Pubul57: Does the 50kohm of my attenuator address that issue, or does it cause problems on the other end into my 100kohm amp load?
Herman's comments accurately addressed the impedance seen by the cdp looking into the resistive volume control, which as he explained is dependent on the rvc, it's setting, and the amplifier input impedance.
A separate issue, which I think is what the quoted question is addressing, is the relation between amplifier input impedance and the output impedance seen looking back from the amplifier into the output of the rvc. That will be dependent on the rvc, it's setting, and the source component's output impedance (if it is high enough to be significant).
With the volume control turned all the way down, the impedance looking into the output of the rvc (I'll call it Zo) will be essentially 0. With the volume control all the way up, it will be the parallel combination of the volume control's end-to-end resistance (in this case 50K) and the output impedance of the source component. If the source component has a low output impedance, such as 50 ohms, then the impedance of the combination will be essentially the same as the source component's output impedance.
Zo will be at a maximum when the volume control is set to the mid-point of its resistance range (which is NOT the 12 o'clock position; it will be just a few steps down from the maximum volume position). At that setting Zo in this case, where the rvc has an end-to-end resistance of 50K, equals the parallel combination of 25K with (25K + 50 ohms), or about 12.5K.
The ratio of your amp's input impedance (100K) to the 12.5K worst case output impedance that is driving it is 8:1. That is slightly short of the so-called 10x rule that you are no doubt familiar with, but considering how close it comes to satisfying that rule, and that the ratio will be better at any other volume control setting, that all seems comfortable.
The other criterion that Zo should satisfy is that it should be considerably lower than the capacitive reactance (which is an impedance, measured in ohms) of the cable that connects the rvc to the amp, at the worst case frequency within the audio band, which is 20kHz.
That number can be calculated by taking the capacitance per foot of the cable, multiplying by the number of feet, and plugging into the formula Xc (capacitive reactance) = 1/(2 x pi x f x C), where f is 20,000 and C is capacitance in farads. The resulting Xc will be in units of ohms.
For example, a 5 foot cable having a low capacitance of 20 pf (picofarads) per foot results in Xc = 1/(2 x 3.14 x 20000 x 5 x 20exp-12) = 79.6K, which would be a satisfactory result in relation to Zo = 12.5K.
Best regards,
-- Al
