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

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    solderblob

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

    Post by solderblob on Mon Aug 27, 2018 6:05 pm

    rjpjnk wrote:

    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?

    This puzzles me a bit too.  Power amps don't have volume controls.  A VTA (or any) power amp with a volume control wouldn't seem appropriate or needed when using a preamp since preamps have volume controls.  I believe the intent of the VTA amp with attenuator is to run it straight off a source such as a CD player.  Maybe a better solution for the VTA amp would be to have separate "preamp" input which bypasses the attenuator.

    Rule of thumb is power amp input Z is a minimum of 10x the preamp's output Z.  Isn't the PAS3x output Z 1k Ohm as per specs I've seen?  I don't understand the 510k Ohm you mentioned?

    Anyway, it sounds like you've solved your situation...

    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 Mon Aug 27, 2018 6:27 pm

    Yes, the stepped attenuator is an option to allow the VTA70 to be used directly with line level input sources without a preamp. Once a preamp is connected this attenuator becomes redundant. Though curiously I don't believe I have read any guidance suggesting its removal.

    If the preamp in question is a PAS, this 100K stepped attenuator actually becomes a problem. This is because the PAS was designed for a 500K+ load not a 75K load. It is a simple matter to fix this by changing the fixed resistors in the PAS and/or removing the stepped attenuator.

    The bottom line is that for the PAS tone controls to work properly the PAS needs to see a 51.7K total load including its internal resistors in parallel with any power amp connection.

    The PAS internal load consists of two resistors in parallel, 62K and 510K. This means the ideal power amp load is around 750K, but 500K is typically quoted as sufficient.
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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 Mon Aug 27, 2018 6:30 pm

    I'd have to have another look at the article, but I think the limits might be different for the X tone controls.

    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 Mon Aug 27, 2018 8:15 pm

    PeterCapo wrote:I'd have to have another look at the article, but I think the limits might be different for the X tone controls.

    That would be good info to included here, thanks.

    My understanding is the main difference with the very unique X tone control pots is that they bypass themselves when centered. (The Bass by shorting both legs of the pot, and the Treble by lifting the center wiper.)

    To work properly, the tone controls need to see a balanced load on each side, (51.7K is used in the NFB path, so 51.7K is needed in the output load)

    When in this special bypass position, there is no need to balance the output load with the NFB path, and the frequency response should be flat for any reasonable output load.  

    However, if the X tone controls are moved off center the need for load balancing comes back into play and they function no better than the non X version.

    This is my current understanding anyway, but I have only been comparing PAS 3 vs 3X so there may be some important details with other PAS I am not aware of.

    Does this sound correct?
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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 Mon Aug 27, 2018 8:27 pm

    Offhand, I think this sounds correct, but, again, I'd want to have a refresher reading of the article.

    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 Mon Aug 27, 2018 9:05 pm

    Sounds good. I just spent some time reading over the section comparing PAS 2/3 vs 3X, and the special X controls operate a little differently than I thought. Because they have two wipers, and one wiper is on a conductive tract and the other is on a resistive track, the response off center will not be exactly the same as the non-X version.

    For the Bass control anyway, it looks like X and non-X will behave similarly for cuts, but for boosts the X version will continue to be insensitive to output load over the entire boost region. I didn't study the treble control part.

    Those are some seriously special purpose pots!

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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 Mon Aug 27, 2018 9:23 pm

    Dynaco innovation, I suppose?

    rjpjnk

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    PAS 3X Line Stage Schematic

    Post by rjpjnk on Mon Aug 27, 2018 11:30 pm

    For reference in our discussion.

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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 Tue Aug 28, 2018 10:02 am

    PeterCapo wrote:Dynaco innovation, I suppose?

    Yes, and continued in most of their later solid-state devices.

    https://www.vintageshifi.com/repertoire-pdf/Dynaco.php

    Manuals here.

    The PAT-5 & 5 biFet had a tone-control bypass switch, and so used conventional pots.

    The PAT4, 80, and 80Q used the specialized pots, however.

    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 Tue Aug 28, 2018 6:26 pm

    PAS 3x went poof last night. Just a pop and then slowly decreasing sound. Haven't taken the lid off yet Sad
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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 Tue Aug 28, 2018 6:39 pm

    Sorry to hear that.  Maybe whatever was going on might be related to some of the sonic degradation you were hearing.  But, I think driving a really low impedance load with the PAS is just going to need a buffer of some kind for the best sonics.

    Did you have a fuse installed?  Google "Radio Shack Catalog # 2700219" Also available on Amazon http://www.amazon.com/1st-Source-In-line-Fuse-Holder/dp/B0002KR8EE
    I use a fuse holder that looks a lot like that one, and it fits snugly between the deck and the bottom plate. Mouser might also have it, but I couldn’t find it there.
    For the fuse, try a 1/4A and see it it holds.  If not, try a 1/2 amp.
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    Bob Latino
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    Re: Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

    Post by Bob Latino on Tue Aug 28, 2018 6:56 pm

    solderblob wrote:

    This puzzles me a bit too.  Power amps don't have volume controls.  A VTA (or any) power amp with a volume control wouldn't seem appropriate or needed when using a preamp since preamps have volume controls.  I believe the intent of the VTA amp with attenuator is to run it straight off a source such as a CD player.  Maybe a better solution for the VTA amp would be to have separate "preamp" input which bypasses the attenuator.


    You do not need a separate "preamp input" on the amp. The VTA stepped attenuator goes between the input jack and the "IN" eyelet on the driver board. If you turn the stepped attenuator all the way to the right, it is basically out of the circuit. (it has now been bypassed) All you have is one extra solder connection between the input jack center pin to the driver board IN eyelet. If you measure the resistance from the input center pin to the driver board IN with the attenuator turned all the way to the right, you will get 0.0 ohms. I have had some customers who use an attenuator later buy a preamp and use the attenuator as a "master volume control". One customer bought an older '90's Conrad Johnson preamp. He liked the preamp but told me it had too much gain. This preamp would give nearly all its gain in the first 1/3 turn of the control. At about 10 o'clock on the volume control, the sound was too loud. What he did was set the attenuator at about 1 o'clock instead of turning it all the way to the right to use with his preamp. Now the sound did not start to get loud with the preamp until 12 o'clock on the volume control and maximum volume was reached at 2 or 3 o'clock.

    Bob
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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 Tue Aug 28, 2018 7:20 pm

    rjpjnk wrote:PAS 3x went poof last night.  Just a pop and then slowly decreasing sound. Haven't taken the lid off yet :(
    The good thing is that all parts are avalable,
    The bad thing is that they may cost although none of the parts will cost arm or leg.

    In addition you will have a crash course of the PAS innards.

    Is the power indicator lamp still lit ? Thats a good sign. If not you might need a new transformer
    as the PAS had no fuse. Whatever happens the transformer is in danger. Dynakitparts has them.


    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 Tue Aug 28, 2018 8:19 pm

    I'm not worried. Probably something minor. I recently put one of these power supplies in it

    http://www.erhard-audio.com/Power_Supply_Kits.html

    It's a great kit, but the two-tube filament voltage under load was about 25V so I added a series resistor to get it down to the Dynaco speced level of 22V. In a pinch, I used a resistor that was just barely suitable for the power draw and I'm guessing it burnt up. Hopefully it's that simple. I have some higher wattage ones now. May take a look tonight.

    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 Tue Aug 28, 2018 8:41 pm

    Bob Latino wrote:
    solderblob wrote:

    This puzzles me a bit too.  Power amps don't have volume controls.  A VTA (or any) power amp with a volume control wouldn't seem appropriate or needed when using a preamp since preamps have volume controls.  I believe the intent of the VTA amp with attenuator is to run it straight off a source such as a CD player.  Maybe a better solution for the VTA amp would be to have separate "preamp" input which bypasses the attenuator.


    You do not need a separate "preamp input" on the amp. The VTA stepped attenuator goes between the input jack and the "IN" eyelet on the driver board. If you turn the stepped attenuator all the way to the right, it is basically out of the circuit. (it has now been bypassed) All you have is one extra solder connection between the input jack center pin to the driver board IN eyelet. If you measure the resistance from the input center pin to the driver board IN with the attenuator turned all the way to the right, you will get 0.0 ohms.
    Bob

    Bob, this is correct of course, but the issue we are examining here is the VTA driver input impedance, and specifically how it loads a PAS preamp. True, the stepped attenuator adds no additional series load at full clockwise position, but it still adds a 100K shunt across the input reducing the VTA input impedance from 280K to 73K. If one were to use a PAS preamp with the stepped attenuator in place would it not be best to adjust the load resistors in the PAS to match this 73K input impedance so the tone controls work properly?

    Alternatively, if one would rather not modify the PAS, then removing the stepped attenuator would eliminate unnecessary load and seems like an important step in the right direction. Do you agree?


    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 Wed Aug 29, 2018 12:18 am

    PAS is up and running again. The 12X4 filament burnt out.
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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 Wed Aug 29, 2018 2:50 am

    rjpjnk wrote:PAS is up and running again. The 12X4 filament burnt out.

    That is exotic and unusual. You mentioned that the filament was a bit high, the 12x4
    is powered by 12 V, whats the voltage supplied to this ? And is it stable ( no peaks
    during power up ?)
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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 Wed Aug 29, 2018 6:57 am

    CL-140 thermistor suggested. I'd recommend monitoring AC mains level as often as possible for a couple of weeks to get an idea of where it runs.

    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 Wed Aug 29, 2018 11:27 am

    peterh wrote:
    rjpjnk wrote:PAS is up and running again. The 12X4 filament burnt out.

    That is exotic and unusual.   You mentioned that the filament was a bit high, the 12x4
    is powered by 12 V, whats the voltage supplied to this ? And is it stable ( no peaks
    during power up ?)

    Is it really unusual?  Sometimes they just burn out, right? I don't know the history of this tube as I just bought it used a couple weeks ago in anticipation of the SP12 project. It was a Tungsol and tested well. I put the original back in last night.

    The 12x4 filament runs from a simple 12V AC filament winding on the transformer. Not sure how that could possibly get messed up.

    I measured the voltage on pins 3&4 of the socket and it was 11 something without the tube installed. I would have guessed it would be higher.

    Mains typically measure 117-119.

    EDIT: Regarding my previous mention of the filament voltage being too high. I was referring to the 12AX7 filaments, and this is too be expected using the power supply board listed above. This board uses SS diodes, so the internal resistance of the selenium rectifier is no longer in the circuit. A 1 Ohm resistor inline seemed to be a good fix for this.


    Last edited by rjpjnk on Wed Aug 29, 2018 11:43 am; edited 2 times in total

    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 Wed Aug 29, 2018 11:28 am

    PeterCapo wrote:CL-140 thermistor suggested.  I'd recommend monitoring AC mains level as often as possible for a couple of weeks to get an idea of where it runs.

    Why a thermistor?
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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 Wed Aug 29, 2018 12:34 pm

    Why a thermistor?

    Soft inrush. Cold filaments have very low resistance.
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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 Wed Aug 29, 2018 12:52 pm

    Also, if AC mains is on the high side, a thermistor can serve to drop it a bit.

    rjpjnk

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    PAS output buffer

    Post by rjpjnk on Fri Aug 31, 2018 9:32 pm

    peterh wrote: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

    Peterh, Did you try this buffer yet? I found another one here that looks to be the same circuit.

    https://www.ebay.com/itm/Dynaco-PAS-Preamp-Impedance-Matching-Module-Buffer-New/292704780919

    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 31, 2018 9:45 pm

    Inappropriate to be promoting/marketing cheap products on Bob and Roy’s site. If you want a buffer, contact Roy.Mottram. This forum isn’t intended to be a place for that sort of thing.
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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 31, 2018 9:52 pm

    Link to webpage featuring Roy's buffer?

    Sponsored content

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

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