a "source follower" in FET language is transparent and will not affect sound
    quality. And it is intended to be used for driving a transistor device so who cares ?
    The interesting is that it can be powered by the PAS own power.

    rjpjnk

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by rjpjnk on Fri Aug 24, 2018 2:46 pm

    Dave_in_Va wrote:I had a similar problem that I brought up in a thread a couple of years ago. I followed PeterH's advice and bought one of these...

    https://www.ebay.com/itm/YAQIN-SD-CD2-Stereo-Audio-Processor-Tube-Buffer/132720795221?epid=1029041475&hash=item1ee6c69e55:g:uA8AAOSwrklVc26d

    It's worked out perfectly (and as I've mentioned before, the tubes that it uses are dirt cheap as NOS).

    Yes, I read that just now. Glad it worked out. Did you end up putting the buffer in the tape loop? It sounded like you were leaning that way.

    By comparison, the FET board mentioned by Peterh above looks like it is installed on the line stage out based on the pictures and would not protect the tape out. Though perhaps it could be installed in either position.

    rjpjnk

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by rjpjnk on Fri Aug 24, 2018 2:49 pm

    peterh wrote:
    rjpjnk wrote:Interesting. I don't have any experience with FETs. I put it in my watch list for future reference. Thanks.

    I just ordered one for evaluation.

    As i understand it a "source follower" in FET language is transparent and will not affect sound
    quality. And it is intended to be used for driving a transistor device so who cares ?
    The interesting is that it can be powered by the PAS own power.

    Great. Let us know how you like it.

    What are you driving with it?

    rjpjnk

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by rjpjnk on Fri Aug 24, 2018 3:53 pm

    Maybe old news to you guys, but new to me. An excellent 1994 article by Norman Koren on the subject. He built two cathode followers. One for the phono stage and one for the line stage. Also sounds like he included them inside the feedback loops in order to maximize fidelity. No schematic here though.

    http://www.gunthard-kraus.de/Spice_Model_CD/Mixed%20Part%20List/Tubes/Spice/Norman%20Koren/PAS94.txt
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    peterh

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by peterh on Fri Aug 24, 2018 4:23 pm

    rjpjnk wrote:
    peterh wrote:
    rjpjnk wrote:Interesting. I don't have any experience with FETs. I put it in my watch list for future reference. Thanks.

    I just ordered one for evaluation.

    As i understand it a "source follower" in FET language is transparent and will not affect sound
    quality. And it is intended to be used for driving a transistor device so who cares ?
    The interesting is that it can be powered by the PAS own power.

    Great. Let us know how you like it.

    What are you driving with it?
    PAS3X
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    peterh

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by peterh on Fri Aug 24, 2018 4:33 pm

    rjpjnk wrote:Maybe old news to you guys, but new to me. An excellent 1994 article by Norman Koren on the subject. He built two cathode followers. One for the phono stage and one for the line stage. Also sounds like he included them inside the feedback loops in order to maximize fidelity. No schematic here though.

    http://www.gunthard-kraus.de/Spice_Model_CD/Mixed%20Part%20List/Tubes/Spice/Norman%20Koren/PAS94.txt

    The schematic is known. It's korens "modified" PAS.
    BUT
    it needs a totally new powersupply as it has more tubes to feed. I have not seen any
    boards or kit's based on this.
    I also think some of his claims was criticized by dave gillespie as not valid ( it was a long time ago)
    The koren article is here :
    http://www.normankoren.com/Audio/Spice_preamp.html
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    PeterCapo

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by PeterCapo on Fri Aug 24, 2018 5:15 pm

    peterh wrote:
    rjpjnk wrote:Maybe old news to you guys, but new to me. An excellent 1994 article by Norman Koren on the subject. He built two cathode followers. One for the phono stage and one for the line stage. Also sounds like he included them inside the feedback loops in order to maximize fidelity. No schematic here though.

    http://www.gunthard-kraus.de/Spice_Model_CD/Mixed%20Part%20List/Tubes/Spice/Norman%20Koren/PAS94.txt

    The schematic is known. It's korens "modified" PAS.  
    BUT
    it needs a totally new powersupply as it has more tubes to feed. I have not seen any
    boards or kit's based on this.
    I also think some of his claims was criticized  by dave gillespie as not valid ( it was a long time ago)
    The koren article is here :
    http://www.normankoren.com/Audio/Spice_preamp.html

    I, too, seem to recall something from Dave Gillespie and/or George Ronnenkamp where some things about Koren's work could not be corroborated. Also, the long thread about the original Stereo 70 by Gillespie over at AK, IIRC, might contain some things that either cast doubt upon, or reduce the significance of, the earlier Van Alstine article that has gotten a lot of attention, though I have not read the AK thread in a while so I am not sure I remember clearly. Worth a look, though.

    audiobill

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by audiobill on Fri Aug 24, 2018 5:39 pm

    Why not add an SP 12 or SP14 as a buffer after the PAS3x?

    rjpjnk

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by rjpjnk on Fri Aug 24, 2018 5:53 pm

    peterh wrote:
    rjpjnk wrote:Maybe old news to you guys, but new to me. An excellent 1994 article by Norman Koren on the subject. He built two cathode followers. One for the phono stage and one for the line stage. Also sounds like he included them inside the feedback loops in order to maximize fidelity. No schematic here though.

    http://www.gunthard-kraus.de/Spice_Model_CD/Mixed%20Part%20List/Tubes/Spice/Norman%20Koren/PAS94.txt

    The schematic is known. It's korens "modified" PAS.  
    BUT
    it needs a totally new powersupply as it has more tubes to feed. I have not seen any
    boards or kit's based on this.
    I also think some of his claims was criticized  by dave gillespie as not valid ( it was a long time ago)
    The koren article is here :
    http://www.normankoren.com/Audio/Spice_preamp.html

    Thanks. I definitely trust Dave Gillespie. I built his modified Magnavox 9300 a while back, including the little input buffer (cathode follower) that he added to it to make it resilient to any input connection loading issues.

    If there was criticism of the Koren circuit I imagine it must have concerned the issue of putting the CF inside the feedback loop.

    Peterh: I read a few other posts of yours describing an external buffer you built. What circuit did you use? Are you still happy with that solution?
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    corndog71

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by corndog71 on Fri Aug 24, 2018 5:55 pm

    audiobill wrote:Why not add an SP 12 or SP14 as a buffer after the PAS3x?

    2 preamps! affraid That's just crazy talk. Wink

    audiobill

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by audiobill on Fri Aug 24, 2018 5:57 pm

    Not as crazy as judging preamps with formulae.....

    rjpjnk

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by rjpjnk on Fri Aug 24, 2018 5:58 pm

    peterh wrote:
    rjpjnk wrote:
    peterh wrote:

    I just ordered one for evaluation.

    As i understand it a "source follower" in FET language is transparent and will not affect sound
    quality. And it is intended to be used for driving a transistor device so who cares ?
    The interesting is that it can be powered by the PAS own power.

    Great. Let us know how you like it.

    What are you driving with it?
    PAS3X

    No, I meant what device follows the PAS 3 + FET buff in your system? You mentioned it was a transistor device. Or were you referring to my subwoofer?
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    peterh

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by peterh on Fri Aug 24, 2018 6:24 pm

    rjpjnk wrote:
    peterh wrote:
    rjpjnk wrote:Maybe old news to you guys, but new to me. An excellent 1994 article by Norman Koren on the subject. He built two cathode followers. One for the phono stage and one for the line stage. Also sounds like he included them inside the feedback loops in order to maximize fidelity. No schematic here though.

    http://www.gunthard-kraus.de/Spice_Model_CD/Mixed%20Part%20List/Tubes/Spice/Norman%20Koren/PAS94.txt

    The schematic is known. It's korens "modified" PAS.  
    BUT
    it needs a totally new powersupply as it has more tubes to feed. I have not seen any
    boards or kit's based on this.
    I also think some of his claims was criticized  by dave gillespie as not valid ( it was a long time ago)
    The koren article is here :
    http://www.normankoren.com/Audio/Spice_preamp.html

    Thanks. I definitely trust Dave Gillespie. I built his modified Magnavox 9300 a while back, including the little input buffer (cathode follower) that he added to it to make it resilient to any input connection loading issues.

    If there was criticism of the Koren circuit I imagine it must have concerned the issue of putting the CF inside the feedback loop.

    Peterh: I read a few other posts of yours describing an external buffer you built. What circuit did you use? Are you still happy with that solution?
    It is a very basic 12au7 from merlin blencov's book.
    It's described in http://n.manet.nu/fm1/index.html

    I have an adjustment coming for it, basically it involves moving the motor-pot to before
    the CF, this to get a really low impedance out of it.
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    peterh

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by peterh on Fri Aug 24, 2018 6:30 pm

    rjpjnk wrote:
    peterh wrote:
    rjpjnk wrote:
    peterh wrote:

    I just ordered one for evaluation.

    As i understand it a "source follower" in FET language is transparent and will not affect sound
    quality. And it is intended to be used for driving a transistor device so who cares ?
    The interesting is that it can be powered by the PAS own power.

    Great. Let us know how you like it.

    What are you driving with it?
    PAS3X

    No, I meant what device follows the PAS 3 + FET buff in your system? You mentioned it was a transistor device. Or were you referring to my subwoofer?
    Oh, that was a misunderstanding. I have a vta70, between the pas and vta is my CF (cathode follower).
    The FET device will be tested instead of the CF. So will a 5670-based board from aliexpress.
    My CF is more then CF, it also contains a remote volume and a power sequencer for the vta70 and
    in addition a second CF that buffers tape-out connected to a MAC soundcard.
    it's unlikely that this box will be replaced.

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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

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