Stupid RC filter question

[the_floyd]

In an RC filter, as per this diagram:



can I assume that if the filter is passing highs (or lows) to the output, then it is dumping whatever it cuts from the signal to ground? Essentially, does the RC filter "split" the signal into a high path and a low path - or is that not at all how these things work? I ask because I'm trying to plan out a BMP-style tone control that feeds whatever it cuts back into the negative feedback loop of the opamp, essentially compounding the effect of the filter.

Help would be much appreciated!

[amptramp]

In an RC filter, as per this diagram:



can I assume that if the filter is passing highs (or lows) to the output, then it is dumping whatever it cuts from the signal to ground?

Yes, it is passing lows.

Essentially, does the RC filter "split" the signal into a high path and a low path - or is that not at all how these things work?

The signal output is a voltage divider with the output dependant on the impedance of the capacitor at the frequency in question.  Look at the reactance of the capacitor as being one side of the right triangle and the resistor as another.  Total impedance is the hypotenuse (third side of the triangle) if you have no resistance to ground on the output side.

I ask because I'm trying to plan out a BMP-style tone control that feeds whatever it cuts back into the negative feedback loop of the opamp, essentially compounding the effect of the filter.

The amplifier will have a natural rolloff at high frequencies.  Putting a low-pass network like this in the feedback path will emphasize the high frequencies.  The response will rise until it meets the falling gain of the op amp.  If the gain is above 1 when the phase shift reaches 180 degrees, it will oscillate at that frequency.  This is why you see feedback lead but not feedback lag used with feedback amplifiers.

[the_floyd]

Thanks for the reply, very informative. I have no electrical background whatsoever, though, so some clarification on the opamp feedback bit may be necessary - I'll see if I can make my query a little more specific here. Let's say I were to take the tone section of a Big Muff, and tack it onto a Rat circuit (minus the filter control, of course). If I took the resistor of the high pass filter portion, and instead of connecting it to ground, connected it to the NFB loop of the opamp, would the gain on the lows decrease as the high-pass side of the control is brought into the signal path? My logic here is that the high-pass filter would dump the lows into the NFB loop instead of to ground, thus cutting bass both at the gain stage and after it. Or would (I'm just taking a stab in the dark here) the phases of the signals misalign somehow, causing this not to happen?

[tubelectron]

Low Pass RC Filter theory :

Fc = 1/2.PI.RC at -3dB attenuation.

[slacker]

If I took the resistor of the high pass filter portion, and instead of connecting it to ground, connected it to the NFB loop of the opamp, would the gain on the lows decrease as the high-pass side of the control is brought into the signal path?

I don't know exactly what would happen, but turning the tone pot probably wouldn't affect it. The tone pot just pans the output of the tone control between the high pass and low pass sides, it doesn't affect the input of the tone control or change anything to do with the individual RC filters. So all you are doing is putting a fixed RC filter in the feedback loop of the opamp, if you wanted the position of the tone pot to affect whatever happens in the feedback loop you would have to put the whole tone control in the loop.

[amptramp]

Don't forget, the op amp uses a negative feedback loop.  This means if there is more highs fed back, there will be less at the op amp output, so a tone control of the type you describe would work backwards - you get less of what you feed back.  And you still have to ensure that you have stability.

[the_floyd]

Thanks for all the replies, everybody. In retrospect, perhaps I shouldn't have used an illustration of a low-pass filter when posing a question about a high-pass. But at any rate, I think I've found the answer to what I seek to accomplish in the "Nature" control of the BJF Honeybee.