Pot is making problems

R2Music · Joined: Aug 23, 2006 Posts: 3 Location: San Jose, CA

Hopefully one of you electrical engineers/circuit bending pros can help me.

I am bending a "Talk and Learn Alphabet" toy that runs from two AA batteries. I have found a couple decent bends but I'm having trouble with my audio ouput. I took the speaker wires and ran a 100k pot in series with the 1/4" jack. Everything is connected and working fine and was working fine before the pot, but my problem is that my output and within the 10 or 15 degrees that I turn the pot it reduces the volume quickly and goes out. Also when I have it at a lower volume the sound is very distorted (that is my biggest problem). Am I using the wrong pot? I also tried a 10K and it acted exactly the same.

If anyone knows the answer you will make my day!!!

seraph · Posted: Mon Aug 28, 2006 3:59 pm Post subject: Re: Pot is making problems

bigtex · Posted: Mon Aug 28, 2006 4:19 pm Post subject:

You probably need a lower value pot. The speaker drive circuitry is designed to drive an 8 ohm load. Now, if you use a really low value pot, you'll end up burning it out because the speaker can probably handle a watt or two, and the pot will probably only be good for a quarter watt. Try a 1k pot or even a 500 ohm pot. You'll probably get better control. Also look for an audio taper pot, because those few degrees where you get most of your control will be expanded on the pot's turning range.

The other thing you could do is get a higher wattage 8 ohm resistor (or just keep the speaker) and use a simple opamp buffer circuit to drive the output. That way the resistor (or speaker) can handle the current of the output amplifier, and the opamp buffer will just read the voltage and send it to your 1/4" jack.

google for "unity gain op amp buffer" or something like that and you'd find a good circuit for that

The only reason I'd recommend removing the speaker in this case is the speaker would act as a microphone and make your recordings less clean. So find an 8 ohm resistor of at least 1 watt power handling (or maybe 10 ohm will be easier to find... you could probably pull one out of an old power supply or something... look for a fatter resistor) and then just use the unity gain op amp buffer. Should work like a charm.

Or just use a lower value pot for a simpler (but likely lower quality) solution.

Good luck!

bigtex · Posted: Mon Aug 28, 2006 4:24 pm Post subject:

http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm

Just use the "voltage follower" on this page. Very simple circuit. All you need is to power an op amp and connect the + of the speaker/resistor to the + input of the op amp, and connect the output of the op amp to it's - input and to your 1/4" jack. Very simple circuit.

R2Music · Joined: Aug 23, 2006 Posts: 3 Location: San Jose, CA

I will try what you have suggested.

What is the difference between a phono and mono 1/4" jack? Will that change anything?

Also I'm not an electrical engineering buff... can you help explain to me which option would give better audio output? Maybe just tell me to for example: "connect the audio output to resistor type a then to the 1/4 jack".

I need some instructions cuz im so confused

bigtex · Posted: Tue Aug 29, 2006 8:02 am Post subject:

I figured I should keep the discussion out where others can see. I hope you don't mind. This way, if others have the same questions they might find what result you eventually come up with as the answer they want too! Very Happy