Capacitor question / help

[Jeep4by4]

Hi all, a little background before I get to my question...<p>I'm a software guy by trade, but I took a few EE classes in school and was completely fascinated by it. I've been trying to learn on my own ever since. So along those lines I bought some mini kits from Ramsey just to see if I could make them work and to figure out how they work. One of the one's I bought was the "stereo super ear"(As seen here) This is where I get to my question.<p>I got it working just fine (man is that thing sensitive!) but when I turn up the amplification I get quite a bit of feedback, kind of a humming sound. Now I expected that since it's not a high quality amp, but the thing that I don't understand is that if I touch two specific caps, but not at the same time, the buzzing is gone completely. It works by touching either of these caps, but if you touch both at the same time the buzzing comes back. Either one seperately and it's quiet. What could be causing that? Bad Caps? Bad solder job? <p>Anyway, I know it won't do you much good without a schematic, but the caps in question are the small one in the upper right, and the one that's hidden behind the volume knob in the picture in the link I posted above. <p>Thanks!<p>[ September 06, 2005: Message edited by: Jeep4by4 ]</p>

[Gorgon]

Hi
Here is the schematics and assembly instruction for the kit. Do I understand you correct that the capacitors in question is C1 and C4?<p> http://www.velleman.be/Downloads/0/Manual_MK136.pdf <p>TOK

[Jeep4by4]

If I'm reading the schmatic correctly then yes, C1 and C4, but I'll double check when I get home tonight.

[Chris Smith]

Your body capacitance is adding to the cap value, and because this changes the value, so does the filtering value or capability. <p>If you also want to cut back on some of the “feed back”, use “Ear Muff” type ear phones [sealed around your ear] so that less of the actual noise/ frequency makes it way back to the input.

[Jeep4by4]

I was using ear muff type headphones. I'm not sure that feedback is the proper term, but it was a buzzing/humming kind of sound.<p>So if my body is adding capacitance, could I replace those caps with variable caps and tune it so that the humming goes away?

[Chris Smith]

I think if its not sound feed back, then you need to RF shield the unit to find out whats going on. <p>Your body might be picking up RF in the air, and leaking into the circuit? <p>Try wrapping the unit in a insulator like paper or plastic, then placing it in a “anti static” bag like computer parts come in, then tightly wrap it with aluminum foil and place it away from your body, and any other electronics, or take it out side. <p>And make sure its battery operated at this stage. <p>If all the noise goes away, either the PS [if you use one] is the problem, or you need good RF shielding. <p>Also keep your body as far away as possible from any leads and the unit, other than the shielded leads to the head set.

[rshayes]

Both of those capacitors are electrolytic capacitors. The can is connected to the negative lead in these. The buzzing may be simply AC hum injected into the amplifier input. Touching the capacitor could be either injecting hum or shorting out a hum signal that is being picked from other sources.<p>Acoustic feedback may be a possibility, but this usually builds up to a level that saturates the amplifier and would be very painful with a set of earphones. That doesn't sound like what you are describing.<p>RF pickup is possible if their is a strong radio station nearby. Normally, you would hear the modulation, so this doesn't sound like your problem. A little bit of filtering can usually take care of this type of problem.

[Jeep4by4]

Thanks for the responses guys!<p>here's a little more info for ya. I double checked and the C1 cap exhibits the behavior the most. It also happens to a much lesser extent on the C2 and C4 cap. The C3 cap, which appears to be the equivalent of the C1 cap, just on the other microphone side, does not exhibit the behavior.<p>Now, the sound I'm getting is definitely not the squealing feedback you get when you put a mic in front of a speaker, but more of a hum like it's picking up a DC motor or something of that sort. I can't think of any DC motors that are around the device, so I'm not sure if that's what it is or not, but it's a repetative buzzing type sound. I've done some more playing around with the C1 cap and if I "pinch" it between my thumb and forefinger from the sides, the hum deminishes to almost nothing. However if I touch the very top of it with my fingertip and press a little then the hum gets louder.<p>I'm pretty sure that it's not picking up RF from a radio station, well to be more precise it is doing that also, but it's very quiet and is drowned out by this humming sound. When I "pinch" the C1 cap I can just faintly here a radio station, but not enough that I can identify the song that is playing.<p>BTW, this is all powered off of 3 AAA batteries. I will try the RF shielding when I get a chance to see if that helps.<p>thanks again everyone! Keep 'em coming, I'm really learning a lot here.<p>[ September 06, 2005: Message edited by: Jeep4by4 ]</p>

[Chris Smith]

Sounds like a bad cap. Either its leaking the out side to the inside, like touching the leads or ...The layers may be exhibiting a physical magnetic push pull because of poor layering?<p>Physical squeezing is bringing the layers closer together and removing any magnetic effect to oscillate. <p> I would replace it with a good brand, high quality cap. <p>If its not a electrolytic, then go with the best type of cap for the job. Perhaps Mylar? <p>If it is electrolytic, go with a top brand name if you can find one. <p>Caps are normally a dime a dozen, or even less, and you get what you pay for. <p>I used to get 5 pound bags for five bucks and 10% of them were junk.

[terri]

Gee, I wonder if it's picking up a horizontal oscillator from a tv or monitor nearby.<p>Or a lamp dimmer.<p>[ September 07, 2005: Message edited by: terri ]</p>