There are actual technical reasons why the performance of the L and R RCA outputs of the X3600H is so good. These outputs have a better design IMO than any other recent model Denon or Marantz AVR or AVP. There is nothing exotic or particularly expensive involved in the design, but it is different. The following explanation is as simple and short as I can make it, but you still likely will have to invest time and thought in understanding it. I’m just a hobbyist so there isn’t anything really challenging here.
There are two major areas of difference between the X3600H L and R RCA outputs and the outputs of other D/M AVR’s and AVP’s.
o Connections to the NJR audio switches and volume controls.
D/M uses the NJR 8-Channel volume control NJU72343.
https://www.njr.com/electronic_device/PDF/NJU72343_E.pdf
Go to page 15 in the datasheet, look at the upper left corner graph, In A1, Out A.
Note that the THD+N at 2V output at 1kHz is about .0005 or -106dB. This is the input used by the L channel directly from the DAC circuitry (DAC IC plus opamp-based filter). The R channel uses a similar input.
The graph also shows performance at other frequencies. The next graph down shows performance at various input levels and attenuations.
D/M uses the NJR 7-in/3-out Analog Switch to route analog signals before and after the volume control.
https://www.mouser.com/datasheet/2/294/NJU72750_E-779912.pdf
In the X3600H the L, R DAC output signals only go through the volume control and not the switches. In all of the other recent, higher-level D/M gear the L and R signals also go through at least one switch and in some cases, two switches.
This website shows measurements from NJR taken when the volume control and switch were first introduced.
http://www.siliconsolutionstream.it/958/
Note that at 2V output the THD+N at 1kHz is .001 or -100dB,
Using these THD+N ratings the best most of the D/M units can do is -100dB vs -106dB in the X3600H.
Noise and distortion accumulate as a signal goes from the output of the DAC IC to the RCA output, so the ultimate performance will of course be worse for either component. The X3600H has a 6dB lead in this horse race. Note that the rated performance of the basic DAC IC in any of these D/M products is at least -106dB, so better DAC IC’s make little to no difference.
o Design of DAC opamp filter circuitry
For a reason only known the Denon the X3600H has the best DAC filter circuitry for the L, R, and C channels of any of their upper level AVR’s or AVP’s.
The DAC filter circuitry contains an opamp in a differential amplifier configuration that combines the + and – signal from each channel from the DAC IC. This circuit functions to boost the signal level from 1V (-1V) on each leg to 2V, remove common mode noise, and remove remaining noise from the DAC process. The design of this circuit is fundamentally (exactly) the same on all D/M units. It uses an NJR 8080 opamp of very modest quality, and associated resistors and capacitors.
o Resistors
Most Denon AVR’s appear to use 5% tolerance, thick film surface mount resistors. Thick film resistors have excess noise and distortion above the normal Johnson noise in a resistor. The tolerance of the resistors affects how well the common mode noise is removed.
- Except -
The X3600H which uses 1% tolerance, thin-film surface mount resistors in ALL of the DAC filters for all channels. This type of resistor now costs little more than thick film resistors and has no excess noise or distortion. The higher precision means better matching between resistors which improves the reduction of common mode noise. Marantz has adopted these resistors on some of its most recent models including the AV7705 for example. At current prices there is little added cost to use these resistors. Better resistors make an improvement, everything helps.
o Power supply filtering
The L, R and Center channel DAC filters have an extra filter on the negative supply rail for the opamps. This will reduce the amount of noise that is transferred from the opamp to the audio signal. No other D/M AVR or AVP that I’ve seen has these filters. The rail voltages for the opamps are (+,-) 8V vs. 7V in other D/M gear. This increase in voltage is to account for the voltage loss in the DAC negative supply rail filter, but provides slightly better performance for all the opamps in the X3600H, and the signal switches and volume controls which also use this supply. Another contributor to better performance.
For interest: Similar filters are used in the power amplifier channels for the SR8012, X8600H and X6500H for (-) rail of the input and voltage gain stages.
Summary:
The L and R RCA outputs of the AVR-X3600H, when disconnected from a power amplifier channel, measure better than those in other D/M products. There appear special features in the design of the X3600H that make the X3600H design different from the design of other D/M units These differences in design would appear to lead to better measured performance and may account for the superior performance of the X3600H.
The added cost of the special features in the X3600H is likely minimal. It is unclear why Denon (or D/M) chose to use a superior design for the L and R channels of the X3600H, a relatively modest product in the Denon lineup.
Note: If this post appears familiar, I previously posted this piece on the AVSForum.