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synth circuit - output to amp? different waveform?

Discussion in 'Electronic Basics' started by Mad Scientist Jr, May 26, 2007.

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  1. I built a couple of different synth circuits, schematics are here:

    They work - they basically play a tone through a speaker. However, can
    someone explain (or point me in the right direction) how I can

    1. add an output for a guitar amp. Right now they play a sound through
    an 8-ohm speaker. If I disconnect the wires from the speaker and
    attach them to a guitar cable into an amp, I don't hear anything
    (evidently doing this hasn't yet fried my guitar amp or the circuits -

    2. alter the waveform to something a little more pleasant sounding
    (such as a sine or square wave)

  2. If you don't hear anything, it's probably because speakers are
    low-impedance devices, so the voltage available at a speaker output is
    low. I don't know what a guitar pickup puts out, but you could try using
    a small audio output transformer driven backwards to get a better
    impedance match.

    If the output is a square wave, you can low-pass filter it to knock down
    the nasty-sounding high harmonics. There are volumes published about
    waveshaping using active filters, but a simple RC section will probably
    make an audible difference.
  3. Shouldn't matter. The input to a guitar amp is high impedence(usually) and
    usually just around 100mV. Chances are you might be output to much(or not
    enough). You definitely should hear something. Guitar amps are not the best
    amplifiers though and depending on your signal you might not get what you

    Chances are your either driving it with a DC signal(which shouldn't matter
    as it should be capacitively coupled) or its just to much voltage(But then
    you would probably hear something like clicking). If the freq is too high
    then chances are it isn't going to work on a guitar amp.... try your
    computer's mic input first and see if that works.

    not sure what you mean here. The timbre of a sound is the simultaneous
    frequencies being heard. Most musical instruments use the overtone
    series(search on google) to form a harmonious sound. Different strengths of
    the individual overtones produce the different timbre's that you hear.

    A frequency, BTW, is a pure sinusoid. So if you have a circuit that can
    generate one then you can duplicate it a few times and combine each one
    using an op amp sumer to get a timbre of something more realistic... the
    more overtones you add and the more real it will sound but only up to a
    point. Real instruments are not perfect and they have inharmonic overtones
    that give them character(and the fact that the timbre depends on time
    too(plus a multitude of other factors)).
  4. Bob Masta

    Bob Masta Guest

    If you disconnect the speaker there is no load on the circuit.
    The 2-transistor tone generator needs that to function.
    But you can easily find a resistor value to replace the
    speaker (and the 10 ohm resistor). To get started,
    connect the input of the guitar amp to the top lead
    of the speaker, and the amp connector shield/ground
    to the bottom of the 10 ohm resistor. I'm assuming here
    that the guitar amp is AC-coupled, which is pretty
    standard. (If not, you will need a series capacitor.)
    You should get plenty of sound from the amp.
    Now (turn down the amp first to avoid a "pop")
    replace the speaker with a resistor. A very low value
    (like 10 ohms) will probably work, but you might want
    to see what happens if you go higher... you should
    expect that this will change the operation of the
    circuit, so you might need to tweak other values.
    In the Olde Tyme days, synth circuits almost always started out
    with a sawtooth (ramp, not triangle) that sounded pretty
    harsh, then used filters to mellow it out. They used voltage
    controlled filters (VCFs) so they could change the timbre of the note
    dynamically. That may be a bit much to start with, but you can
    easily make simple low-pass filters to alter the tone.
    After you get the circuit modified to drive the guitar amp,
    you can add a series resistor between the tone generator
    and the amp, and a parallel capacitor at the amp input
    to ground. The "cutoff" frequency of the filter is given
    by f = 1 / (2 * pi * R * C), so pick some appropriate
    R and C values. Say, if you want the filter to start
    cutting frequencies above 500 Hz, and you want to
    use 0.1 uF for C, then rearrange the above to get
    R = 1 / (2 * pi * f * C) and plug in 0.1*10^-6 for C
    and 500 for f. R comes out to be 3183 ohms.
    Pick the closest value you have on hand... this
    is just to get you in the ballpark. You might want
    to use a 10K control pot for R, to make playing
    around easier.

    As it turns out, the real key to synthesizing different
    sounds is surprisingly not the waveform or harmonic
    content, so much as the envelope - the way the sound
    builds up (attack) and dies away (decay) as well as any
    sustain portion. (If you've ever seen an old-fashioned
    organ with voice tabs, you may have wondered why
    none of them sounded much like their names. That's
    because the envelope was always the same, not because
    the voices didn't have the proper harmonics.)

    So, if you want to proceed beyond tone generation,
    you have to look up envelope generators.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
  5. Rich Grise

    Rich Grise Guest

    If you disconnect the speaker, you've broken the circuit, and there's
    nothing to pull Q2's collector low. leave the speaker connected. Connect a
    wire from Q2 collector (and the top of a speaker) to the "top" of a
    10K or so pot (fully CW rotation); ground the CCW end of the pot, and from
    its wiper, connect a 0.1 cap, and on its other end, connect to the tip of
    the guitar plug (ground the sleeve, of course), and optionally a 120K (I
    pick these valuse because you seem to have them on hand) to ground, in
    case the amp's input is capacitively coupled. The pot is the volume
    Google "Oscillator circuits", oscillator tutorial, that sort of thing.

    Have Fun!
  6. Bob Monsen

    Bob Monsen Guest

    The first circuit won't work without the inductance in the speaker. The
    second one will work, but you get a square wave, which sounds ugly.

    There are various sound generator ICs you can use to generate cool
    sounds. You could also use a PC to generate the sounds in mp3 format, and
    buy a cheapo MP3 player to output them. There are mp3 mini-boards at,, but I've never
    used them. For $20, seems like it might be the easiest the way to go. You
    would probably have to figure out how to interface to it.

    If you want to build a nice adjustable sine wave, try a wein bridge. I
    found one that you might like after a search of the internet here: Oscillator.htm

    It only takes a few more parts than yours, but I'm guessing that it works
    much better, particularly into a high impedance like a guitar amp. Make
    sure you don't forget to start with the amp turned down to 0, and slowly
    turn it up.

    If it sounds like your 555 circuit, you are probably overdriving the amp.
    In that case, use a high value potentiometer (100k?) on the output of
    your circuit to lower the amp input voltage to something that the amp

    Bob Monsen
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