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Builders: noob potentiometer question

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  • Builders: noob potentiometer question

    So, I bought my nephew this cheesy little voice changer deally last week, and he starts playing with it when I realize the darn thing is octaving and sputtering glitchy awesomeness, so I bought one to rehouse.... And have a course of questions - any help would be great:





    What pot value is average for a volume knob to go before the output jack? Any resistors necessary?



    Could I use a pot to control the input signal, sort of like a gain knob? Values, caps?



    The toy itself has a mic, a trigger to turn the effect on or off, 4 switches and undoubtedly a board behind them (we'll find out when it arrives), and a speaker. Swapping the mic and speaker for jacks, and the trigger for a switch is a start, but I want to be able to adjust the output volume, and hopefully the input strength in order to control some of that feedback (which may or may not work, but we'll hopefully find out).







    This is all I can forsee needing to know, but I am noob-ish (noob-ian?) so please ask me questions about the toy if there is more to it.....
    <div class="signaturecontainer"><b>Yes Spa'am:</b><br />
    <a href="http://acapella.harmony-central.com/showthread.php?2953126-two-Squier-basses-and-Fender-amp-FS&amp;p=46857831#post46857831" target="_blank">CHEAP Bass gear FS</a></div>

  • #2
    dont get your hopes up, almost zero percent of newer toys are built with through-hole components. try 100k for the volume pot if you must.
    <img src="http://i45.tinypic.com/208cflz.png">

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    • #3
      The mic is probably thru-hole, low-level with a bias voltage and low input impedance. Need a buffer or attenuating input resistor and a series cap. Output level, dunno but 1K-500K pot probably would work, try somewhere in middle.
      --------------------------------------------------------------
      www.spenceramps.com

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      • #4
        clips are mandatory.
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        • #5
          It's a foregone conclusion that thing doesn't have a custom DSP in it. I've seen a dozen similar items with similar features in the toy section at Walgreens, and in the "impulse mile" leading to the cash registers at Frys. First thing I'd do is download a data sheet for that chip. Chances are the circuit will match, or nearly match, the reference circuit in the chip data sheet. These Chinese toy makers don't waste any time designing a circuit if the chip maker gives them a circuit design that already works.



          First, it almost definitely has an electret condenser mic. Those kinds of mics are low quality, and require DC power. You can't simply pull the mic off and replace it with a guitar jack - you'll have DC power going to your guitar. Get your ohmmeter out. One of the mic wires is probably going to +DC through a resistor (that's the signal input), and the other is going to ground. Connect a series blocking capacitor between the jack tip and the signal input to block that DC voltage. 0.1uF ought to be enough. Connect the jack sleeve to ground. The impedance of an electret is typically on the order of a couple of K ohms, so hopefully the input impedance of the preamp is high enough that a guitar pickup won't load it down.



          On the output side, if there's a separate amp chip to drive the speaker then skip the amp chip, and pick the signal directly out of the signal processor chip. It's likely to be a lot closer to the voltage range you're looking for. If there isn't already a capacitor in series with that line then add one to block any DC voltage.



          If the speaker driver is built into the signal processor chip; i.e., the signal processor connects directly to the speaker, then you'll need to provide a load to simulate the speaker or the impedance mismatch between the chip and the amp or next pedal is going to be too much, and it's going to sound like poo. One of those little impedance matching transformers from Radio Shack would be a cheap and easy solution (catalog number 273-1380). First, check the resistance of the speaker. If it's about 6 ohms then you're ready to go - connect the low ohm side of the transformer to the speaker driver lines coming from the PCB. If the speaker impedance is higher then you may want to a resistor in series with the transformer to keep from overloading the power amp section of the chip. The total resistance through the resistor and transformer should be approximately the same as the resistance through the speaker.



          Hang a 50K or 100K pot on the other side of the transformer and connect the wiper to the tip of your output jack. Connect the sleeve and one end of the transformer output to ground. If you find you're getting too much drive with the pot turned all the way up (good chance, because the transformer ratio is 125:1) then adjust the pot for the maximum range you want, measure wiper to ground, and replace the pot with a combination fixed resistor and pot. For example, if you find that 10K on the wiper gives you plenty of gain with a 100K pot then replace the pot with 10K pot in series with a 91K fixed resistor. If you can't find a 91K then used a 100K fixed resistor. The ratio will be close enough.



          You can also connect a conventional bypass switch in between the jacks and the circuit board.



          Sounds like a fun project. Let us know how it goes!
          <div class="signaturecontainer">Owner/Engineer, <a href="http://www.wattson-fx.com" target="_blank">Wattson Classic Electronics</a></div>

          Comment


          • #6






            Quote Originally Posted by amp_surgeon
            View Post

            It's a foregone conclusion that thing doesn't have a custom DSP in it. I've seen a dozen similar items with similar features in the toy section at Walgreens, and in the "impulse mile" leading to the cash registers at Frys. First thing I'd do is download a data sheet for that chip. Chances are the circuit will match, or nearly match, the reference circuit in the chip data sheet. These Chinese toy makers don't waste any time designing a circuit if the chip maker gives them a circuit design that already works.



            First, it almost definitely has an electret condenser mic. Those kinds of mics are low quality, and require DC power. You can't simply pull the mic off and replace it with a guitar jack - you'll have DC power going to your guitar. Get your ohmmeter out. One of the mic wires is probably going to +DC through a resistor (that's the signal input), and the other is going to ground. Connect a series blocking capacitor between the jack tip and the signal input to block that DC voltage. 0.1uF ought to be enough. Connect the jack sleeve to ground. The impedance of an electret is typically on the order of a couple of K ohms, so hopefully the input impedance of the preamp is high enough that a guitar pickup won't load it down.



            On the output side, if there's a separate amp chip to drive the speaker then skip the amp chip, and pick the signal directly out of the signal processor chip. It's likely to be a lot closer to the voltage range you're looking for. If there isn't already a capacitor in series with that line then add one to block any DC voltage.



            If the speaker driver is built into the signal processor chip; i.e., the signal processor connects directly to the speaker, then you'll need to provide a load to simulate the speaker or the impedance mismatch between the chip and the amp or next pedal is going to be too much, and it's going to sound like poo. One of those little impedance matching transformers from Radio Shack would be a cheap and easy solution (catalog number 273-1380). First, check the resistance of the speaker. If it's about 6 ohms then you're ready to go - connect the low ohm side of the transformer to the speaker driver lines coming from the PCB. If the speaker impedance is higher then you may want to a resistor in series with the transformer to keep from overloading the power amp section of the chip. The total resistance through the resistor and transformer should be approximately the same as the resistance through the speaker.



            Hang a 50K or 100K pot on the other side of the transformer and connect the wiper to the tip of your output jack. Connect the sleeve and one end of the transformer output to ground. If you find you're getting too much drive with the pot turned all the way up (good chance, because the transformer ratio is 125:1) then adjust the pot for the maximum range you want, measure wiper to ground, and replace the pot with a combination fixed resistor and pot. For example, if you find that 10K on the wiper gives you plenty of gain with a 100K pot then replace the pot with 10K pot in series with a 91K fixed resistor. If you can't find a 91K then used a 100K fixed resistor. The ratio will be close enough.



            You can also connect a conventional bypass switch in between the jacks and the circuit board.



            Sounds like a fun project. Let us know how it goes!






            wow. thank you. I'll see how it goes when the rest of my parts come in, and post results



            helping me decide on a name and graphics will be mandatory
            <div class="signaturecontainer"><b>Yes Spa'am:</b><br />
            <a href="http://acapella.harmony-central.com/showthread.php?2953126-two-Squier-basses-and-Fender-amp-FS&amp;p=46857831#post46857831" target="_blank">CHEAP Bass gear FS</a></div>

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