AUDIO AB SELECTOR WITH GAIN CONTROL
Explore the innovative technique of virtual masses, where signals don't pass through the switch, but the ground takes on the role of "Mute/De Mute." This project features clever circuitry using OP amplifiers and resistors to seamlessly switch between microphone and line inputs, with a built-in gain compensator for maximum control. Read on to learn how this project can also serve as a clever By Pass function for your audio effects!
Initially, this project complements my two Labs publications:
It serves as Micro/line switch using the technique of virtual masses. The advantage, the signals do not pass through the switch, it is the ground that makes the function "Mute / De Mute" Here is how it works:
The OP 1/2 5532 amplifier is mounted as a reversing sump. For the microphone to be active (IN A), the line (IN B) must be inactive. It is the role of R1 to put to the ground by a resistance of 10K. R2 is also at ground level, no current will circulate at node R2 R5 for line input (IN B). On the other hand, R3 & R4 being free, we can find in the output of 1/2 5532 a calculated voltage such as: -V Ina x ( R5/ R3+R4 ) or -V Ina x (22K/ 20K). For the line to be active (IN B), the microphone (IN A) must be inactive. It is the role of R3 to put to the ground by a resistance of 10K. R4 is also at ground level, no current will circulate at node R2 R5 for Micro input (IN A). On the other hand, R1 & R2 being free, we can find in the output of 1/2 5532 a calculated voltage such as: -V Inb x ( R5/ R1+R2 ) or -V Inb x (22K/ 20K). The second OP 2/2 5532 amplifier is a gain compensator of ± 10 dB.
The potentiometer is a model with a central knob. In this position, the gain is calculated as follows: - 20 log (R7 +10K) / (R6 +10K) or – 20 log (20K/20K) or 0 dB
For +10 dB, 20 log (P1+R7) / R6
For – 10 dB, 20 log R7 / (R6+P1)
If you want a ratio of +/- 6 dB, the values of R6 & R7 will be 20K
The output phase is respected since there are 2 inverter OP amplifiers in cascade.
This mount can also be used to assume the By Pass function of an effect if you cable, for example, IN A to FX IN & IN B to FX OUT
- https://www.elektormagazine.fr/labs/microphone-preamplifier-with-phantom-power
- https://www.elektormagazine.fr/labs/line-receive-with-rfi-and-dc-protect
It serves as Micro/line switch using the technique of virtual masses. The advantage, the signals do not pass through the switch, it is the ground that makes the function "Mute / De Mute" Here is how it works:
The OP 1/2 5532 amplifier is mounted as a reversing sump. For the microphone to be active (IN A), the line (IN B) must be inactive. It is the role of R1 to put to the ground by a resistance of 10K. R2 is also at ground level, no current will circulate at node R2 R5 for line input (IN B). On the other hand, R3 & R4 being free, we can find in the output of 1/2 5532 a calculated voltage such as: -V Ina x ( R5/ R3+R4 ) or -V Ina x (22K/ 20K). For the line to be active (IN B), the microphone (IN A) must be inactive. It is the role of R3 to put to the ground by a resistance of 10K. R4 is also at ground level, no current will circulate at node R2 R5 for Micro input (IN A). On the other hand, R1 & R2 being free, we can find in the output of 1/2 5532 a calculated voltage such as: -V Inb x ( R5/ R1+R2 ) or -V Inb x (22K/ 20K). The second OP 2/2 5532 amplifier is a gain compensator of ± 10 dB.
The potentiometer is a model with a central knob. In this position, the gain is calculated as follows: - 20 log (R7 +10K) / (R6 +10K) or – 20 log (20K/20K) or 0 dB
For +10 dB, 20 log (P1+R7) / R6
For – 10 dB, 20 log R7 / (R6+P1)
If you want a ratio of +/- 6 dB, the values of R6 & R7 will be 20K
The output phase is respected since there are 2 inverter OP amplifiers in cascade.
This mount can also be used to assume the By Pass function of an effect if you cable, for example, IN A to FX IN & IN B to FX OUT
Updates from the author