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    Do I need to add a DC blocking cap to VTAs input?

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    dyna70

    Posts: 25
    Join date: 2010-03-30
    Location: Hampshire, England

    Do I need to add a DC blocking cap to VTAs input?

    Post by dyna70 on Mon Apr 19, 2010 6:22 pm

    I have a Little Dot III headphone amp, which apparently can also be used a valve pre-amp. I'd love to partner this with my recently VTA'd Dyna-70.

    As I understand it, the VTA driver board input is DC coupled? The Little Dot III manual says not to use the pre with DC coupled amps.

    So in order to use this I take it I need a cap on the input between phono + and input on the driver board? How big would I need to make this? From a little reading I've done with those using PAS pre-amps it's often recommended to use a 1uF cap, so would this serve me well here?

    With it's lowest gain setting selected, the pre-amp has a gain of x3 (that's probably pushing it with my high gain VTA driver board but should be useable).

    The output impedence of the Little Dot III is quoted at 600 ohms.

    Would the Little Dot III make a reasonable match for the VTA if I add input DC blocking caps?

    Many thanks.

    - John

    Bob Latino
    Admin

    Posts: 1902
    Join date: 2008-11-27
    Location: Massachusetts

    Re: Do I need to add a DC blocking cap to VTAs input?

    Post by Bob Latino on Tue Apr 20, 2010 6:02 am

    dyna70 wrote:I have a Little Dot III headphone amp, which apparently can also be used a valve pre-amp. I'd love to partner this with my recently VTA'd Dyna-70.

    As I understand it, the VTA driver board input is DC coupled? The Little Dot III manual says not to use the pre with DC coupled amps.

    So in order to use this I take it I need a cap on the input between phono + and input on the driver board? How big would I need to make this? From a little reading I've done with those using PAS pre-amps it's often recommended to use a 1uF cap, so would this serve me well here?

    With it's lowest gain setting selected, the pre-amp has a gain of x3 (that's probably pushing it with my high gain VTA driver board but should be useable).

    The output impedence of the Little Dot III is quoted at 600 ohms.

    Would the Little Dot III make a reasonable match for the VTA if I add input DC blocking caps?

    Many thanks.

    - John

    John,

    Every preamp output that I know of has a DC blocking cap at the end of the circuit. The preamp can't send any DC to the input of the ST-70. I don't see the need for an input DC blocking cap on the input of any Dynaco amp. Dynaco did include one on the ST-35 amp though for whatever reason. I routinely bypass these on the ST-35.

    Personally I would just try your Little Dot headphone preamp with the ST-70. It may not be ideal but it will probably work Ok ..

    Bob

    dyna70

    Posts: 25
    Join date: 2010-03-30
    Location: Hampshire, England

    Re: Do I need to add a DC blocking cap to VTAs input?

    Post by dyna70 on Tue Apr 20, 2010 6:22 am

    Understood Bob, it's just that in the manual Little Dot specificaly warn not to use the pre-amp with a DC coupled power amp, so I assume this means the pre has no output blocking cap.

    I'll e-mail them and find out for sure.

    - John

    GP49

    Posts: 489
    Join date: 2009-04-30

    Re: Do I need to add a DC blocking cap to VTAs input?

    Post by GP49 on Tue Apr 20, 2010 9:00 am

    Have you tried using your DVM and measuring the outputs of the Little Dot for DC? If your DVM has selectable ranges, choose the lowest, i.e., most sensitive.

    dyna70

    Posts: 25
    Join date: 2010-03-30
    Location: Hampshire, England

    Re: Do I need to add a DC blocking cap to VTAs input?

    Post by dyna70 on Tue Apr 20, 2010 1:00 pm

    Yeah have tried it - after warm up (with source connected but not playing anything) the DC on the output fluctuates wildly, worse case being 200+ mV.

    Bob Latino
    Admin

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    Re: Do I need to add a DC blocking cap to VTAs input?

    Post by Bob Latino on Tue Apr 20, 2010 1:12 pm

    dyna70 wrote:Yeah have tried it - after warm up (with source connected but not playing anything) the DC on the output fluctuates wildly, worse case being 200+ mV.

    John,

    Probably a blocking cap around .10 uF would get rid of nearly all of the DC. Try different values and measure the amount of DC. Dynaco used a .10 uF @ 16 volts blocking cap on the ST-35. You may not be able to get it down to zero but a couple of millivolts shouldn't hurt anything ..

    Bob

    dyna70

    Posts: 25
    Join date: 2010-03-30
    Location: Hampshire, England

    Re: Do I need to add a DC blocking cap to VTAs input?

    Post by dyna70 on Tue Apr 20, 2010 1:19 pm

    Thanks Bob - I have quite a few spare 0.1uFs of varying quality so will try some. Will 0.1uF curtail bass response in any way, or should I purchase a pair of 1uF (or higher?) types?

    Thanks.

    Bob Latino
    Admin

    Posts: 1902
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    Re: Do I need to add a DC blocking cap to VTAs input?

    Post by Bob Latino on Tue Apr 20, 2010 1:24 pm

    dyna70 wrote:Thanks Bob - I have quite a few spare 0.1uFs of varying quality so will try some. Will 0.1uF curtail bass response in any way, or should I purchase a pair of 1uF (or higher?) types?

    Thanks.

    John,

    I don't believe it will affect the bass at all. Again, Dynaco used a .10 on the ST-35. I REMOVED the .10 @ 16 volt DC INPUT blocking cap on my ST-35 and I did not notice any affect on the bass.

    Bob

    dyna70

    Posts: 25
    Join date: 2010-03-30
    Location: Hampshire, England

    Re: Do I need to add a DC blocking cap to VTAs input?

    Post by dyna70 on Sat Apr 24, 2010 10:15 am

    Remembered I had some 4.7uf Black Gate N caps going spare so opened up the Little Dot III and inserted those on each output phono.

    Fired it up 10minutes ago and the Little Dot III/VTA ST70 sound fantastic together! Didn't have to touch the ST70 at all in the end.

    Very pleased!

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