15 x12.5mm 1/4" 6.35mm Round Shaft Multicolor Pedal Effect Metal Aluminum Pointer Potentiometer Control Knobs

What’s Wrong with Digital Volume Controls?

Many audiophiles have systems comprising only digital sources. That may be a CD player or, increasingly, a computer connected to a DAC. More complex systems have multiple means -- e.g., disc transport, computer, and network streamer -- of feeding digital data to a single box for converting all those ones and zeros into the analog waveform required by the amplifier. In such cases, a new type of component has come to the fore: the digital preamplifier. These devices are not actually preamplifiers, in that they’re not providing an amplification stage. Instead, they offer source selection and volume control, so that they can be connected directly to a power amplifier with no need for the traditional analog preamp. Often, but not always, the volume control is implemented in the digital domain, which is what gives some audiophiles pause in using these devices as intended by their designers. Does a digital volume control really represent a sacrifice in performance?

Audiophiles first became acquainted with digital volume controls on CD players more than two decades ago. The first examples operated with a 16-bit word length -- the same as the "Red Book" CD standard. That meant that any reduction in volume also reduced the resolution and dynamic range of the source signal. Whether or not these effects were audible, this method was often discarded on theoretical grounds: If the sufficiency of CD resolution was already in question, then further reducing the resolution was a nonstarter.

The next step was volume controls that operated in the 24-bit domain. Since most playback material was still 16-bit, a volume reduction of 48dB -- quite a lot -- sacrificed none of the content of the original digital signal. With the obvious theoretical challenges overcome, 24-bit digital volume controls started to gain traction. Now that 24-bit sources are readily available, 24-bit volume controls seem unacceptable. Fortunately, volume controls with 32-bit or better precision are finding their way into DACs at a variety of price points.

That would seem to be the end of the story. However, there is more to digital volume control than truncating bits. Just lopping off unneeded bits is a very crude way of adjusting the scale of a digital signal. As the volume is reduced, the use of a proper dithering algorithm is necessary, not to introduce highly unpleasant quantization distortion into the reconstructed signal. Some of these algorithms are better than others, and the results can be audible.

Of course, the digital signal must eventually be translated into the analog domain, and that is where the understanding of digital vs. analog volume controls becomes more complicated. The crucial parameter is dynamic range, which is the relationship between the peak amplitude of the audio signal and the underlying noise floor of the circuit. When a DAC with a fixed output is followed by a passive attenuator -- e.g., a potentiometer -- the peak signal and noise are reduced by the same amount, so that the dynamic range remains constant. With a digital volume control, the peak signal level is reduced, but the underlying noise remains the same -- consequently, there is a reduction in dynamic range. The degree to which this loss of dynamic range is a problem depends on the inherent noise level of the DAC.