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Thread: XLR DI on valve amp, capacitor value

  1. #1
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    XLR DI on valve amp, capacitor value

    Hi
    I copied this circuit for the DI output section for another Fender amp.
    The DI transformer is on the top right corner of the page of page 6.
    My question is how do I determine the right capacitor value on the input prim of the transformer. In the attached schematic they used a 0,1uF.
    When I connect the cap with my chosen transformer, an OEP A262A3E, I get a slight boost of the higher frequencies.
    Attached Files Attached Files

  2. #2
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    The cap forms a resonant circuit as it's in parallel with the inductance of the transformer. I'm wondering what the original reason was for including it in the Mesa design. Unless you know the inductance of the transformer, you'd need to arrive at the 'right' value cap by trial and error. Smaller=higher peak frequency. For me, I'd test the circuit without it.
    Last edited by Mick Bailey; 04-04-2017 at 06:42 PM.
    J M Fahey likes this.

  3. #3
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    The cap forms a resonant circuit as it's in parallel to the inductance of the transformer. I'm wondering what the original reason was for including it in the Mesa design. Unless you know the inductance of the transformer, you'd need to arrive at the 'right' value cap by trial and error. Smaller=higher peak frequency. For me, I'd test the circuit without it.
    What Mick said ^^^. And, to some degree, you can tune your amp's DI output response to match what you want to hear. Often they're terribly bright sounding. With a transformer and parallel cap, there's the possibility you'll wind up with a tone that sounds like a stuck wah-wah. As a mixer person I'm never sure what's going to come down the DI line unless it's a Countryman 85 direct from the instrument. DI from an amp? Sometimes it sounds great, sometimes not so much.
    J M Fahey likes this.

  4. #4
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    Quote Originally Posted by diydidi View Post
    ...When I connect the cap with my chosen transformer, an OEP A262A3E, I get a slight boost of the higher frequencies.
    Is it a 'perceived', or measured, treble boost?

  5. #5
    Old Timer J M Fahey's Avatar
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    I bet its a resonant peak.
    Personally Id do without the cap, and, IF neccessary, add treble at the mixing desk.
    Juan Manuel Fahey

  6. #6
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    Thanks for all the replies.
    I decided to keep the cap idea, but made it as small as possible so as to not have an effect on the response.

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