RC filter question (with diagrams)

[the_floyd]

Could someone please enlighten me on the frequency response of these RC circuits? How do these differ (if at all?) in the way through which they attenuate frequencies? 'A,' I understand, is a simple low-pass filter. 'B' I have seen as the frequency leg in op-amp gain circuits, and I assume it is a high-pass filter of some sort. I've seen 'C' in a few BJF designs, where it is used to direct signal from the audio path into the NFB loop of an opamp; and I'm assuming it's also a high-pass?



This topic has been one of endless confusion for me, and any help would be greatly appreciated.

[alanlan]

If you are assuming a source with zero impedance and a load with infinite impedance (i.e. open cct) then B and C do nothing.

[therecordingart]

A is a low pass filter, B is just a resistor in series with a cap, and C will pass a portion AC to ground.

In A the equation R*C gives you the time constant in seconds. The equation 1/2*pi*R*C gives you the cut-off frequency for the low pass filter. It's the 3db down point or .707 x the input voltage.

[the_floyd]

So only A affects frequency response, then? Hmm... so then how does the Tube Screamer tone control do its filtering, then?



I understand this:

The tone control operations are easiest to see if you assume that the tone control is at one end or the other of its range. When fully toward the (+) end, the capacitor shunts the frequencies above 3.2KHz to ground; when fully toward the (-) end, the capacitor shunts feedback frequencies above 3.2KHz to ground. This means that at the (+) side, the signal gets another -6db/octave high frequency rolloff, while when it's at the (-) side the signal finally gets some treble boost, +6db/octave above 3.2KHz. Note that the "boost" actually just levels off the -6db/octave induced by the 1K/0.22uF network ahead of the active control stage, so the treble is just not being cut any more above the turnover frequency for the tone control stage when fully at "treble".

But why is it that the filter's response is determined not by the 1K resistor in the signal chain that, if the pot's resistance towards that side were zero, the .22uf cap would connect to, but the 220-ohm resistor between it and ground? This doesn't seem to make sense.

[stringsthings]

.... why is it that the filter's response is determined not by the 1K resistor in the signal chain that, if the pot's resistance towards that side were zero, the .22uf cap would connect to, but the 220-ohm resistor between it and ground? ...

the 1k resistor forms a simple passive filter with the .22uF cap that's connected to ground .... the audio input signal to the op amp must go thru this RC network ... but, you'll notice that the 1k resistor is not in either feedback path of the op amp .... the pot is ...

and as pointed out, the active op amp filter is more or less compensating for the filtering that the 1k/.22uF RC network has already "performed" on the audio signal ....

isn't filtering fun?