Preamp for soundcard drawn in LTspice IV giving me unexpected graph results

Discussion in 'The Projects Forum' started by GregJ7, Aug 28, 2015.

  1. GregJ7

    Thread Starter New Member

    Jun 7, 2014
    18
    2
    I am planning on building the Preamp for Soundcard shown at http://www.learningelectronics.net/circuits/preamplifier-for-soundcard.html (parts ordered) to use with my Xonar DGX. I want to be able to plug an electric guitar into my sound card as well as use spectrum analysis software. (I know there are products available that already do this.) I drew the schematic in LTspice IV and am not getting the output I was expecting. I'm newb enough that the problem could be anything, including my expectations for the output. I am expecting the output wave to look the same as the input wave, except with a greater amplitude, and no added bias.

    I did some new things with LTspice that I haven't done before:
    (1) I got .MODELs off the internet of the two transistors used.
    (2) The default capacitor it draws is not polar, so I randomly selected an electrolytic from the ones included with LTspice by default, then changed the capacitance value.

    (I also got a model and .asy for a potentiometer off the Internet, but got an untraceable error, so I deleted it and put in two resistors instead.)

    I triple-checked the schematic, the parameters for my voltage sources, and the simulation parameters, but haven't seen any errors. Can anyone suggest what I might have missed about getting this model into LTspice correctly or something I haven't realized about the circuit design? I have attached snapshots of the schematic, the input+output graph, and the LTspice file itself.

    Soundcard preamp LTspice input-output graph.jpg Soundcard Preamp schematic in LTspice snapshot.jpg
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Then you need to quadruple check your schematic. :rolleyes:
    Errors I found:
    5V should be connected to the line labeled Signal_Out, not the junction of R9 and R8 labeled From _USB (which is actually the output).
    R6 and R1 are not connected to Q1's base as they should be (notice the lack of a connecting dot).
    The input signal is too large. Reduce it to 10mV.
    C1 polarity is incorrect. Reverse it.

    Note that the output will have a DC bias of about 2.5V. Too eliminate that you can run the signal through a series DC blocking capacitor, its value determined by the output load resistance and how low a frequency response you want, the low frequency -3dB point being 1 / (2*pi*R*C).
     
    Last edited: Aug 28, 2015
  3. GregJ7

    Thread Starter New Member

    Jun 7, 2014
    18
    2
    Thank you very much for looking at this in such detail! Obviously, I needed a second set of eyes on it. Your comment that my input signal was too large sent me down an investigative path (which I am still on). It looks like this might not be the right circuit for my needs. The phrase in the schematic description that says the circuit is for "inductive pick-up elements" comforted me too much. Guitar pickups fall into that category, but my guitar output is typically in the 200mV to 400mV (and 700mV+ for some guitar pickups).

    Various web pages make it appear that max typical sound card input should be no more than 0.7 to 1.2V. (Why the sound card manufacturers don't tell us this spec is a mystery to me.) One problem is, why does this circuit design amplify more than that? Another is that my guitar, when plugged into the sound card directly, is too quiet to hear. But why? Given my understanding of the specs, the guitar already provides about the right voltage for the sound card.

    Also I have seen that a DC bias is sometimes used to reduce clipping, but I don't know whether it is desirable or not to remove it from the output. (And this circuit doesn't seem to clip at acceptable voltage levels for my purposes.) Now I wonder what an audio signal I am familiar with sounds like with an added DC bias, as well as how a circuit can be modified to modify clipping behavior. Ask one question and it creates 10 new ones! Gah! Back to drawing board.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    The writeup for that circuit mentions dynamic microphones or tape recorder heads as sources and these apparently only output millivolts of signal so the circuit has a gain of about 400 to accommodate that.
    You could reduce its gain to operate with your guitar pickup, but I would suggest you consider a simple op amp circuit to give the needed gain.
    A low noise op amp such as the MC34071 will operate down to a 3V supply but must be biased to use with a single supply as shown here.
     
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