The Mad Professor Snow White Auto Wah is a cool little envelope filter. Designed by Bjorn of BJF effects it always was going to be a decent design. Check out the demo video to see what this ice-cool pedal can achieve:
And a PCB layout by DimebuGG: http://www.mediafire.com/?wntjz1k5kj0jkjq
And a vero layout by Harald Sabro (www.sabrotone.com):
My7of9 (from http://www.guitarpcb.com), has made a great post detailing some mods/improvements that he has made to the circuit. I've summerised them here, along with the circuit block schematic he provided:
"Here are a couple Mods with an explanation from our site (GuitarPCB.com) which some really seem to favour.
Install a 1N34 germanium diode in D6 and a 1uf electrolytic cap for C8 and the thing really came alive.Sounds much funkier clean and before a light overdrive it cops a great Garcia-Like tone.
In Regards to adjusting R18 as well as much more here is the explanation.
R18 mod - short answer - increasing R18 will allow you to set the Sensitivity more towards the 12 o'clock position. I'd go with a 47kΩ resistor here. The only advantage is being able to turn the signal down a bit further (50% to 0% is better than 20% to 0%) at the expense of turning the signal up (20% to 100% is better than 50% to 100%)
The circuit can be broken down into five main blocks:
Input buffer stage (blue) a high impedance input to get as much guitar signal as possible going into the circuit and a low impedance output to drive the next two stages.
State variable filter (yellow). Without getting too techie, this is the part that goes "Wah". If you are interested, look at the application notes (Google NE5517 or LM13700 data sheet) - warning !! this can be rather boring for normal sane people.
While we need to keep the signal going into the State Variable Filter (yellow) quite low, a slightly higher level signal might be needed for the envelope follower stages (green, pink and lilac).
The "green" stage boosts the signal, the "pink" stage converts the signal into a DC voltage, the DC voltage is proportional to the signal coming from the "green" stage - higher signal level, higher DC voltage level.
The DC voltage output of the "pink" stage drives the transistors in the "lilac" stage into producing a control current (more DC voltage at the base of Q2 means more current), the control current, which is connected to pins 1 and 16, tells the State Variable Filter (yellow) how much it should "Wah".
Since the question is about R18, a quick look at the "green" stage might be in order.
Nothing more than a gain stage. The gain is determined by whatever the Sensitivity pot is set at divided by the value of R18.
If the pot is set to 50% rotation, the resistance value of the pot (for a 100kΩ linear pot) is 50kΩ. Divide 50kΩ by 20kΩ - the value of R18 - and you have a gain of 2.5. If the input signal is 100mV, the output signal is 250mV.
So how did I arrive at a value of 47kΩ in the short answer ?
It would appear from many posts that setting the Sensitivity pot at the 9 o'clock position gives good results.
Since "9 oclock" is about 20% rotation, the value of the Sensitivity pot will be about 20kΩ.
The gain is therefore pot value divided by R18 which is 20kΩ / 20kΩ giving a gain of 1.
If I set the pot to 50% rotation, the pot value is now 50kΩ and if I change R18 to 50kΩ (47kΩ nearest standard value), the gain is 50kΩ / 50kΩ which is also a gain of 1.
It is possible to leave the value of R18 as it is (20kΩ;) and change the pot to a 47kΩ pot and still achieve the same results.
There is one other mod possible - called "side chain" which allows you to place effects such as fuzz and distortion before the auto wah and achieve good results. The only disadvantages are that you will require a simple buffer / splitter circuit (if you have got a Tonmann ParaMix available from GuitarPCB.com you won't need the extra circuit) plus you will need enough space on the enclosure to install another jack socket."
Here's a nice internal picture of the Mad Professor Snow White Auto Wah:
Here, for reference is the freestompboxe.org topic: http://www.freestompboxes.org/viewtopic.php?f=7&t=7012