Made a Twin T notch filter today. The problem with acoustic pickups on overdriven sounds is that you get too much hi end from them and the overdrive makes it all fizzy and horrible. With this switched on, you can play overdriven sounds with no problems. Makes a piezo much more versatile. You probably need a preamp before it though. I'm told the Q would get softened if it went straight after the piezo. You could try it. This one notches out 11k and a fairly wide slope around it. I tried one at 7.8k but it took too much high mids out.
To notch 11k, use two 0.001u caps (C2 and C3) and a 0.002u cap (C1). The resistors are 7.5k (R3) and 15k (R1 and R2). I doubled up on some components in the one in the pic because I didn't have the .002 cap or the 15k resistor here, so ignore the photo.
You won't want to notch unless you are using overdriven sounds, so put it on an on/on switch so you can bypass the Twin T when you don't need it.
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Diagram and calculator here: http://sim.okawa-denshi.jp/en/TwinTCRtool.php
To notch 11k, use two 0.001u caps (C2 and C3) and a 0.002u cap (C1). The resistors are 7.5k (R3) and 15k (R1 and R2). I doubled up on some components in the one in the pic because I didn't have the .002 cap or the 15k resistor here, so ignore the photo.
You won't want to notch unless you are using overdriven sounds, so put it on an on/on switch so you can bypass the Twin T when you don't need it.