Omnidrive - rat

[l_w_b]

The omnidrive is the very first pedal I have built and I am very pleased with it but I was wondering if it was possible to swap the filter switch with a pot to try and give it the capabilites of a rat pedal.
Thanks
Liam

www.geofex.com/PCB_layouts/Layouts/omnidrv.pdf

[Mark Hammer]

Hi Liam,

The short answer is yes....with some no thrown in.

R6 and R7 form a simple 2-pole lowpass filter in conjunction with C3 and C4. R8 and R9 set the gain and Q of the filter to provide a little bump at the corner frequency.

R6/R7 can be made variable by replacing them with a dual-ganged 100k pot. Stick a 10k fixed resistor in series with each section so that at max treble there is always a minimum 10k resistance to sub for R6/R7. Because the rolloff frequency will be higher due to the reduced pot value, C3/C4 should probably be bumped up to 1500pf or 1800pf to compensate. Alternatively, if you can find a dual-ganged 250k pot, instead of the much more widely available 100k, you can then leave the caps the same. Since such a variable filter can achieve the same bandwidth as having the circuit bypassed, you can omit the switch if you opt for a variable control.

The "no" part is because of the placement of that filter. Filtering and eq-ing BEFORE a clipping locus will produce different outcomes than the same filtering/EQ placed AFTER it. In the case of the Omnidrive, the filter is unfortunately placed before any clipping circuitry, which means that the tone-shaping you expect from a Rat will not be attainable in this context, or at least using the posted layout.

If the Big Muff stye tone control shown (C8-10, R18-20) is not to your taste, let me suggest you consider the Stupidly Wonderful Tone Control (http://img.photobucket.com/albums/v4...mmer/SWTC.gif). This will function muh like a Rat style tone control, without requiring a buffer or eating up too much signal.

Here's what you do:
1) Remove C9, C10, R19, R20.
2) Replace R18 with a 470R to 1k resistor.
3) Unsolder the wire linking the Blend control to the Tone control and reroute it to the point where the Tone control used to tie to R19/R20/C9.
4) Run a cap from the wiper lug of the Tone pot to ground. If the chassis of the pot is grounded, you an siply do what they do in guitars and solder the cap between the wiper lug and chassis.

Done!

[l_w_b]

Sorry for my naivety but I dont know much about electronics (we havent covered it in physics yet) but will this turn the blend pot into the filter pot and when you say remove resistors and capacitors do I just remove them or do I put resistor leads where the used to be.
thanks Liam

[Mark Hammer]

No need to apologize for what you haven't learned yet.

The suggested change replaces the BMP-style tone control with a Rat style, and changes nothing else about the functioning of the circuit. (I.E., the Blend pot will still pan between clean and distortion as before)

The Big Muff type control essentially pans/blends between two different signals. One is a severely treble-cut version, and the other is a severely bass-cut version. Set to the middle, there is a slight midscoop but full bass and treble. Turning it in either direction gets you more of one and less of the other.

The Rat-style control always retains full bass, and simply trims back on the amount of treble by rolling off the treble lower and lower in the frequency spectrum. Because it is a rather shallow filtering action, you still get some buzz coming through even when it is set to what seems, on paper, like a ridiculously low cutoff frequency.

In the Rat, there is a FET stage in between the Filter control and the Volume pot. This lets them operate independently of each other. If you skip that FET stage, volume levels depend on the interaction of volume pot and tone control. What I did with the SWTC is to make volume level independent of where you set the tone cntrol without having to require a FET stage between them.

This will not produce the robust tonal changes and revoicings that the stock Big Muff type control does. Not by any stretch. But I find most often that people like to be able to round off or warm up their distortion tone, the way they do on their guitar with that tone control, and this suggested change will do that. Try it out, and if you find it doesn't do enough, switch back. The nice thing is that it does not require wholesale destruction of the board.

[l_w_b]

Thanks so much.
There is no way I would have come close to thinking of any of this for myself.
Liam

[Mark Hammer]

Originally Posted by l_w_b

Thanks so much.
There is no way I would have come close to thinking of any of this for myself.

Well that's why we need forums, isn't it?
I'm still not overly thrilled with the Omnidrive, but it's a nice little learning lab for people starting out. Hope these suggested changes make it a go-to pedal for you, or at least a now-I-know pedal.