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    coupling cap on SP 14

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    bscp

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    Post by bscp on Sat Feb 23, 2019 4:08 pm

    Hi!  In a few weeks, I plan on purchasing a VTA SP 14. I have an AVA Fet Valve 400R amplifier. The 400r has an input impedance of 1M ohms. I need to know the perfect coupling size cap for optimal performance with the SP 14. Any help would be very much appreciated.

    1M ohm = 1000k if that helps Very Happy


    The 400R runs class A up to about 20W/channel.

    Rated power is 225 W/Channel into 8 ohms, more into 4 ohms.
    The voltage gain of AVA power amps is about 25
    It takes 2V to drive the 400R to full power
    The amps also have a very high input impedance so they are easy loads for a preamp to drive.
    The input impedance is 1 megohm, it's a triode input


    according to Frank


    Last edited by bscp on Sun Feb 24, 2019 10:46 pm; edited 1 time in total (Reason for editing : explanantion)
    deepee99
    deepee99

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    Post by deepee99 on Sun Mar 03, 2019 6:37 pm

    Consider two sets of output RCA jacks on the back of your SP-14; easily done if you're buying it new. Then you can install coupling caps of different flavours, in my case one set of caps for s/s/ amps (20k ohm Bedinis -- low even for solid-state gear) and the other of a higher value for tube amps.
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    bscp

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    Post by bscp on Sun Mar 03, 2019 6:44 pm

    Thanks for the response. I ordered a couple of sizes of some basic mkp's to get an idea of what I need. The 400r was designed to be an easy load for any preamp, but I am a little worried about getting a full bass response with anything under 2 or 3uf.
    deepee99
    deepee99

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    Post by deepee99 on Sun Mar 03, 2019 7:08 pm

    bscp wrote:Thanks for the response. I ordered a couple of sizes of some basic mkp's to get an idea of what I need. The 400r was designed to be an easy load for any preamp, but I am a little worried about getting a full bass response with anything under 2 or 3uf.
    That's why I suggest two outputs of different values, so you don't lose any bass hooking up to an s/s amp should you choose.
    I hear that Van Alstine is a sweet-heart of an amp.
    Interestingly, I have an Aqua La Scala hybrid DAC that integrates FETs and 12AT7 tubes in a proprietary ladder configuration. In other words, the tubes are part of the main circuitry -- not just for show as buffers.
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    bscp

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    Post by bscp on Sun Mar 03, 2019 7:15 pm

    The AVA amp is nice! I like to use Gold Lion 12AT7 tubes. I have some Mesa Boogie's to try.
    I think I will try at least a .47uf cap in one output and a 3uf in the other to start.
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    bscp

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    Post by bscp on Sun Mar 03, 2019 7:58 pm

    Does anyone know if these coupling caps have to be at least 400Vdc, or can a 200Vdc be used, such as Arizona Capacitor 3.3uF 200Vdc Blue Cactus Series? I know that Audiocap Theta caps have been used, but the larger 1uf and up seem to be acceptable.
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    mijohn

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    Post by mijohn on Sun Mar 03, 2019 9:09 pm

    bscp wrote:Does anyone know if these coupling caps have to be at least 400Vdc, or can a 200Vdc be used, such as Arizona Capacitor 3.3uF 200Vdc Blue Cactus Series? I know that Audiocap Theta caps have been used, but the larger 1uf and up seem to be acceptable.
    They must be at least 400Vdc.
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    bscp

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    Post by bscp on Sun Mar 03, 2019 10:08 pm

    Thanks!
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    bscp

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    Post by bscp on Mon Mar 04, 2019 3:17 pm

    I have another question. Why would you ground all the rca's together when this is a dual mono design?
    Wouldn't it make more sense to keep those left and right signals separated to maintain the dual mono design?
    tubes4hifi
    tubes4hifi
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    Post by tubes4hifi on Wed Mar 06, 2019 5:34 pm

    a 0.22uF capacitor should be sufficient to driver any ampifier with an input impedance of greater than 100K.
    Since your AVA has an input impedance 10X that high, you COULD actually get by with a tiny 0.022uF capacitor !
    On the other hand, if you connect an amplifier with an input impedance of 10K (a few solid-state amps MAY be that low)
    then you'd want to use a 2.2uF capacitor.
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    bscp

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    Post by bscp on Thu Mar 07, 2019 12:25 am

    Thank you so much!!

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