This doesn't make sense to me

[ExpAnonColin]

So as you guys may know, I use my little kids keyboard as a signal generator, just because it's small and the frequency range is pretty good and I can just get a drum loop going and play with the circuit.  The problem, though, that I've just found that grounding the keyboards signal doesn't destroy the signal-it only quiets it.  What's going on here?  I can literally connect it to the ground and it still comes through, just quieter.

-Colin

[Ansil]

hmm could it be that its ground is at a different point than true ground

[ExpAnonColin]

hmm could it be that its ground is at a different point than true ground

No, it's certainly true ground.  It works right when I use my guitar.

-Colin

[smoguzbenjamin]

Wierd. try connecting a resistor in the signal path after your connection to ground, maybe that helps, like this:

Code Select Expand


----signal----------------||||||------
                  |
                  |
----ground----------------------------

[Jason Stout]

Colin,
need more information..
Is this happening with a high gain circuit on your breadboard?

[drew]

http://www.epanorama.net/documents/groundloop/basics.html

Having all your "ground" points connected in some manner does not mean that every "ground" point in your system is properly grounded.

Look at it this way: If you connect the output of the signal to "ground" but your "ground" point has a resistance of 1 ohm to the lowest potential in your system, and your device has an output impedance of 1k ohms, you've just set up a voltage divider, and you're tapping it ~1/1000th of the way up from ground. So you're going to get a signal that's -30dB w/r/t the output of your keyboard.

IANAEE (I am not an EE) so my math is not exact... also note that if you're taking the output off the speaker, you've only got an impedance of 8 ohms so you're at about -10dB assuming a ground resistance of 1 ohm.


drew
www.toothpastefordinner.com

[drew]

bump (did you see this?)

[ExpAnonColin]

Drew, thanks for the link and all, and I think you're probably right...  Oh well, guess I need to start using my guitar again. This isn't a high gain circuit at all.  Quite simply, the keyboard was still quite audible when it was plugged into the ground.  The signal was grounded via my amp and the shaft of my output jack.

-Colin