So I am designing a distortion pedal which would go input -> variable gain opamp -> RC filter -> clipping section -> output buffer. What I am worried, is that the RC filter will "eat" too much of my signal for the clipping to work properly. Is there any "rule of thumb" formula to calculate, how much voltage will be lost in the RC filter? I could go input buffer-> filter -> opamp -> clipping, but that would use another opamp(or a tranny), so I would like to keep the schematic to 2 dual opamps(it's going to be a dual band distortion so each opamp stage will take a tl 072)
About half a thumb. Safe bet is twice the gain needed, but at 9volt supply you might well run the risk of the pre-amp clipping first.
Why don't you include the RC filter in the op amp feedback loop?
I mean, an equivalent filter.
Not worried about the preamp clipping that much, I am planning to use a charge pump for a whopping +-9V :)
Great idea about the feedback loop filtering, but wouldn't that mess with the gain when I have the RC filters R variable to vary the frequency?
Yes, I was going to suggest putting the filter in the preamp - BUT, if it's going to have variable gain, it will affect the filter cut-off if it's in the feedback loop Rat style.
But now I see you want the cut-off frequency variable? That's a tough one without changing the gain.
It could be enough to have the tone pot select depth of a second feedback filter cap. Say a fixed one at 3Khz and the pot brings in one at 300Hz.
Your gain pot would be in the "-" amp input
...and now we know you have 18v to play with, go with the passive filter after the pre. Give the clipping section a high input impedance and it won't cause excessive loading loss on the filter.
One suggestion, the Timmy OD pedal seems to have a clever solution for including filters in a non-inverting opamp stage without affecting gain:
(http://tapatalk.imageshack.com/v2/15/01/25/0baf5513a2a72f080b8c89ad18b9ffd8.jpg)
On the bass side, when the 50k pot is set to 50k, C3 is basically out of the circuit and the bass cutoff frequency is set by R3 and C4 alone. As the pot is dialed down to 0k, C3 and C4 act in parallel and the cutoff frequency is set by R3 and C3+C4. The overall gain doesn't change much because at high resistance settings the signal has a lower impedance path to follow in R3/C4, and at low resistance settings the pot/C3 combination starts to act like an independent capacitor so the signal flows through the parallel combination of caps.
Similarly, you can include an additional parallel connection between the output and -ve input of the opamp consisting of a series capacitor/pot combination (say 10k-50k). When the pot resistance is high, the original capacitor and the gain pot set the high frequency cutoff. When the pot resistance is low, the two capacitors act in parallel to lower the cutoff frequency. Again, the gain doesn't change much because low frequencies continue to pass through the gain pot and get blocked by the caps thus maintaining their level, while high frequencies pass through the original cap or the parallel combination of caps depending on the pot setting.
Thanks for both ideas! I will try both of these out, when my opamps and charge pumps arrive(breadboard is too small for 741s of each stage, which is the only OPamp I have) ... wait I also have these weird power opamps, maybe try them out