Harmony Central Forums
No announcement yet.

noob potentiometer question


  • Time
  • Show
Clear All
new posts

  • 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?

    This is all I can forsee needing to know, but I am a noob so please ask me questions about the toy if there is more to it.....
    Yes Spa'am:
    CHEAP Bass gear FS

  • #2
    Not sure from your post whether this is a guitar pedal or something for a microphone.

    If this is a guitar pedal feeding an amp a 250 or 500K should work nicely the same way as it would for a guitar.

    If this something mic related you have devices that feed a PA at mic level and line level.

    A 1K ~10Kohm pot might be needed but I'll need more details on what the device is.

    Also, using a pot before the output jack can be bad. You are essentially shorting the units output.

    If you use say a dual pot to provide a true bypass or use a pot to attenuate the input you'd be OK.

    This octaving and sputtering may be Feedback from what I read.

    A mic or guitar will do that if the gain is too high. You can drop the output with

    an attenuator so it matches the amp its driving but I need to know specifically what device were're

    talking about here. Post a link to whet you bought your nephew and I can give a better reply to your question.


    • #3
      sorry, i spend so much time in EHFX that I forget there are other topics floating around here...

      It's definitely octaving. It's a voice changer


      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).

      So, from what you're saying 250/500k's should handle the signal appropriately in both suggested uses, but placing a pot before the output might not work?
      Yes Spa'am:
      CHEAP Bass gear FS


      • #4
        Suppose we treat the device as a black-box, but we are able to measure the impedance of the speaker and the microphone. We can design our inputs and outputs to mimic those impedances.

        Let's deal with the output first. We shouldn't quite treat it as a blackbox, because it is important to know whether the speaker output is single-ended or whether it uses a bridged amplifier. Both are common in cheap consumer electronics, because they are easier to do than implementing a dual-voltage supply. Bridging isn't done for better fidelity (removing an electrolytic from the audio path) but simply to save space. There exist inexpensive mass-production audio amplifier IC's that do the bridging internally and provide outputs that can be attached directly to a speaker.

        In either case, suppose the speaker is 8 ohms. We can replace it with an 8 ohm resistor and take the output voltage from that. (If the level is too great, we can use a voltage divider to reduce it. For instance, a 4 ohm to 4 ohm divider to cut it in half for a -6 db reduction.)

        A low impedance like that has the advantage of being a low source impedance, which is good for driving a wide range of impedances. We can take the signal and drop it across a 10K linear pot whose wiper goes to a 10K line input.

        If the amplifier is single ended, it couples to the speaker via a big electrolytic capacitor, and the other side of the speaker goes to ground. We should leave the capacitor there, and we can couple that ground to the next device. But we have to know which terminal is ground.

        If the amplifier is bridged, then both speaker terminals have swinging voltages. We must connect our load resistor to the rest of the circuit using blocking capacitors.

        See attached schematic, keeping in mind it's just an example. The load resistor doesn't have to be as low as the speaker impedance, but less than 1/10th the pot value.

        This second schematic shows the voltage divider idea to reduce the output. The toy probably doesn't send that much voltage to the speaker, but if you go to a sensitive line input, you never know.

        Music DIY mailing list: http://www.kylheku.com/diy
        ADA MP-1 mailing list: http://www.kylheku.com/mp1


        • #5
          The input side is more tricky. Now this being a cheap toy, the mic is probably a high impedance type. It probably faces a high impedance input inside the device. Why not; the signal isn't going over a 30 foot mic cable.

          If the mic input has a reasonably high impedance, like 50K, you can feed a line output into it, and you can use a pot anywhere from 10K to 200K. For pot values lower than the input impedance, use an audio pot. For higher values, you don't have to: the loading effect of the lower input impedance (lower than the pot) creates a curve that resembles audio taper.

          You can guess the impedance input from the mic. It is probably several times higher than that of the mic.

          Let's look at the worst case. Suppose that the mic has a low impedance and the input is something like 600 ohms, or less. That is going to be hard to drive by your previous device, unless it has very good circuitry, like an op-amp which can easily drive 600 ohms. In that case, we would build a buffer circuit which can drive that 600 ohms, and if we wanted an input level control, we could stick it into that buffer, as an attenuator or gain control.

          E.g. along these lines:

          The two amps on the NJM4556 can drive 150 ohm loads, so that would be a good chip to reach for if you have to drive a low-impedance mic input (unlikely to be present in this toy).

          The input gain is controlled in the feedback circuit of the inverting stage (where I forgot to put in a high frequency compensation capacitor, oops). The power supply is single voltage with a bypassed voltage divider providing the middle reference voltage. For that reason we have to decouple our inputs and outputs with caps. The input resistor is 20K and that determines our input impedance. For that, a 5uF capacitor provides good low frequency response. A smaller value can be chosen if we don't care about that.

          Note that the feedback only goes up to 10K, but the input is 20K. So this circuit has less than unity gain. The idea is that the mic input is probably sensitive, so it behooves us to bring down the level.

          If you use only one op-amp of a dual IC, always hook up the other one properly: here that means pulling the + input to VREF, and connecting output to -.
          Music DIY mailing list: http://www.kylheku.com/diy
          ADA MP-1 mailing list: http://www.kylheku.com/mp1


          • #6
            Just put a 10K ~100K pot across the mic wires and attenuate the mic instead of the speaker and vary the mic input.

            Something like this might work http://www.parts-express.com/pe/show...number=023-628

            If you want to vary the speaker you need to use an L pad. This one is 50W which over kill

            for a toy that puts out maybe one or two watts but should work


            You may be find an 8 or 16 ohm wire wound pot that can handle a few watts instead of an L pad.

            You'll cook a normal carbon pot and it will become scratchy vn at 1~2Watts and finding one under 50 ohms will be difficult.

            The difference in tone from a megaphone is the input preamp is high gain and the mic tends to

            overdrive the preamps for at driven sound. Putting a pot on the mic will cause the mic

            to loose sensitivity and drive but it will clean up the sound a bit before it goes dim.

            An L pad will attanuate the sound and it will retain its drive. It will just go down in volume

            and any feedback from room reflection will deminish.

            The best placement for a pot would be between the

            preamp and power amp to retain the sound quality, but wiring it in might not be possible.

            They often use a single chip on cheap devices and there no access points between the

            preamps and power amp stages.


            • #7

              Simple question. Complicated answers.

              Kazinator: "if the amplifier is bridged"?? Really? In an two-bit toy noise-maker?

              Just a low-resistance pot on the output will do fine. It won't be "cooked". This little toy couldn't even muster 1/2 watt at the output. So no problems.


              • #8

                Quote Originally Posted by lhorwinkle
                View Post


                Simple question. Complicated answers.

                Kazinator: "if the amplifier is bridged"?? Really? In an two-bit toy noise-maker?

                Yes, amplifiers can be bridged in mass-produced, battery-powered consumer crapware. There are inexpensive audio amplifier IC's that do this all on one chip, and even in stereo. No large electrolytics to the speakers means you save space and cost.
                Music DIY mailing list: http://www.kylheku.com/diy
                ADA MP-1 mailing list: http://www.kylheku.com/mp1