Finished Pieces

CM2 Bending

    ——————WET/DRY MOD DETAILS——————

    I’ve been building these megaphones for a few years and I’ve always been annoyed by the fact that there is no way of mixing the clean (dry) signal with the fully modified (wet) signal. I finally sat down with some data sheets for the output amp and figured out how to make it work.

    It requires some board lead cutting and component addition, but all told it’s a pretty simple procedure.

    This mod allows you to pass a dry signal and adds a volume adjustment for fading in the wet signal.

    The amp used in the megaphone is an LM386 IC. It’s a very useful and widely used amp chip and is worth looking into.

    Look at the diagram above before you read through the following steps. Many of the points and components refered to below are labeled in the diagram.

    First we’re going to increase the total amplitude so that the dry signal gets a little bump.

    1) Remove capacitor labeled “C4”. De-solder it or just cut it off.

    2) Sever the lead running from pin 5 of LM386 as shown in the diagram.
    Use a knife to cut a line through the lead. Make sure it is all the way through the metal.

    3) Solder in a jumper wire between pin 8 and the contact two points above pin 1

    4) Solder one lead of a .1uF capacitor to the microphone input pad.
    Solder the other lead of the cap to the amplifier input pad.

    5) Sever the lead labeled “voice changer output” this is running along the top of the circuit board.

    6) Scratch a small patch of the green lacquer away to reveal a surface large enough to solder a wire to.

    7) Install a 10k pot as shown in the diagram.


    -It’s a good idea to disconnect the speaker from the positive power supply and connect it to ground. If you look at the two wires coming from the speaker you will see that one wire is connected to a pad below the amplifier and the other is connected to the right where a red wire is also attached. The red wire is your positive power supply. Disconnect that speaker wire and connect it to ground (the black wire from the battery). The reason for doing this is stupid and boring but important. I’ve made an attempt to explain it below. It’s the last NOTE in the list.

    -The wet output is a lot louder than the dry level. You may want to limit the volume a bit by adding a 4.7k resistor before the volume pot. I decided not to limit the volume because if you don’t want it loud, you just don’t have to turn it all the way up. And if you do turn it all the way up, it effectively masks the dry level if you want a fully wet sound.

    -You may want to add a volume adjustment for the dry level. I haven’t tested this, but you should be able to wire it up with a 10k pot just like the wet volume. It’s not totally necessary, but may be a nice feature if you are a knob twiddling maximalist.

    -You may ALSO want to add a total input volume. This would be most useful if you are using the ‘phone as a plug in fx processor rather than as a voice changer. Be aware though that the processing IC uses a very aggressive volume gate and will cut off all signal below a certain volume. This could be cool though if you just want to grab volume peaks or something like that
    The signal goes to the mic in pad then through a 1uF cap labeled “C10”. After C10 the signal makes it’s way to the main processing IC (the black blob on the perpendicular board). I have not tried this yet, so don’t take my word for it, but I think that the best way to proceed would be to sever the line from C10 to the main IC. Then wire the C10 side of the lead to the right lug of a 10k pot, the center lug to the IC side of the line and the left lug to ground. confusing enough?

    -I’m not sure why, but in the original configuration, one lug of the speaker is connected to the amp output and the other lug is connected to the positive power supply. Disconnect the second lug from the positive power supply and connect it to ground (the negative power supply, the black wire coming from the battery). It’s a good idea to do this especially if you are adding audio in and out jacks.
    It can be dangerous under certain circumstances if you DON’T switch the lug to ground and you DO install in and out jacks.
    It’s a little confusing, but I’ll attempt to explain…

    The mic has two wires coming off of it. One is connected to the mic in the other is connected to ground. Lets say your input jack is connected to the input and ground and your output jack is connected to the amp out and the positive power supply. In many scenarios this will work fine, but then one day you decide to plug your megaphone into a mixer, then you plug one of the FX send channels from your mixer into the your megaphone input jack. That means that the ring of each jack (in and out) are now connected together through the mixer. You’ve just created a power short by connecting the positive power of the megaphone directly to the negative. Best case, this will quickly kill your battery, worst case it will break your ‘phone.


    Sorry, no sound files for this piece.

    7 Responses to “CM2 Bending”

    • 1

      * Casper Electronics » Archive » Megaphone Schematics said:
      July 10th, 2009 at 7:29pm #

      […] uploaded schematics for installing wet/dry balance in the CM2 model voice changing […]

    • 2

      Travis said:
      September 29th, 2009 at 2:01pm #

      Bad news for everyone! The newer revision of these voice changers that have just been released are SMD components. FUN TIMES! The circuit board components are also situated differently. On top of all this it seems the switch (which is still a 3 position slide switch) functions as normal sound and then the other two positions are effected. The packaging for some reason is the same as previous models-stating the three effected modes, but this is not the case. I don’t think this is a manufacturing error because the circuit board switch notation reads NRL, ROB, GND.

    • 3

      sloromon said:
      November 5th, 2009 at 10:23pm #

      i’m currently adapting these schematics for use with a very inexpensive voice changer kit from velleman. seems to be pretty much the same schematic!

    • 4

      Ryan McKay said:
      November 18th, 2009 at 5:35pm #

      Hi, been trying my hand at the voice changing megaphones and I have a couple quick questions:
      -I’ve got a circuit verrry similar to the cm2 schematic, but it’s laid out a bit different and the parts aren’t numbered…when to remove ‘resistor R7’ what exactly is that doing and how can I find the similar resistor on my megaphone?
      -I can get the pitch to go higher by touching the two ends of the clock speed resistor w/ a wet finger, but for some reason connecting a pot (10K, tried 250ohm, 1k, 10k, 50k, 250k, 500k, & 1m) does not work…any idea of what I’m doing wrong here?
      -Any chance of putting up a schematic for the megaphones that have a row of 5 switches on the side (labeled a-e)? Seem to be quite common, I’ve had trouble getting the pitch on those (and fired two), and I’d be interested to see what you’ve done with them.
      Thanks a lot! You are the shit!

    • 5

      alienmeatsack said:
      March 5th, 2010 at 9:13pm #

      I second the schematic call for the A-E switch units. I have one, sold by Toysmith, and have been struggling with it. So far all I’ve done is add an input and start the feedback mods.

      I’ve poked and proded on it for a while and am having problems getting any pitch shift anywhere. Oddly enough, its similar to the problem Ryan has… I can touch it and it goes up. A resistor however has no effect.

    • 6

      casper said:
      March 11th, 2010 at 1:36pm #

      @ Ryan & Alienmeatsack:
      I’m looking at that megaphone right now. There is a small resistor right next to the black blob. The black blob is on the green side of the board, the resistor is on the orange side of the board. If you are looking at the green side, the resistor is just to the right. Remove this resistor and replace with a 100k potentiometer. Done.

    • 7

      Mr. Moriarty said:
      October 29th, 2010 at 11:43pm #

      Any news on the adaptation, sloromon? I should probably just order one because they are oodles easier to come across. Just wanted to know what specific things have changed before I order that kit. THANKS!

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