Passive attenuator impedance question

Your analysis is well done! And accurate to a very close approximation.

The DAC will see a load impedance of 9K + (1K in parallel with 100K), which is extremely close to 10K.

The amp will see a source impedance (the impedance seen "looking back" from its input) of 1K in parallel with (9K + 100 ohms), or about 0.9K, which in terms of impedance compatibility is slightly better than the 1K you assumed.

The one other consideration is that if the DAC utilizes a coupling capacitor at its output, which is particularly likely if its output stage is tube-based, its output impedance may be significantly greater at deep bass frequencies than the specified 100 ohms. But even if that were the case, the nominal ratio of 100x pretty much assures there won't be an issue.

Good luck. Regards,
-- Al

Regarding "bit stripping," I would not extrapolate experience with Wadia models to models from other manufacturers, and perhaps even between different Wadia models.

The manual for the Wadia 121 states as follows:

Does the Wadia Volume Control compromise resolution?

The Wadia 121 Decoding Computer uses the latest generation of Wadia’s proprietary digital volume control. The volume level can be varied in the digital domain by means of mathematical manipulation of the signal, eliminating the distortion and noise that are inevitable with even the best analog volume controls. While conventional thinking indicates that reducing the volume digitally can sacrifice low level resolution, Wadia has created an innovative solution. Wadia’s patented digital filtering algorithm produces a 32-bit output. This high-resolution signal is then used in the computations that in turn reduce the volume level. This new signal is fed directly to the DAC chips. Through this innovative method, the Wadia 121 Decoding Computer maintains high resolution even at the lowest volume control settings.

While the manual for the 321 has no such statement but says:

A Volume Setting of 88% on the Wadia 321 will provide optimum performance when it is connected to a Preamplifier (Integrated Amplifier or Receiver).

Best regards,
-- Al

07-06-15: Davehrab
The IC from your passive attenuator should be very low in capacitance and as short as possible to keep from rolling off the highs ... Al can expound on this

Thanks, Dave. Good point, which is certainly often a major consideration when a resistance-based passive attenuator is being used. In this case, though, given that the cable is being driven from an impedance of less than 1K (about 900 ohms, as I mentioned earlier), I would not expect the capacitance of that cable to result in audibly significant effects under most circumstances (i.e., unless cable length is particularly long and capacitance per unit length is particularly high).

Also, the OP may wish to consider constructing the attenuator such that it can be connected directly to the input jacks of the amp, with no intervening cable. That is how the Rothwells and other such fixed in-line resistive attenuators are typically used. Although eliminating the cable, or at least keeping the cable short and using one having low capacitance, assumes much greater importance with something like the Rothwells, because the resistor values they employ are much higher than the 9K/1K values the OP will be using. Low values such as 9K/1K are suitable for his particular application because of the low output impedance of his DAC.

Something not mentioned or considered is the shift in impedance as the V/C is turned up and down and again Al can easily explain this better than I

In this case, though, note that the resistive attenuator would be fixed, consisting of just a pair of resistors. The digital volume control in the DAC would continue to be used.

... we don't know anything about the V/out of the source and your V/in of the amp.... Al can expound on ... the V/out V/in through the passive

But note the OP's reference to "the desired 20 db attenuation." Presumably he has determined that to be what is necessary.

Best regards,
-- Al

Gmudunuri, I suspect that either of those approaches would work well, but I'd suggest going with the three resistor approach. Not only because it would use one fewer resistor per channel, but because of some probably very minor but perhaps significant theoretical advantages.

One being that the four resistor approach may cause or contribute to signal current flowing in the ground connection between the two components (i.e., in the shield of the balanced cable), which may to some extent counteract the potential advantages of a balanced interface. Another being that with the four resistor approach if the exact values of the two 1K resistors are not well matched the result might be a slight degradation of common mode noise rejection. In the three resistor configuration the exact value of the 1K resistor is non-critical. You would still want to purchase 4.5K's having relatively tight tolerances, though, perhaps 1%.

There are also differences of a factor of 2 in impedance between the two configurations, both looking forward from the DAC (favoring the four resistor configuration), and looking back from the amp (favoring the three resistor configuration). But I don't think either difference would be significant with your particular components.

Good luck. Regards,
-- Al