#### CurtZHP

##### Well-known member

Well, I've managed to get in over my head again, sort of. Fortunately, this time around I haven't actually started soldering anything together.

I've been wanting to learn how to build an EQ circuit, so I started reading up on them. Bought a copy of Self's "Small Signal Audio Design" and found a pdf of Lancaster's "Active Filter Cookbook."

One circuit that really interested me was found in Self's book. I've attached a picture below. It's a 4-band EQ that is basically two 2-band Baxandall tone controls in series. I'd like to build something similar, but I'm trying to get my head around a few of the component choices. How are the different frequency bands determined in a circuit like this? Obviously, it has a lot to do with capacitor values, but I'm having a time figuring out how. It's one of those situations where you're trying to find something, and it's right in front of you.

Take the LF section for starters. I get that C4 essentially shorts out RV1 at high frequencies, taking it out of the circuit as far as they are concerned. But at what frequency does this start to happen? I know that a low pass filter is basically frequency-dependent voltage divider with the cap as the shunt element. But I can't quite figure out which resistor(s) to include in the calculation.

It gets even more fun when trying to figure out the HM or LM sections, since they are bandpass filters, and now you're dealing with the interaction between two capacitors, I assume. The "bottom" frequency seems like it would just be a matter of calculating a high pass filter comprised of, say, C5 and RV2. Or is it C5 and R11? C5, and the combination of RV2 and R11?

I know I'm missing something simple here. Just need another pair of eyes on the problem. (And a smarter brain behind them....) Maybe I have a lot more to learn before trying this.

I've been wanting to learn how to build an EQ circuit, so I started reading up on them. Bought a copy of Self's "Small Signal Audio Design" and found a pdf of Lancaster's "Active Filter Cookbook."

One circuit that really interested me was found in Self's book. I've attached a picture below. It's a 4-band EQ that is basically two 2-band Baxandall tone controls in series. I'd like to build something similar, but I'm trying to get my head around a few of the component choices. How are the different frequency bands determined in a circuit like this? Obviously, it has a lot to do with capacitor values, but I'm having a time figuring out how. It's one of those situations where you're trying to find something, and it's right in front of you.

Take the LF section for starters. I get that C4 essentially shorts out RV1 at high frequencies, taking it out of the circuit as far as they are concerned. But at what frequency does this start to happen? I know that a low pass filter is basically frequency-dependent voltage divider with the cap as the shunt element. But I can't quite figure out which resistor(s) to include in the calculation.

It gets even more fun when trying to figure out the HM or LM sections, since they are bandpass filters, and now you're dealing with the interaction between two capacitors, I assume. The "bottom" frequency seems like it would just be a matter of calculating a high pass filter comprised of, say, C5 and RV2. Or is it C5 and R11? C5, and the combination of RV2 and R11?

I know I'm missing something simple here. Just need another pair of eyes on the problem. (And a smarter brain behind them....) Maybe I have a lot more to learn before trying this.