Guitar Amp Prototype

Discussion in 'The Projects Forum' started by tracecom, Oct 9, 2012.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    In another thread ( http://forum.allaboutcircuits.com/showthread.php?t=74828 ) I said I was going to try to build a guitar amp using a TDA2005R. Tonight, I finished the perfboard prototype, and it works. As you can see from the photo, I don't yet have a permanent connection for the input, and am currently using a (white) cliplead. This is because I haven't yet built the pre-amp stage. (I am planning an MPF102 circuit with gain and tone control.)

    For test purposes, I am using my MP3 player as an input source, and must turn the drive from the MP3 player way down in order to avoid really loud audio from the speaker. In fact, the gain of the amp is so high that I am not sure if I really need the preamp. I am also building a cigar box guitar, but it's not far enough along to connect to the amp. I hope to finish it within a week or two.

    Anyway, here's the circuit (from the ST datasheet) and the prototype as it currently is built.
     
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  2. MrChips

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    Oct 2, 2009
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    Good for you. You are further along than I am. I am now acquiring the cabinet parts. I have 8 corners and an 8-inch speaker grill. I just ordered a yard of speaker fabric. I still cannot find a handle. I will probably cut out the boards this weekend.
     
  3. #12

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  4. tracecom

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    Attached is the schematic for the TDA2005 guitar amp with an MPF102 preamp, gain control (VR1), and tone control (VR2) added.

    If you are interested, please look it over and post your comments and corrections. Note that everything to the right of VR2 is straight from the TDA2005 ST datasheet. I plan to use this amp with a piezo pickup in the guitar.

    Thanks for your help.
     
  5. Audioguru

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    Dec 20, 2007
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    I don't know if a piezo vibration sensor needs a very high 1M load from the Jfet preamp.

    The MPF102 Jfet is not suitable because it has a very wide range of very high IDSS current from 2mA to 20mA. A J201 Jfet should be used instead because its range is much less from 0.2mA to 1mA.

    C12 is only 0.047uf and is feeding the 10k volume control so it cuts low frequencies below 340Hz worse than a telephone. C12 should be 0.47uF for a cutoff frequency of 34Hz.
     
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  6. tracecom

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    I have ordered some J201 JFETs; will the circuit need to be changed to accommodate this transistor?

    Thanks.
     
  7. Audioguru

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    A J201 Jfet will work well in that circuit. Some MPF102 Jfets won't work and some will work poorly in that circuit unless you match the source resistor value to the Jfet IDSS.
     
  8. tracecom

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    Is that the procedure described here?
    http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp5.htm

    I actually tested my 10 pieces of MPF102 according to that procedure. The results are attached.
     
    Last edited: Oct 10, 2012
  9. Audioguru

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    Are you going to use the circuit with the added drain resistor and adjust the value of the source resistor to match the Jfet? It has voltage gain when the output is from the drain but you said you do not want gain. If the output is from the source then the added drain resistor will reduce the amount of output swing.

    Your original circuit does not have a drain resistor. It is a source-follower with no voltage gain. You can adjust the value of the source resistor so that the source voltage is half the power supply voltage.
     
  10. tracecom

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    As usual, I am confused. I was thinking that if I am going to change to a J201, should I just use the Tillman preamp design?

    The drawing below is supposed to be the Tillman design optimized for the MPF102. What do you think of it?
     
    Last edited: Oct 10, 2012
  11. Audioguru

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    Use one of your MPF102 Jfets with a 1M resistor to ground at the gate and a resistor to ground at the source. Connect two zener diodes in series back-to-back from the gate to ground.
    Connect the drain to the supply voltage. Connect the output capacitor to the source. Adjust the value of the source resistor for half the supply voltage at the source.
     
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