This time I’m going to look closer at the part of the effect pedal that really shouldn’t affect the sound in any way, but still is essential for proper functioning. Let’s start with the least cool but maybe most important part of the build: the power section!
Because the LFO is going to be based on a schmitt trigger (square wave generator) there is a major chance there will be a ticking noise in the pedal, unless the power section is executed properly. My first version of the tremolo (the hardly, but still, working one) had a major issue with this. Back then I had installed a potentiometer that seamlessly mixed the square and triangle waves (I now consider mechanical switching between the two waveforms) and unless I used only the triangle, the ticking noise from the LFO was almost louder than the audio signal. Enter the RC-filter!
This filter is connected between the power source and the power input the the circuit board. Basically it’s a frequency dependent voltage divider. If the signal is strictly DC (could be considerd AC at 0 Hertz), no current will flow to ground through the capacitor. As the frequency grows the capacitive reactance through C will decrease, and this way higher frequencys will be more dampened. This will keep the voltage at the output as even as possible. Suitable values for R and C are 100 ohms and 100 microfahrads.
In the case of the LFO, the schmitt trigger will momentary draw a lot of current as it changes between its two states. An ideal power source would be able to keep the voltage constant under any conditions, but real power sources have a tendency to lower the voltage when the current draw is large. These short voltage drops tend to spread through the audio circuit as loud ticks. The RC prevents this to some extent. The capacitor stores some charge and thereby keeps up the voltage when the power source isn’t able to keep up the voltage at an even level. The resistor on the other hand puts a limit on the total current that can pass through the system and thereby also on how fast the capacitor can “reload”. This might sound counter-productive but as it turns out, the combination of these two filters out most of the LFO-ticking.
Another good idea is to place a diode reversed between the voltage input and ground. This prevents voltage of wrong polarity to pass through the system as it will conduct any negative voltage to ground.
I’m going to use separate RC-filters for the LFO-section and the audio section (seriously, this is how annoying I find the ticking!), so the whole power section will look something like this:
At last, a block diagram of the whole circuit:
Next post will be all about the in- and output buffers. Stay tuned!
What's wrong with this circuit? 