Impedance mismatch guitar amp footdrum

Discussion in 'The Projects Forum' started by twmahoney, Aug 10, 2011.

  1. twmahoney

    Thread Starter New Member

    Aug 10, 2011
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    Hey relative newby here,
    I am building a foot drum "stomp box" that uses two speakers as the transducer's. i want to be able to run the guitar through the stomp box so that only one lead is run back to the amp. the guitar signal is very low through the amp when the speakers are in parallel and non coming through when they are in series, when the speakers are switched "off" the guitar is strong through the amp but there is mains hum. here is the schematic, please help

    thanks Tom
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    I'm afraid it's not going to work like that.

    The guitar pickup is at a level lower than "line level" audio, which is high impedance; and your speakers are low impedance devices designed to be used on the power side of an audio amplifier.

    A brief intro to various audio signal levels is here:
    http://en.wikipedia.org/wiki/Line_level

    You would need some sort of impedance matching network to get them to work together.
     
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  3. twmahoney

    Thread Starter New Member

    Aug 10, 2011
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    Thanks Sgt, would it be best to increase the impedance of the speaker branch, as the guitar amp prefers a high Z input? If not any ideas on matching the impedance?

    regards Tom
     
  4. #12

    Expert

    Nov 30, 2010
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    A guitar pickup driving a speaker is like a flea pushing an elephant. It just won't work. To prove it to yourself, connect the guitar directly to a speaker and see how much sound comes out.

    This is why amplifiers were invented.
     
  5. twmahoney

    Thread Starter New Member

    Aug 10, 2011
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    I understand this, the speakers aren't being driven by the pickup's they are in parallel and used as transducers. I guess my question is, what would be a way to match the impedance so that the levels are the same and both signals are 'loud' though the guitar amp? Or would i be better to amplify the guitar signal first?
     
  6. #12

    Expert

    Nov 30, 2010
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    Oh. Now I get it.

    The impedance of a guitar is a few thousand ohms at "10" but it can be over 100,000 ohms at "5". The obvious choice is an output transformer run backwards, something out of a vacuum tube amp. Like, 8 ohms to 10,000 ohms. I think you will need to put the transformer in a metal box and use a guitar cord to avoid hum pickup. Still, there is the problem of adjustability. The guitar is adjustable. The transformer isn't. That's going to limit how your volume control works.

    The other way is to have an amplifier for what the speakers pick up and another channel for what the guitar puts out.

    I'll go see what I can find for transformers.

    Antique Electronic Supply
    P-T1750A
    $16.30
    Impedance 8 ohms to 22,800 ohms

    I do not promise that this will work the way you want it to work.
     
    Last edited: Aug 11, 2011
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  7. twmahoney

    Thread Starter New Member

    Aug 10, 2011
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    Thanks for the feedback #12, ill have to look into the transformer. Space is a bit of an issue with this build, so maybe it will have to do for now.
     
  8. Hi-Z

    Member

    Jul 31, 2011
    157
    17
    Guitars like to "see" a high impedance, otherwise the sound ("tone") will be adversely affected.

    The reason for this is that the guitar's pickup(s) are inductive and this combines with cable etc. capacitance to "lift" the response at, typically, 2-4kHz (depending on pickup and cable details). Loading with an impedance less than about 500k ohms will damp this effect, and will deaden the tone.
     
  9. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    Hi twmahoney,

    Try putting the speaker(s) in series with the guitar signal. This configuration won't change the guitar signal circuit impedance much, but will still allow the voltage generated across the speaker coils to be added to the guitar signal. Ground the speaker frames to reduce hum pickup.

    The voltage level from the guitar should be comparable with the voltage levels from the speaker when "stomped". If not, you may have to put a resistor across the speakers, or reduce/increase the guitar output to get the effect you are looking for.

    Have fun with it,
    Ifixit
     
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  10. twmahoney

    Thread Starter New Member

    Aug 10, 2011
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    Thanks Ifixit, I have a bit of time today so i will have a play around with it.
     
  11. twmahoney

    Thread Starter New Member

    Aug 10, 2011
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    Just an update, putting the guitar pickups in series worked well. Both the guitar and drum signal's are both strong through the amp. The only thing is the mains hum is quite bad. I have ground the back of the pot and the speaker cages cant think of where it is coming from.
     
  12. #12

    Expert

    Nov 30, 2010
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    The problem with using a speaker for a microphone is that the speaker has very little shielding and no "humbucking" windings. The speaker never had a thought about avoiding hum. You have to provide shielding, and that's really hard to do because the magnetic fields can come in through the paper cone, even if you have a metal box around 5 sides.

    I can suggest grounded brass window screen on all six sides. Build a box and shield it. You don't have to port the box like a real speaker because the cone movement is very small.
     
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