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Thread: Adding guitar preamp to the line input

  1. #1
    Join Date
    Mar 2019
    Hoboken, New Jersey, USA

    Adding guitar preamp to the line input

    I have added a guitar input to the Audio Board (rev. D). It's based on TS922 op amp.
    I put a capacitor on the preamp output, 10uF was all I had, just before connecting it to the Line In of the Audio Board, in parallel with the line input jack.

    I works, but when I plug my phone into the line in jack, the guitar volume drops about 30%.
    Do I need to add caps to the line in jack? What capacity should I use for both?

    Also, I'm going to make it all as an SMD board. How do you pick the footprint for all resistors and capacitors?
    On Paul's Audio Board I see them being smaller and bigger. How do you know the size?


  2. #2
    Junior Member
    Join Date
    Oct 2019
    By wiring the guitar input in parallel with the line-in, when both are connected, the output impedance of your phone presents a load to the output of the guitar. Guitars typically want to drive input impedances of about 1M-ohm the output impedance of the phone is a couple of k-ohm. To sum the two inputs you'd really want to use a simple audio mixer to add the guitar output and the phone output and use the output of the mixer to drive the line-in of the audio board.

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