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    Best way to wire an ST70 with Input Attenuator and Input Modifications

    mrb5217
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    mrb5217


    Posts : 2
    Join date : 2022-04-19
    • Post n°1

    Best way to wire an ST70 with Input Attenuator and Input Modifications Empty Best way to wire an ST70 with Input Attenuator and Input Modifications

    Post by mrb5217 Tue Apr 19, 2022 9:11 pm

    I received my 100K input attenuator today from tubes4hifi and I'm wondering what the best way to combine it with the DC blocking / bandwidth filter modifications from Van Alstein would be, or if that is even possible.  I'm not allowed to post a direct link, so you'll have to copy/paste. http www enjoythemusic com/magazine/manufacture/0902/index.html


    It seems there are really 3 options:

    1. Put the 100K Attenuator immediately after the RCA jack and before the DC blocking/band-pass circuitry
    2. Put the 100K Attenuator after the DC blocking/band-pass circuitry
    3. Don't use a 100K attenuator with the Band-Pass circuitry as the extra (variable) resistance will screw up the range of frequencies being filtered out by shifting around the R values in the RC filters.

    I don't understand the electronics well enough to really understand how this will impact things, hopefully someone more knowledgeable than me can help me understand.

    Thanks,
    Matt
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    Bob Latino
    Bob Latino
    Bob Latino
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    Location : Massachusetts
    • Post n°2

    Best way to wire an ST70 with Input Attenuator and Input Modifications Empty Re: Best way to wire an ST70 with Input Attenuator and Input Modifications

    Post by Bob Latino Wed Apr 20, 2022 5:17 pm

    mrb5217 wrote:I received my 100K input attenuator today from tubes4hifi and I'm wondering what the best way to combine it with the DC blocking / bandwidth filter modifications from Van Alstein would be, or if that is even possible.  I'm not allowed to post a direct link, so you'll have to copy/paste. http www enjoythemusic com/magazine/manufacture/0902/index.html


    It seems there are really 3 options:

    1. Put the 100K Attenuator immediately after the RCA jack and before the DC blocking/band-pass circuitry
    2. Put the 100K Attenuator after the DC blocking/band-pass circuitry
    3. Don't use a 100K attenuator with the Band-Pass circuitry as the extra (variable) resistance will screw up the range of frequencies being filtered out by shifting around the R values in the RC filters.

    I don't understand the electronics well enough to really understand how this will impact things, hopefully someone more knowledgeable than me can help me understand.

    Thanks,
    Matt

    Matt,

    Just my opinion here but > I don't agree with Mr. Van Alstine on the need of some form of DC blocking using some type of resistor/capacitor combination on the input stage of any VTA or ST-70 amp. All modern preamps that you might use your amp will have some form of DC blocking on their output stage to prevent any DC from reaching the input jacks on your amp. Placing some type of a resistor/capacitor DC blocking filter on the input stage of an amp WILL alter the frequency response of the amp.

    The VTA stepped attenuator may be used with or without a preamp. If you have the VTA stepped attenuator on your ST-70, ST-120 with a preamp, the best advice is to turn the attenuator all the way clockwise and use the preamp's volume control. Turning the stepped attenuator all the way to the right will give a complete unaltered pass through of the audio signal from the input jacks to the driver board input eyelets.

    Bob
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    mrb5217
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    mrb5217


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    • Post n°3

    Best way to wire an ST70 with Input Attenuator and Input Modifications Empty Re: Best way to wire an ST70 with Input Attenuator and Input Modifications

    Post by mrb5217 Wed Apr 20, 2022 9:10 pm

    Hi Bob,

    Thanks for taking the time to reply to my question.

    I'm planning to use a Yamaha C4 as a preamp with my ST70, so I'm not sure if that qualifies as "modern", but a look at the schematic shows that it does have some DC blocking capacitors on the output, so I guess it's not necesaary on the amp in my case.

    Do you think there is anything to be gained from the input bandwidth limiting filters on the input?
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    Bob Latino
    Bob Latino
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    • Post n°4

    Best way to wire an ST70 with Input Attenuator and Input Modifications Empty Re: Best way to wire an ST70 with Input Attenuator and Input Modifications

    Post by Bob Latino Thu Apr 21, 2022 8:30 am

    The "theory" behind bandwidth limiting is that you remove frequencies going into the amp on the input stage that maybe the output transformer can't reproduce properly especially at higher volume levels. Years ago, I once placed a bandwidth limiting filter on a stock Dynaco ST-70 and the amp sounded just about the same to me. I didn't really hear any difference in the sound.

    Bob

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