Passive Guitar Pickup Output vs. Soundcard

Discussion in 'The Projects Forum' started by GregJ7, Oct 5, 2015.

  1. GregJ7

    Thread Starter New Member

    Jun 7, 2014
    As part of my effort to learn about transistors I want to be able to use a simple transistor circuit to produce a desired simple waveform in LTSpice IV. To keep me focused, I am working toward designing a single transistor NPN common collector preamp to sit between my electric guitar and my sound card (or perhaps more accurately, to understand the obvious design well enough that I could design it).

    Without a preamp, I can't hear anything out of my speakers from my guitar with everything turned up to max. Unfortunately, it appears to be impossible to get exact specifications from the sound card manufacturer (ASUS/Xonar DG). Initially I assumed that the voltage generated by the passive pickup needed to be amplified. However, recently I stumbled across some info that made me think that my guitar already provides enough voltage for the sound card, so now I am wondering if I need to be amplifying the current (whatever that means). Normal strumming results in about 50 to 200 mV, with extreme strumming topping out around 750 mV. However, my digital multimeter shows 0.00 mA, possibly because it won't measure anything under 50 µA. Does anyone have any idea what I actually need the preamp to do?
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Your preamp needs to have a high 'input impedance' (you can google that), otherwise it will load the guitar pickup down and could even attenuate the signal rather than amplify it. It is unlikely to need much voltage gain, so it's function is mainly as a buffer between the pickup and your sound-card input. A single common-collector transistor stage is not ideal, because the base bias resistors add to the loading on the pickup. Have you considered using a junction FET?
  3. Veracohr

    Well-Known Member

    Jan 3, 2011
    A guitar pickup can have up to 10k ohm output impedance (it varies by frequency), so it needs a high input impedance. If you're running into a line level input on your soundcard it may only have 10k-20k input impedance. a mic input may have even less. Many guitar circuits have in the area of 500k-1M ohm input impedance.

    Also, a common collecter circuit isn't really a "preamp", it's a buffer. Useful for your purposes, but not a preamp.