Charles Eric LaForest, PhD., GateForge Consulting, Ltd.
A better power supply and voltage bias source for audio circuits using TL072 (and TL074) op amps.
Effect pedals most often have a single 9V power supply, so the input signal must be biased upwards so it is only positive over its entire swing, and that swing must also stay inside specified limits on the input and output range of the op amp.
The two limits we care about are how close the output of the op amp can get to the power supply rails before clipping, and what is the common mode input range, outside of which the behaviour of the op amp is not guaranteed to be linear (or even stable!). Since we will have op amp outputs feeding op amp inputs, we have to find a voltage range that stays inside both limits.
For the TL072 op amp fed by a 9V supply, the output can get to within 1.5V of the positive rail, or 7.5V, and the common mode input range spans 4V to 9V. So our usable voltage range is 4V to 7.5V. This places the mid-point bias at 5.75V, so we can have signals reaching up to +/-1.75Vp around the bias voltage, which is well into the "line level" range. Given we start with "instrument-level" signals (about 50-100mVp), that's plenty of room if we keep an eye on cumulative gain.
Here is a circuit which takes in a 9V supply and generates the supply and bias we need. (I am omitting the usual RF and decoupling capacitors for clarity.) The 9V input passes through a fuse (a resettable one would be best) and a normally reverse-biased 1N4148 diode to provide reverse voltage protection. The voltage divider then generates a 5.79V bias voltage (close enough!) whose Johnson noise is filtered by a 1uF capacitor (the value is not too important) then buffered by a TL072 op amp.
This circuit protects against reverse supply voltages without a diode forward voltage drop, which maximizes our usable signal range, and creates a bias voltage source which is extremely insensitive to load ("stiff"), without having to use a large electrolytic capacitor. Otherwise, dynamic loads on a plain voltage divider will cause its voltage to fluctuate and introduce significant distortions later on, even with a large capacitor.
You can try out this circuit in the interactive CircuitJS1 simulator: tl072_supply_bias.cjs1