# Do passive RC filters need ground?

So, after a bit of searching I can't seem to find an answer to this anywhere, at least not in an easy to understand manner.

Ok, lets set a scenario, if we have an audio input (in my project it would be buffered by a op amp in a 3403 quad op amp IC) and have that go into a passive RC filter (possibly 3 band) and have the output of that go into an overdrive circuit.

Question: Does that RC filter need a ground to filter to, or could I instead wire that to a second "identical" overdrive circuit and have part of my signal filter to od1 and the other part to od2?

Just a curious question, since all I can find are explanations of how RC filters filter to ground, but as I understand it, the filter works by having the "unwanted" frequencies follow a path of least resistance to ground, by that logic I would think that it doesn't matter whether that path is to ground or to some other part of the circuit.

Edit 1: Hypothetical schematic added.

simulate this circuit – Schematic created using CircuitLab

• The filter generally connects to the audio return path. In a single ended connection, yes that is ground. But in a balanced audio connection, it is the inverted leg. – Brian Drummond Oct 20 at 11:35
• and is the ground absolutely necesary in a unbalanced connection? – Nook Oct 20 at 11:55

## 1 Answer

It needs to be able to return current to the source. This is to form a complete circuit for current from the source to flow around.

That return is most often ground.

It could conceivably also be the positive supply rail, provided the positive supply is well-bypassed to ground near the source circuit.

If your "overdrive" circuit has low output impedance and shares ground with the buffer circuit, that will most likely also work.

• I'm not entirely sure what you mean with the possitive supply rail, because i have a bias voltage supply, the audio signal comming in, and the supply that powers the op-amps. – Nook Oct 23 at 8:27
• The overdrive circuit is designed around the same op-amp as the buffer in the schematic, followed by a active filter using also the same op amp, at the end there is an output buffer, my plan was to split the audio into treble and bass, and have them each go through their own overdrive/active filter section to late be joined again right before the output buffer – Nook Oct 23 at 8:33