YourGuitarist
New member
It has recently come to my attention that the Common Mode Input voltage range of many opamp (especially TL07xs) substantially limits the voltage swing.
fpgacpu.ca
Despite this, many many pedal designs across experience levels still use 4.5V as a midpoint bias, which would limit headroom. For example, the TL072 has a recommended input voltage range of (V-)+4V -> (V+)+.1V. With a midpoint of 4.5V, you only have -+500mVp of headroom before clipping/undesirable behavior. With a higher midpoint of 7V, you'd quadruple the headroom to +-2Vp (+12dB).
Why is this not more common? Am I missing something here?
Additionally, in my experimentation, I've found that the voltage drops across virtual ground resistors tend to be substantial and proportional to the the resistance (Ohm's law ofc), outside of separating the source circuit from the bias point generating circuit, why are such high values used (M ohms). I often find myself feeding my bias voltage via 100-500k rather than mega ohms.
Finally, when a voltage drop is incurred across the midpoint feed resistor, should I consider my midpoint the value preceding the drop or following it? I see this as relevant for inverting opamps and IC's which accept a signal ground input.
This is my first post, so hello to the PedalPCB community! Let me know if there are some key figures to look out for. My main interest is in designing custom pedals from the ground up.
Thank you.
Modifying and Improving the Behringer BDI21
How to modify and improve the Behringer BDI21 to reduce noise, add bass overdrive, compression, and a mid boost, and optionally remove the high mid scoop.
Despite this, many many pedal designs across experience levels still use 4.5V as a midpoint bias, which would limit headroom. For example, the TL072 has a recommended input voltage range of (V-)+4V -> (V+)+.1V. With a midpoint of 4.5V, you only have -+500mVp of headroom before clipping/undesirable behavior. With a higher midpoint of 7V, you'd quadruple the headroom to +-2Vp (+12dB).
Why is this not more common? Am I missing something here?
Additionally, in my experimentation, I've found that the voltage drops across virtual ground resistors tend to be substantial and proportional to the the resistance (Ohm's law ofc), outside of separating the source circuit from the bias point generating circuit, why are such high values used (M ohms). I often find myself feeding my bias voltage via 100-500k rather than mega ohms.
Finally, when a voltage drop is incurred across the midpoint feed resistor, should I consider my midpoint the value preceding the drop or following it? I see this as relevant for inverting opamps and IC's which accept a signal ground input.
This is my first post, so hello to the PedalPCB community! Let me know if there are some key figures to look out for. My main interest is in designing custom pedals from the ground up.
Thank you.
