electrolytic capacitor question

[newperson]

What causes touching the top of an electrolytic capacitor to stop humming?  I am a ruby board that when I touch the top of the 220uf cap it stops any humming.  Or it also stops when I put a resistor touching the top of the cap to ground. 

[Minion]

Well when you touch the cap you are creating a high impedance path to ground , I don"t know the mechanism for why it is getting rid of the hum or why this cap needs to be grounded but I have heard of this problem before but usually only in high frequency and digital curcuits....,
You can try takeing off the plastic cover on the cap and wrap some wire arround it and connect it to ground (Possibly through a resistor) and put some shrink tubeing over it to keep the wire in place and see if that gets rid of the hum .....

good luck

[amptramp]

Are you certain you don't have a cold solder joint there?  Touching the capacitor may change it from "not connected" to "connected".  Only one solder joint has to be bad for this to happen.

[PRR]

Nothing to do with "cap".

Ever have a guitar that hummed, but stopped when you touched the strings or other metal?

It buzzes because there is no connection to a general ground. Maybe it is out in the open? Maybe you don't have a good connection to a metal box?

Your body is a hum antenna. When you are near, the hum on your body influences the circuit.

The outside of a large electrolytic cap is very often also circuit ground. When you touch it, your body and the circuit are on the same ground.

[newperson]

The board is not touching anything.  It is mounted to a screw post.  Is the outside metal body connected to the ground side on smaller caps like these 220uf's?  I tried to heat up the body of the capacitor to solider a resistor to ground but it does not want to stick. 
Thanks for the replies so far.
Paul.

[R.G.]

What causes touching the top of an electrolytic capacitor to stop humming?  I am a ruby board that when I touch the top of the 220uf cap it stops any humming.  Or it also stops when I put a resistor touching the top of the cap to ground. 

A far more useful question is "what causes a circuit to hum when I'm NOT touching the top of this electrolytic cap?"

The board is not touching anything.  It is mounted to a screw post.  Is the outside metal body connected to the ground side on smaller caps like these 220uf's? 

The outer can of electrolytics is usually the (-) side, not the (+) side, all right. But there is no "ground side" of a capacitor. This may your perception because most modern circuits use the (-) side of the power supply connected to signal ground. But many electros are connected with neither side attached to ground.

I tried to heat up the body of the capacitor to solider a resistor to ground but it does not want to stick. 

That's because these are aluminum electrolytics, and aluminum is notoriously difficult to solder to. And there are semi-fluids in that cap, making it very, very difficult to heat one spot to the temps needed to solder. You'd have to boil the electrolyte in the cap to get a good solder joint, not to mention using aggressive, special-purpose fluxes and high temps to solder just because the case is aluminum.

[newperson]

Quote from R.G.
A far more useful question is "what causes a circuit to hum when I'm NOT touching the top of this electrolytic cap?"

I guessing the mess of wiring.  It has two 18" runs of wire.  One 18" going from 1/4" in to the board then 18' output going back to a switch and then back to a speaker.  Not really great wiring, but needed for the spot of the speaker vs 1/4 and the board.  The ruby board is in a small synth project and is powered by a 9v tap from a +-12 volt power source.  The speaker also will pick up a LFO thump from a LFO board.

When I put the body of the cap to resistor to ground it takes away the LFO and hum noise.  Is there another way to solve my issue?

It has a 1.5M to ground from the MPF102 transistor.  Would lowering or raising this value help?  This is where I am making the additional connection from the C4 220uf cap to R1 1.5M to ground.  The 1.5M is made up with (1) 1M and (1) 470M resistor to make up the value.  I am putting the connection in the middle of the two.

Paul.

[R.G.]

When I put the body of the cap to resistor to ground it takes away the LFO and hum noise.  Is there another way to solve my issue?

I'm guessing that there is no ground path to the rest of the circuitry, and that when you touch the cap, you're making a ground-ish path which stops some of the humming. While I don't have a schemo of the circuit in question handy (which is why I ask for links to the schemos in the "what to do when it doesn't work" item I posted) I'm guessing that the 220uF may be a power supply filter, and the negative side may be intended to be ground. Maybe. Can't tell from here.

You could possibly check for ground continuity with a meter set to ohms. Maybe.

[newperson]

Hello again,
The probe test says .5 Ohm from panel to ground on board.  The body of the 220uf capacitor does not test to ground.  The (-) speaker tests .7 Ohm to ground and the (+) speaker terminal tests 7.7 Ohm to ground.  The speaker is rated at 8 Ohms.

There is a 100uf next to the 9V input so I am guessing that should be the power filter.  The 220uf goes to the (+) speaker then to pin 5 on the 386 IC. 


Here is the schematic.  http://runoffgroove.com/ruby.html
http://runoffgroove.com/ruby.html


Thank you for your help,
Paul.

[newperson]

I have found also it I put the speaker (+) side to 1 Meg to ground it will also quite it down.  I guess I should also add, this noise is coming when nothing it plugged into the input, or it is unnoticeable when something gets plugged in.

Will putting this resistor from the speaker side to ground harm anything?

[solderman]

My guess is that it is something like this. Some one more knowledgeable than me, like R.G, will be able to describe this more correctly though.

In your normal guitar amplifier, the input jack signal (top) is shorted to ground (ring) when noting is inserted in to the jack. This grounds the signal and cancels any hum (magnetic radiation) picked up by the signal leads inside. No resistance between signal and ground lowers the potential to the same level as ground= Zero. When you put in your guitar cord in to the amp and in to your guitar the pickup coil completes the circuit. However, the coil has impedance and a resistant and the cable has a capacitance. This will let the guitar pickup and cables etc. act as an antenna and insert magnetic waves that flutes around like a radio receiver does with the difference that it only receives on the "hum channel" and amplifies this "channel".

If you insert the guitar cable in to your amp but not in to the guitar an touches the guitar side you will also have a hum noise.

So if you see to that the signal is grounded in the input jack when nothing is inserted in to it the hum will stop

[newperson]

So you think I will be changing the input 1/4 jack to short to ground when unplugged? 
Thanks for the answer.