Originally Posted by nbp_civic
To bypass ADC just make sure pure direct or direct is used. DAC is used when inputs like toslink and coaxial digital is used. I've noticed that when using the 7.1 multi ch inputs, there is a slight buzz and higher noise floor than using a regular input such as CD analog in. Don't know why but it's like that for all my av receivers. Always get better results from all other analog ins compared to multi-ch-ins. The DAC I use is a emotiva xda-2 gen2.
Most people will not notice a difference between the 2 inputs. I'm just very picky with noise floor and other line interference. This is why I use Toslink SPDIF instead of USB with my DAC.
From what I've seen in some Marantz AVR's and AVP's, and this is like true of other manufacturers as well, it makes sense that you would hear differences between various inputs.
In the gear I've seen, the overall volume is controlled by one or more eight channel, active, volume controls, such as the discontinued Renesas R2A15220 (a JRC NJW1299 is likely similar). These chips allow direct input of single-ended signals and can switch from one set of eight channel inputs to another and often preform more complex switching as well. The chips allow amplification of the signal if desired. They are low cost and are designed specifically for this purpose. The output from each channel is from an opamp that is internal to the volume chip.
Multi-channel inputs typically go directly to these chips, then to the output stage - often HDAM's in the case of a lot of Marantz gear.
Two channel inputs are landed ahead of an ADC on a switch where can go to the ADC or often directly to the volume control. This input is different from the volume control and may be of higher quality: lower noise and distortion since the two channel inputs are presumed to be from higher quality sources.
The ADC output is routed to the DSP chips along with any digital inputs; HDMI, Toslink, etc. The DSP chips do their thing, but don't do overall volume control because this would have a negative effect on the signal to noise ratio, and would also increase distortion. Feeding an attenuated signal to the following DAC chips, which have OK, but not exceptional S/N ratios, would lower the S/N ratio of the whole unit. A digitally attenuated signal will also increase the % distortion from the DAC chip, especially if it is attenuated by 10dB or more.
The digital signal is then sent to one or more, eight channel or two channel DAC's, depending on the unit. These are voltage output DAC's with limited head room. The signal goes from the DAC's to the volume control(s) and then to the output stage as described above.
All of this works quite well. The performance from relatively low cost units is amazing.