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    Dynaco PAS 3 510k output resistor / impedance

    Midwestside
    Midwestside

    Posts : 71
    Join date : 2019-03-20

    Dynaco PAS 3 510k output resistor / impedance Empty Dynaco PAS 3 510k output resistor / impedance

    Post by Midwestside on Wed May 27, 2020 10:25 pm

    PeterW wrote: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.


    I sort of pulled this out of another thread and created my own thread before i really knew what i was trying ask.
    I had been  wondering if the 510k ohm resistors on the outputs of my PAS 3  were 'holding it back' paired with my vta st 70, being that this was intended for the original ST 70.
    I was really curious and just snipped both connections.
    Wow! what an improvement.  
    Im still learning, so forgive me for sort of duplicating a thread in a way.

    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.

    With this above statement in mind, having exactly this combination of amp / pre.  im a little confused by the  'to get 50k (150k)" part.
    I dont seem to see 25kohm resistors, so essentially i need to build like 24k + 1k = 25kohm x 2  (one per output) to get 50k ohms total?

    Mods feel free to merge this new thread back into the thread from which i quoted.
    peterh
    peterh

    Posts : 1440
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Dynaco PAS 3 510k output resistor / impedance Empty Re: Dynaco PAS 3 510k output resistor / impedance

    Post by peterh on Thu May 28, 2020 3:52 am

    Where is the figure "75K on the VTA70" comes from ?
    Mine has 280k ( 270k + 10k) as input.

    Said that; an unmodified Dynaco PAS(X) will drive an unmodified VTA70 without problem.
    Midwestside
    Midwestside

    Posts : 71
    Join date : 2019-03-20

    Dynaco PAS 3 510k output resistor / impedance Empty Re: Dynaco PAS 3 510k output resistor / impedance

    Post by Midwestside on Thu May 28, 2020 8:44 am

    peterh wrote:Where is the figure "75K on the VTA70" comes from ?
    Mine has 280k ( 270k + 10k) as input.

    Said that; an unmodified Dynaco PAS(X) will drive an unmodified VTA70 without problem.

    Not sure, i just plucked it from Peter W's quote there in my post.
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    rjpjnk

    Posts : 264
    Join date : 2018-07-18

    Dynaco PAS 3 510k output resistor / impedance Empty Re: Dynaco PAS 3 510k output resistor / impedance

    Post by rjpjnk on Sat May 30, 2020 12:38 am

    peterh wrote:Where is the figure "75K on the VTA70" comes from ?
    Mine has 280k ( 270k + 10k) as input.

    Said that; an unmodified Dynaco PAS(X) will drive an unmodified VTA70 without problem.

    If the ST70 has a volume pot installed like on the VTA models, (Bob uses 100K) this pot will make the input impedance around 75K. I assume that is where this number is coming from. To us a PAS with a VTA amp with the optional volume control you need to change the output resistors in the PAS for ideal match.

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    Dynaco PAS 3 510k output resistor / impedance Empty Re: Dynaco PAS 3 510k output resistor / impedance

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