Technical Articles and Editorials

AVR - Audio Video Receiver - Build Quality: Part I

ARTICLE INDEX

Improved Distortion and Noise Performance with Balanced DAC Output

All but the lowest cost DACs provide two outputs per channel. Each output actually comes from two separate DACs. One DAC output is out of phase with the other. This is called a balanced signal. A circuit called the balanced to single ended converter is connected to the balanced outputs of the DAC IC.

The balanced signals are subtracted from each other to form a single-ended output. In the process of converting the balanced analog signal to the single-ended output, some distortion produced by each DAC, in the balanced pair, is partially cancelled. Comparing the THD levels for single ended and balanced output DACs shows significant improvement for the balanced DAC. This will be seen on the chart below. In addition to improving distortion, balanced DACs may also show an improvement in SNR level.

The balanced-to-single-ended converter is also designed to provide the final reconstruction filtering in the analog domain. This low order filter removes folded tones that remain after the digital reconstruction filter. They are typically above 100 kHz and low in level, so only low-order filtering is required to remove them. Additional details on this analog filter and its typical frequency response can be found at the following link:

http://www.hometheaterhifi.com/integrated-amplifiers/integrated-amplifiers-reviews/harman-kardon-hk-990-stereo-integrated-amplifier-with-digital-room-correction-and-dual-subwoofer-bass-management-part-ii/page-8-hk-990-backend-analog-circuitry.html

The analog reconstruction filter must be in the signal path for single ended and balanced DACs. Since the reconstruction filter uses an opamp in all but the lowest cost (below $350) AVRs, only a few passive parts are needed to add the balanced-to-single-ended conversion function.