I'm a complete noob with electronics, trying to design a guitar effects pedal. There is a bit of crackle when I connect a pair of headphones to the output, but the audio signal is still clearly audible. When connected to a valve amplifier, there is a much louder crackle, and it's constant. The crackle in both cases reminds me of what I used to think was dirty potentiometers, but now believe is related to varying the DC component in an audio signal.
The circuit looks like this
The VCA and buffer are from the VCA datasheet (though I've swapped OP275 for TL072)
The clean boost is from here
The microcontroller uses an LM78L05 for power, and the only common ground with the analog side runs through that LM78L05. The MCU supplies a control voltage to the Control Voltage Booster (another TL072), which boosts the voltage before sending it to the VCA (which has a virtual ground, hence the need to boost the control voltage).
What might be causing the huge increase in noise when connecting to an amplifier instead of headphones? My first thought was the impedance of the headphones versus that of the amplifier. But then I thought that the TL072 boost should be equally comfortable sending its output signal to an amplifier.
Here's the pedal, for reference. The screen is blank because, despite my ignorance with electronics, I have a pretty cool idea for the software, and I don't want to spill the beans before my product is ready to launch.