I'm trying to modify the high pass filter in a microphone preamp to drop its -3dB point from 75Hz to 40Hz. The original schematic is attached.
Should I simply increase the values of R24 and R25 keeping them equal, or is that likely to mess something up? Online filter calculators I've looked at seem to use a 2:1 ratio for these two resistors. What is the significance of this ratio and what are the effects of varying it?
Should I simply increase the values of R24 and R25 keeping them equal, or is that likely to mess something up? Online filter calculators I've looked at seem to use a 2:1 ratio for these two resistors. What is the significance of this ratio and what are the effects of varying it?
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assuming that everything about the first filter was as you wanted, except for the cutoff frequency, you could just increase the resistors by a factor of 75/40.
Alternatively, you could increase the capacitors by the same ratio...It's just usually easier to get closer with resistors.
Alternatively, you could increase the capacitors by the same ratio...It's just usually easier to get closer with resistors.
Thanks for the replies. I had already searched the web - including Wikipedia - but the answer seemed to be buried in layers of stuff way beyond me. Spent a few minutes with a design engineer this morning and problem solved.
Apparently changing both resistors (or capacitors) by the same amount just shifts the cutoff frequency, whereas changing their ratio alters the shape of the knee.
Apparently changing both resistors (or capacitors) by the same amount just shifts the cutoff frequency, whereas changing their ratio alters the shape of the knee.
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