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Auto-resonant Acoustic Guitar Amplifier

  • Greetings,

    One of my mad ideas: Take a guitar amplifier but instead of a speaker, use a surface transducer to resonate the body of an acoustic guitar. In theory, if you're using a coil pickup, the amount of energy required to cause destructive feedback would be much greater than the energy delivered to the guitar and you should have a nice resonance from the strings and the body - you could say the frequency response of the system is determined by what is being played - IN THEORY (find out in the guitar demo at the end of this project).

    But first there was the learning curve involved with building an amplifier. After much help from the awesome people here and reading a ton of (often conflicting) articles. I'm pretty satisfied that I've got a working approach for pre-amps and power amps. Here is the first prototype design. What I'm looking for here is really comments and criticisms on how the sections are put together as well as my choices of component values (I'll attempt to explain each aspect):


    - Output power should be in the region of 4W

    - I stuck a fuse on the battery because those LiPo things are scary.

    - According to guidelines I've read for input offsets, they should deliver as much current as is practical and 100K is considered a good tradeoff, so I decided to start with 68K and get progressively lower towards the power section. The bypass capacitors are essential in eliminating noise (as I've found).

    - Following a suggestion from here, I have 2 initial gain stages. The first one is 10x and the second is approx. 20x with variable input. This should deliver upto 200x gain and be enough to drive the signal into saturation (as is desireable in guitar amps).

    First test:


    Good news: The amplifier works beautifully - great noise immunity and stuff.

    Not so good news: That theory I had about feedback was a load of bollox. There is feedback most foul meaning this project is dead in the water.

    Its not a total loss though. Its still a decent audio amplifier that can be used for other things, and theres always the possibility of putting other things through it as long as its not the guitar itself (like a microphone or a backing track or something).

    And here's the proper build:


    I managed to get the treble pot backwards but I can live with that (my nephew can have this one when I build the next version).

    The guitar definitely gets a sound boost, but the higher freqencies need to be kept down otherwise they cause feedback.

    I think the best way to use of this idea would be to make a digital version. Compensating for 'rogue' frequencies would still be complicated and require a fast processor, but effects should be no problem - things like delay (minus the original signal) and the phasing effects (chorus, flange etc) would be very easy.
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