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David Rick · 19th April 2008

Actually converters often do have sweet spots. It's usually related to the analog circuitry of the ADC input or the DAC output stages, but there are also digital causes. Here are some possible reasons:

The designer has put 0 dBFS too close to the supply rails so that some opamp is in distress at high levels.
The gain-bandwidth product of an opamp is reduced at high output swings. With less loop gain, it is less linear.
Above a certain signal level, a discrete analog stage transitions from class A to class AB.
Very low levels put the signal too close to noise floor.
Some device has a fixed amount of crossover distortion, which is relatively less at higher signal levels.
High signal levels modulate the supply rails, causing intermodulation distortion.
A switchmode power supply's operating mode depends on the current demand. It may change operating frequency, skip cycles, or develop a subharmonic loop oscillation. The noise spectrum on the supply rails may change dramatically, and this may affect devices powered from those rails.
The modulator of a high-order sigma-delta loop is prone to instability above a certain signal level.
The decimation filter in a PCM converter overloads, even though all its input samples are below full scale.

Many of these causes can be reduced or eliminated by careful circuit design. Therefore one may expect that better converter sets may be less particular about signal level than garden-variety "prosumer" stuff.

In my own experience, my Lynx II cards prefer not to be driven especially hard. In this case, I think it mostly has to do with the opamp supply rails.

David L. Rick
Seventh String Recording

19th April 2008	#16
David Rick Lives for gear Joined: Mar 2004 Posts: 561	Actually converters often do have sweet spots. It's usually related to the analog circuitry of the ADC input or the DAC output stages, but there are also digital causes. Here are some possible reasons: The designer has put 0 dBFS too close to the supply rails so that some opamp is in distress at high levels. The gain-bandwidth product of an opamp is reduced at high output swings. With less loop gain, it is less linear. Above a certain signal level, a discrete analog stage transitions from class A to class AB. Very low levels put the signal too close to noise floor. Some device has a fixed amount of crossover distortion, which is relatively less at higher signal levels. High signal levels modulate the supply rails, causing intermodulation distortion. A switchmode power supply's operating mode depends on the current demand. It may change operating frequency, skip cycles, or develop a subharmonic loop oscillation. The noise spectrum on the supply rails may change dramatically, and this may affect devices powered from those rails. The modulator of a high-order sigma-delta loop is prone to instability above a certain signal level. The decimation filter in a PCM converter overloads, even though all its input samples are below full scale. Many of these causes can be reduced or eliminated by careful circuit design. Therefore one may expect that better converter sets may be less particular about signal level than garden-variety "prosumer" stuff. In my own experience, my Lynx II cards prefer not to be driven especially hard. In this case, I think it mostly has to do with the opamp supply rails. David L. Rick Seventh String Recording