variable bandpass filter question

[Fenris]

Hi, I haven't posted in quite a while but this board has been a lifesaver in the past! I need a variable mid boost guitar pedal for a recording project. The JHS Haunting Mids is surface-mount and not conducive to modding, so I'm going to build the "Malificent Mids" clone and lower the frequency range somewhat so it goes down to 200 Hz instead of 500 Hz. The pedal has Volume, Frequency, and Gain knobs and switchable Q.

I'm having some trouble understanding the schematic. I thought a multiple feedback bandpass filter required two capacitors, but I only see one. Should I increase the value of C4? Schematic is attached.

 

[abbey road d enfer]

Hi, I haven't posted in quite a while but this board has been a lifesaver in the past! I need a variable mid boost guitar pedal for a recording project. The JHS Haunting Mids is surface-mount and not conducive to modding, so I'm going to build the "Malificent Mids" clone and lower the frequency range somewhat so it goes down to 200 Hz instead of 500 Hz. The pedal has Volume, Frequency, and Gain knobs and switchable Q.

I'm having some trouble understanding the schematic. I thought a multiple feedback bandpass filter required two capacitors, but I only see one. Should I increase the value of C4? Schematic is attached.

But there are two capacitors! C3 and C4.
BTW, it's not a multiple feedback filter. It's a Wien-bridge derived filter.
You could increase the value of both.
Beware that, as per schematic, the lowest setting is 1kHz, not 500.
If you really want to go down to 200 Hz, you have to replace them with 7.5nF; since they probably don't exist, you may use the next higher value, 8.2nF, or with 10nF for a minimum frequency of 150Hz.

 

[Fenris]

Thanks! I used to know the filter types but I'm rusty. A minimum frequency of 150 Hz sounds good. If I change C4 to 10 nF, what should I change C3 to?

 

[abbey road d enfer]

Thanks! I used to know the filter types but I'm rusty. A minimum frequency of 150 Hz sounds good. If I change C4 to 10 nF, what should I change C3 to?

Same. they are of equal value in a Wien bridge.

 

[Fenris]

It looks like they used unequal values for C3 and C4 to increase the Q and gain. It makes it more complicated, but I can try some different values.

 

[abbey road d enfer]

It looks like they used unequal values for C3 and C4 to increase the Q and gain. It makes it more complicated, but I can try some different values.

OK, my mistake, I didn't see they were different.
So actually, with the original values, the lowest frequency is 350Hz. If you're content with this shape, you can increase the cap values to 3.3n and 22n.

 

[Fenris]

The pedal works great! It has some hiss and I'd like to try running it off 18v. All the caps are rated for 35v. Do I need to change anything in the power circuit?

(Upload isn't working, had to use imgur.)

https://imgur.com/a/8jMH3P3

 

[abbey road d enfer]

The pedal works great! It has some hiss and I'd like to try running it off 18v.

Running it at 18V won't improve noise. What do you expect?
5532 may not be the best choice for Hi-Z sources. You may want to try OPA2134, both for TL072 and 5532.

All the caps are rated for 35v. Do I need to change anything in the power circuit?

No.

 

[Fenris]

Thanks, I'll try the OPA2134. Does that come in a dual version?

 

[abbey road d enfer]

Thanks, I'll try the OPA2134. Does that come in a dual version?

2134 is a dual. 134 is a single and 4134 a quad.