I seriously experienced the problem of zero crossings on my old preamp volume control card that I made with mechanical relays. Whenever I changed the MSB, a serious plop explosion was coming from the hops. Of course, this was also affected by the mechanical relays being at an uncertain logic level during the flight between the contacts.
Coming to the problem; yes, in order to minimize the problem in MSB transitions, I set the minimum dist level with a small value trimpot parallel to the 2R resistor in the section where the highest bit is. The low pass filter at the output is not very effective. Because I do not keep it at a point close to the audio frequency.
For this reason, it is not a good idea to do software volume control in r2r dacs at the input.
Coming to the problem; yes, in order to minimize the problem in MSB transitions, I set the minimum dist level with a small value trimpot parallel to the 2R resistor in the section where the highest bit is. The low pass filter at the output is not very effective. Because I do not keep it at a point close to the audio frequency.
For this reason, it is not a good idea to do software volume control in r2r dacs at the input.
The practical solution I found is not to control the volume before or within the DAC, but to handle it with a conventional preamp at the DAC output.
By then its too late to correct for intersample overs. Those are signals which after reconstruction exceed 0dBFS. Also, for many or most dac chips, distortion can be substantially reduced if the digital input is pre-attenuated before the dac by maybe 6dB to 10dB. Beyond that initial pre-attenuation, then volume control in the dac chip or after it is probably preferable. Also, analog volume pots can be a dominant source of distortion in otherwise low distortion systems. Goldpoint stepped attenuators may be the best choice for volume control after the dac but they are not especially low cost.
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