I'm trying to build a 'panner' to mix two audio signals, basically a dry/wet potentiometer for an audio effect. It works as intended when I use a dual power supply like this:
But stops working when I try to use a single power supply like that:
Any ideas?
Try adding input coupling capacitors and a wiper decoupling capacitor like this: -
This prevents the new DC mid-rail of 6 volts being affected by ground referenced input voltages and the ground reference of the pot affecting the DC gain of the circuit.
You should also use a capacitor across the left-side 100 kΩ resistor AND you should use an op-amp that is capable of working with a supply rail limited to 12 volts of span.
Something in the region of 1 μF to 10 μF will probably work for all the capacitors. I don't recommend electrolytic caps.
In the second schematic, the non-inverting input is referenced to Vcc/2, while the inverting input is referenced to GND. There is a 6 V DC difference in potential between the two opamp inputs. Trying to amplify this is saturating the output stage.
To fix this:
Connect the pot wiper to the junction of the two 100K resistors. << Reference Designators! >>
Add two decoupling capacitors from that node to GND, a 0.1 uF ceramic and a large value electrolytic. This node has to look like zero ohms at all audio frequencies. The electrolytic cap value is based on the lowest possible combined impedance of the pot and the four R resistors.
Add decoupling capacitors across the opamp power supply pins. Analog Devices often shows two caps in parallel, a 0.1 uF ceramic and a 10 uF electrolytic.
Add input coupling capacitors.
