Design a Balancing Converter
In the latest addition to the Workbench column, we delve into the construction of an unbalanced-to-balanced audio converter. This project aims to provide flexibility in uses and setup, while keeping complexity and fabrication to a minimum.
The design of this converter is inspired by the RDL ST-UBA2 Two Channel Unbalanced to Balanced Amplifier, a widely referenced standard model for such conversions. The converter is designed to match the enclosure of the cable tester for a uniform look on the workbench.
The process involves taking the unbalanced input (usually single-ended RCA or similar consumer-level output), buffering and amplifying the signal to achieve the target professional level and impedance, creating a balanced output using a differential amplifier stage that produces two signals of opposite polarity, and including gain trimming controls to match signal levels precisely. Visual level meters, such as dual-LED VU meters, may also be included for monitoring signal strength.
The audio converter project advances the previous project by incorporating simple active circuitry. The first op-amp section in the device provides gain and can also generate negative gain to help achieve mic level output. A bridge rectifier is included to handle AC or reverse polarity power sources.
The circuit design for the unbalanced-to-balanced audio converter includes a DIP switch to change the output resistance between two choices. A 220 uF series capacitor isolates the audio in the converter, stabilizing the gain of the first active stage. A transformer can be connected to the balanced output for full isolation if needed.
The op-amp sections in the device are configured in the inverting format, providing great flexibility and stability. The + and - 15-volt operating point is a choice of the author as it produces a slight but measurable improvement in Signal-to-Noise Ratio (S/N).
The finished version of the audio converter has a remarkably flat frequency response with Total Harmonic Distortion (THD) no higher than 0.4%. RCA (phono) connectors were used on the unbalanced side, while XLR-3 males were used on the output for high-level broadcast inputs.
The power source for the audio converter is a wall-wart with an output of about 8 volts DC, but anything between 8 and 24 volts should work. A DC to DC converter follows and converts the regulated 5 volts into + and - 15 volts for use by the op-amps.
The project received a positive response from readers in the profession. The parts list for the project is provided at the bottom of the story.
Stay tuned for more projects in the Workbench column!
[Parts List] (List the parts required for the project here)
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