DAC with WM8740


Posted on Feb 7, 2014

It is high performance DAC with oversampling, delta-sigma architecture and maximum sample frequency 192kHz/24bit. It has balanced voltage output. It is connected to high-quality operational amplifier which has low-pass filter function and buffer with single-ended output. Board also includes complete power part of circuit except transformer. In supply part I used two bridge rectifiers similar to other my dac boards.


DAC with WM8740
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For supplying of analog and digital part of dac I used low-drop voltage regulators LE50. For supplying of operating amplifier I used common 78L12 and 79L12 voltage regulators. We must use transformer with two secondary windings with 12 to 15V. Winding for positive supply branch AC1 can be paralelly connected to S/PDIF decoder when we haven`t individual winding for them. All voltage regulators are properly blocked with ceramic capacitors 100nF and on the output filtered with electrolytic capacitors 47uF/16V. DAC and operational amplifier have also blocked all supply pins according to datasheet with ceramic capacitors 100nF and small electrolytic capacitors 10uF/16V. DAC is set to HARDWARE mode with pin MODE. It means, that settings are changed with setting right pins to LOG 0 or 1. With pins CSBIWO and ML/I2S is configured data format for I2S/24bit. This format is most common and is supported on all S/PDIF decoders and USB soundcards with I2S output. DAC has balanced voltage outputs. To them is connected 2nd low-pass filter with dual operational amplifier with very low distortion OPA2134. This amplifier simultaneously converts balanced output to unbalanced. Schematics of filter is adopted from AK4396 datasheet. On the output is connected small resistor which could prevent oscillations when not ideal load is connected. The output doesn`t need coupling capacitor, when parts are properly selected. I measured DC-offset voltage...




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