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This is an Advanced Build with many uses and capabilities.
Not recommended for beginners.
We have modified the Circuit to what we believe enhances it's capabilities.
Board Dimensions (W x H) 2.15” x 2.45” ca.54.6mm x 62.2mm
We still recommend a 125B or larger enclosure for less experienced hands.
Tonmann Layout and build info here:
SWAH Build Documents <- Older versions
New Versions see below for Version Vn
Check the Forum for answers to many questions before posting a troubleshoot thread.
Do not miss the Easy Pedal Wiring Guide.
* TR1 - The bias trimmer controls the filter resonance frequency.
When Sensitivity is turned fully off the Bias can be used as a sweepable filter.
We found as you have found out this is a very minor change and not really worthy of an extra potentiometer knob on the enclosure.
If you wish to make it external that is always easy to do, but that is up to the individual.
If it sounds good with your rig then leave it there and then you are left with the more useful
Note: If you have a now very old V3 board and not V3B then you will need to run a single jumper wire along the backside of the board. That's it.
For further consideration:
SENSITIVITY: Sets the filter trigger level, tune this carefully to fit your guitar/bass output and your playing touch.
You can further change the sensitivity from your guitar volume knob while playing.
BIAS: Controls the filter resonance frequency. When Sensitivity is turned fully off the Bias can be used as a sweepable filter.
RESONANCE: Controls the sharpness or Q-factor of the filter.
DECAY: Controls how fast the filter frequency falls back to resting point (that is set with the Bias control). This can be set fast (CW) so you get the wah effect on every note or slow for a more traditional auto wah sound.
PLAYING: Please take time to get used to the controls and as this is a voltage control filter it will react to your playing touch. The envelope control circuit is carefully designed to follow the dynamics of guitar/bass and it might take some time to master the Auto Wah playing technics.
Try also adjusting guitar volume between 8 and 10 to further expand use."
Another thought. I have two types of BC550 transistors, one set is labeled BC550B, the other is BC550C . While is is not specified on the BOM, I find that the circuits I have built that call for BC550 do better with the "C" version. Presumably the hFE is somewhat higher. BC550 is shown as hFE 420-800, although I don't know where the two fall within that range.
*Again we stress just like a normal wah, pedal placement is critical.
Do not place anything in front of it or you could lose 50%- to 60% of its range. Also depending on guitar pickups, cable length, impedance, etc.. a quality buffer can be helpful. We sell a quality 3PDT Buffer Board.
Here are a couple youtubes:
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%)
I hope the following will give people a general idea of what is happening in the circuit:
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".
In regards to 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 Sensitivty 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 the above 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.
And just to show you what a clever monkey I am, 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.
Think timing, think capacitors !!!!
In this circuit the time it takes to switch Q2 on (attack) and off (decay) is determined by the amount of time it takes C8 to charge and discharge.
C8 is charged from the output of IC2D through R13 and D6 and is discharged to ground through R27 and VR4.
If you reduce the value of C8, it will charge and discharge faster. Socket C8 and lower the value to something you like, you can then make the C8 switchable either on vero board or one of Barry's 2PDT switch boards.
Another thing you can try is to change D6 from a silicon diode to a germanium diode, this will allow C8 to start charging once the voltage across D6 hits 0.2V instead of 0.6V - this will speed up the attack
You could also reduce R27 to speed up the decay time when VR4 is set to minimum rotation. If the decay time at maximum rotation is too much then reduce the value of VR4.
If you need to increase the sensitivity, reduce R18 - 10k perhaps is a good starting point
In early 2016 we introduced the newest version of the board which is slightly smaller to fit more easily inside enclosures but is built exactly the same. It is called version Vn.
There are a few changes in comparison to the schematic above that divides the sections into colors. Since that chart is helpful I will list the changes from the colorful schematic above to the schematic in the current build documents below. The changes are cosmetic and have nothing to do with how the board functions:
WAS - NOW
C8 -> C9
R27 -> R25
D6 -> D3
R18 -> R17
R13 -> R24