Orners Corner

While verifying the dynamic range of an audio compressor project, I needed a convenient way to rapidly switch between +10 dBu, 0 dBu, and -10 dBu reference levels without constantly adjusting the signal generator.

The goal was to establish a fixed reference level and then instantly move above or below that reference by precise amounts. Although originally developed for audio testing, the circuit has many additional applications in signal conditioning, instrumentation, and interfacing between consumer and professional audio equipment.

The circuit can easily be adapted for:

• Professional to consumer audio level conversion
• Precision gain and attenuation switching
• Instrument calibration references
• Signal path testing and verification
• Analog front-end experimentation

The basic form of a resistive voltage divider follows the equation below:

Resistive Divider Equation

Referring to the non-inverting amplifier configuration, we can see that this same divider network appears between the op-amp output and the inverting input as part of the negative feedback system.

Figure 1 — Non-Inverting Op-Amp

Non-Inverting Gain Equation

Therefore the gain equation of the non-inverting amplifier becomes the inverse of the resistive divider equation.

A slight modification of the standard non-inverting op-amp by adding a 3-way switch into the feedback loop provides three separate operating modes from a single circuit.

• Voltage follower driving a resistive divider, providing controlled attenuation
• Unity-gain voltage follower for reference level operation
• Standard non-inverting feedback amplifier providing gain equal to the inverse of the attenuation ratio

Figure 2 — Gain / Reference / Attenuation Circuit

Figure 3 — 3-Way Switch Configuration