Low pass filter (EHX POG)

[fuzzo]

Hi,

I'd like to know if you know some scheamtic to build a Low pass filter like the one in the POG ?  it's really cool , and maybe it would be better to have it in a wah wah pedal to control the frequency cut with the foot. 

There's something close in the Boss ME50 called "resonnance filter" which I really liked .

I looked into the POG schematic but it is really messy and hard to find a specific part (plus I think even that part in digital).

thanks !

[aron]

What you want is a VCF filter like in a synthesizer. The closest thing we have is something like the Oberheim VCF or much simpler, the DOD Envelope Filter or MXR Envelope filter.

[clsnacky]

Parapedal?

[fuzzo]

Hi,

I went seeing what the oberheim is and looks a little bit different , no ? 

the dod and MXR are more close to a "auto-wah" enveloppe filter. I built a dod 440 and that's not the same sound.

Actaully I want to keep the same effect the pog has but housed in a wah shell. 

[DougH]

The HOG has an expression pedal which can be used, in addition to other things, for controlling the frequency of the filter.

You want an active filter with a steep cutoff slope and frequency that can be varied via variable resistance.

[fuzzo]

You want an active filter with a steep cutoff slope and frequency that can be varied via variable resistance.

I think so , yes. But isn't similair to a simple low pass filter, it acts like a Wah , boosting a part of the frequencies.

But like aron said we're more in synth world than guitar effect.

Anyway, I'm still looking for a scheamtic close to that kind of effect.

[Taylor]

The HOG/POG is a digital emulation of a Moog-like filter, which is a 4-pole filter with negative feedback from the output of the last stage to the input of the first stage. It's a tough build, but you can find the schematic for the Moog ladder filter all over the net. The resonance, AKA negative feedback, is what gives that wah-ish boost at the cutoff frequency.

If you want a simpler build, you can put 2 MS20 filters in series, then do an inverting opamp buffer from the output of the last stage, and connect the output of this buffer to the input of the first filter.

http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/sk.gif

[fuzzo]

yes that's the think I'm looking for ! thanks Taylor !

Actually I thought about the Moog filter but I never heard it , in real, so I wasn't sure.

What's the particularity of the LM13600 (in the schematic)  ?

[Taylor]

yes that's the think I'm looking for ! thanks Taylor !

Actually I thought about the Moog filter but I never heard it , in real, so I wasn't sure.

What's the particularity of the LM13600 (in the schematic)  ?

Hmm, I don't understand what you mean by "what's the particularity" - could you say this another way?

[WhenBoredomPeaks]

Coincidence but i finished the MS-20 filter this morning. It's bottom is lower than the guitar frequencies but i think it could go a bit higher, i feel like i lose highs even when the freq is maxed. I use it with a guitar.

[fuzzo]

yes that's the think I'm looking for ! thanks Taylor !

Actually I thought about the Moog filter but I never heard it , in real, so I wasn't sure.

What's the particularity of the LM13600 (in the schematic)  ?

Hmm, I don't understand what you mean by "what's the particularity" - could you say this another way?

Sorry,

I meant why using that chip ? it does something special ?

[caress]

it's an OTA (Operational transconductance amplifier)  http://en.wikipedia.org/wiki/Operational_transconductance_amplifier
some more info, although i'm sure someone more knowledgeable will chime in.

[Taylor]

Yeah, you can't use a regular opamp. But if you can't find the LM13600, the LM13700 works the same.

[fuzzo]

How use a regular opamp instead of the OTA ? it seems really necessary in the schematic .

Thanks Caress, it cleaner for me now.

Otherwise, any other similair schematic for something like that ?

[Taylor]

You can not use a regular opamp in this schematic. You must use a dual OTA chip like 13600 or 13700. The 13700/13600 is a dual version of a CA3080, so if you couldn't find the right chip, you could use four 3080 chips instead, replacing pins 7+8, and 9+10 with darlington buffers.

[fuzzo]

Ok , I never used that kind of special IC.

I tried to find some other schematic (in synth world) without results.