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Hello all,

I'm building a DOD440 clone, the one by GGG, and I want to implement some mods. One of them is a three-way switch to select between lowpass, bandpass, and highpass filtering.

Does anyone have any advice as to where I should tap into the circuit? Perhaps C6 and C7? I'd really appreciate the help!

Here's a link to the schematic: http://www.generalguitargadgets.com/pdf ... 440_sc.pdf


I also want to add a resonance control, which I believe can be done by putting a 100K pot in series with R12.

Can't help but I'm interested to see where this goes as I've been looking for a nice, versatile auto filter to build...

yes i would also say c6/c7 is the go to. try opening c6 and or shorting c7.

At a cursory glance, C6 and C7 are worth messing with. Canonically, a filter with the ability to do LP/BP/HP is something like a state variable filter - which could be envelope controlled with an OTA implementation or using VACTROLs.

stradivari310 wrote:At a cursory glance, C6 and C7 are worth messing with. Canonically, a filter with the ability to do LP/BP/HP is something like a state variable filter - which could be envelope controlled with an OTA implementation or using VACTROLs.

I would disagree. a LP is a resistor and a capacitor, HP is a resistor and a capacitor, BP is two resistors and two capacitors. where in this post did he say that he requires voltage control? as a general rule, every guitar pedal every made has high pass filters formed at the input at the output by having a cap in series to the jack tip. so all we need now to make a band pass filter is a low pass filter present anywhere in the pedal. this can be in the feedback loop of an inverting opamp configuration. or this can be a simple cap to ground. it may be harder to see it but a BMP tone stack contains caps to ground. any negative feedback on a transistor will decrease bandwidth even if it is done through a resistor. you may find that there are multiple LP, HP filters and therefor BP filters in every guitar pedal ever made. at the extreme, we could even argue that no guitar pedal can amplify anything in the MHz range if you really want to discuss what is definitively a HP/LP/BP filter.

woops! I just read the schematic. I think my first response was maybe missing context sorry. this is a vactrol filter, you can definitely convert low pass to high pass but that might be all you can get. you can get really fancy with a dual vactrol that you have in this circuit but you might need to make a lot of changes to get a switchable LP/HP/BP. have you seen this mod done before? why don't you have a link to where you saw it?

eatyourguitar wrote:woops! I just read the schematic. I think my first response was maybe missing context sorry. this is a vactrol filter, you can definitely convert low pass to high pass but that might be all you can get. you can get really fancy with a dual vactrol that you have in this circuit but you might need to make a lot of changes to get a switchable LP/HP/BP. have you seen this mod done before? why don't you have a link to where you saw it?

the circuit already is a bandpass filter. It looks like IC1B is setup as a wienbridge oscillator/filter.

c6 in llel with r9 is the low pass and r10 in series with c7 is the high pass portion of the filter in the +ve feedback loop.

so maybe shorting c7 results in an lpf and opening c6 results in a hpf...

...

Bartimaeus wrote:
I also want to add a resonance control, which I believe can be done by putting a 100K pot in series with R12.

yes this is also ok but at some value of r12 it will just become an oscillator and oscillate even with no input. this should happen around a gain of 3 or higher, it looks like the gain is already very close to 3? (430/220 + 1) ~ 2.95 ? but im not sure, experiment and see. r12 is the place to hit though or r11 (make r11 smaller or r12 bigger).

Thanks for the replies, everyone! I've been busy, but I've had enough time to build the basic clone... only problem is that it doesn't work. It passes signal fine, but there's no filtering – I think I may have fried the dual vactrol when I hit it with the iron (the edge of the vactrol is visibly melted) when wiring up the pedal. I'm going to try to sort this out, and once I do it'll be on to the mods!