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Speaker Noise

Greetings fellows,

Still plugging away on my big project. All goes well, except my 8ohm speaker is giving off a trememdous amount of noise while it's on. It works fine, but there is always a high-pitched "white" noise.

In addition, my project uses small servos, and i've noticed a "click" sound come out of the speaker whenever I turn the servo off (low 1). I assume the problems are related.

Is there too much power going to the speaker? Do I need some sort of voltage regulation or supressive diodes or who knows what?

The over-simplified schematic:


I'm at work, so I can't remember the value of the 'big' resistor, just that it was big. 

Ok, go!

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Captain Tuna's on the right track with suppresion capacitors. The problem is that your servos and even your processor generate electrical noise which a sensitive amplifier will pick up. Put a 0.1uf cap across the supply pins of your processor, speakjet, and each servo / motor driver.

Shielded cables should also be used on long runs (more than 100mm / 4inches).
A larger capacitor, (100uF or more) should be placed on each small circuit board across the power supply.

Low ESR capacitors are best. ESR = equivalent series resistance. Monolithic caps are great for the 0.1uF.

and also the cheapest i think. One other thing i hear of is that having it soldered is better than having it wired on the breadboard, for the noise reason again. But one thing i am not sure of is this: would a PCB be even better than a Perf board (not with wires, just solder) as for noise?

I hope the de-coupling does the trick. The amp setup that I have worked terriffic on the breadboard with the same amount of power (i used the full circuits recommended for both the speakjet and the lm386 but didn't notice any improvement from my own).

the module:


I've already invested enough into my little module that if the decoupling doesn't work, I'll just have to deal with it.

Thanks for yer help!


One other thing, you may be interested. You can use the LM386 in different setups. This site shows some of them (e.g. you can regulate the gain, set the boost and other cool stuff maybe): http://www.hobby-hour.com/electronics/lm386-power-audio.php

Decouple means "isolating" the various ICs you've got, this way you reduce electrical noise and that should lead to a better sound quality. To do this you use a capacitor which filters out this noise. Some guys suggest putting one decoupling capacitor for every IC chip you have in your project.

Now the real deal: what you have to do is take a capacitor, 0.1 uF (i use multilayer ceramic capacitors, i think they are the cheapest) and put one of its legs soldered to the V+ pin of the IC and the other one on the GND pin of the IC. Remember: stick the capacitor as close as possible to the IC ( else it won't be of much help).

Oh, and of course, if you're using polarized caps be careful of polarity (goes without saying). 

I built an Audio Amp for Ratbert that's documented, with a circuit diagram, here. I haven't tried it with servos, but it is pretty noise free. Note the two 27K / 0.1u pairs on the input to the amp. I think these help to keep the signal clean.

From my experience the audio amplifier needs some decouplers, try checking its datasheet. I am not sure that's the reason but you should try.

 You could also put one between the V+ and GND pins of the picaxe. The value's always the same: 0.1 uF, i'd say monolithic.


That's funny, a datasheet for a RadioShack part. That'll be a cold day in analog hell.

How do I "decouple"? Put it between the V+ and GND pins of the audio amp?