How does a DOD 440 filter work?

[Josh?]

Hi everyone!

I plan to build and modify a DOD 440 (http://www.generalguitargadgets.com/pdf/ggg_ef440_sc.pdf) soon, but to do that, I need to know how its filter (from R2 to the output of IC1B) works. Does anyone know anything about how it works?

Also, am I correct in assuming that the VTL5C4/2 is made up of a single LED and two regular LDRs, with a pin of one connected to a pin of the other?

Thanks!

[mth5044]

I can help with the second question.

https://www.alliedelec.com/m/d/350b8613fa7c1e545249ca9cecf75d88.pdf

Looking at the data sheet, it could be interpreted as one LED and two photocells, but those photocells have two of their legs tied together. Could also look at it as one centertapped photocell. I guess they are functionally the same

[Kipper4]

Welcome Josh.

Google up multiple feedback filters and compare it.
I might be along the right lines.

[GibsonGM]

First question:  IC1A 'samples' the incoming signal, rectifies it at the output to DC, and the resulting varying DC evelope controls the brightness of the LED in the optocoupler. 

IC1B at bottom is an active filter.  The resistance portion of the optocoupler is controlled by the varying light of the LED, causing the filter sweep that you hear as the 'wah' sound.   It is like turning a tone control knob (or rocking a wah pedal), just done electronically, and of course with more depth and vocalness due to the way the active filter is set up.    This is the jist of it, as simple as I can lay out.

Hope that helps you! Welcome to the forum.

[PRR]

It is a Wien Bridge.

Drawn funny.

[ElectricDruid]

Hi everyone!

Hi Josh, Welcome!

I need to know how its filter (from R2 to the output of IC1B) works. Does anyone know anything about how it works?

PRR is right, it's a Wein bridge. The best page about them is this one:

http://sound.whsites.net/project150.htm

This talks about parametric EQ, so they're interested in cut and boost, but for a wah, we only want boost, so that removes one control and simplifies things a bit. Figure 3 is pretty close to the Dod440 filter.

The Wein Bridge makes the op-amp have gain, but only at a specific frequency.
The design is (a) a bandpass filter, and (b) has 6dB rolloff on both slopes.

I think this Electrosmash article is useful too - it shows the way the "Wein bridge" is built out of two similar highpass and lowpass shelving filters:



(From https://www.electrosmash.com/boss-ce-2-analysis - you can skip most of it except the bit about shelving filters)

The LDR resistance (both of them) are moved into the right range by putting a resistor in parallel with them - that's R9 and R10. C6 and C7 are the two caps. R7/R8/C5 set up a "virtual earth" midpoint bias voltage for the bottom of the feedback divider. The input is also biased to this midpoint level through R3. This drops the level to half because R2/R3 make a voltage divider, so then the R11/R12 combination add a gain of roughly two to get it back to where it was.

Also, am I correct in assuming that the VTL5C4/2 is made up of a single LED and two regular LDRs, with a pin of one connected to a pin of the other?

Yes, you are. mth5044 said you could also regard it as a centre-tapped LDR, which is true, but remember that it's not like a "LDR pot" despite the central tap. If it were, the resistance of one bit would get larger while the other got smaller, and that's not what it does. Both bits get smaller or both bits get bigger. The centre-tap stays in the centre in both cases, at least within LDR tolerances.

HTH,
Tom

[Mark Hammer]

I built mine using a homebrew dual-LDR unit.  It's been a few years since I made it, but I think I had a square LED sitting around that had been removed from an old cassette deck or something.  The flat sides permitted the LDRs to be placed beside the LED, and affixed snugly.

It's a nice vocal-sounding filter in my experience.

[Josh?]

Thank you for all of the information, everyone! Tom's links are also super helpful. Regarding what he said, however, I have one question:

The design is (a) a bandpass filter, and (b) has 6dB rolloff on both slopes.

What about a filter determines its rolloff?

Thanks again everyone!

[ElectricDruid]

The design is (a) a bandpass filter, and (b) has 6dB rolloff on both slopes.

What about a filter determines its rolloff?

Without going into a lot of complicated filter theory, basically "the number of RC pairs it has". Each pair gives 6dB/oct rolloff. If it's a lowpass filter and you can see two resistors and two capacitors, then it's a 12dB/oct filter (2 x 6dB = 12dB). If you can see three pairs, it's a 18dB/oct filter. Similarly for highpass.

For bandpass filters, the situation is slightly different because you have to have a lowpass and a highpass slope, so two pairs of RCs means that you have a 6dB slope at the low end and a 6dB slope at the high end.