I used to think passive preamps were superior to active preamps given right the setup, but

^^ the above statement is false if taken as a generalization. You may refer to my last two posts as to why.

But if for example the source component has a coupling capacitor at its output, resulting in a substantial rise in output impedance at deep bass frequencies, or if the output impedance varies significantly in other parts of the spectrum for whatever reason, using that same passive preamp may result (depending on the specific numbers that are involved) in objectionable irregularities in frequency response and/or phase response.

This comment by Al gets to the issue that I often hear about- when a passive is used, bass impact suffers (I hear about this far more often than a high frequency rolloff, for which George supplied pretty good numbers earlier in this thread).

This is because many sources, analog and digital, use an output coupling capacitor. The series resistance added when a passive is inserted into the signal chain can exacerbate the increased output impedance at low frequencies caused by the coupling capacitor. This explains why the bass often has less impact at any setting below full volume.

This is not to say that this happens in all cases! But it happens often enough that the passive/active 'conversation' is not likely to go away anytime soon.

Saying that I hate caps in the signal path, give me direct coupling any day over it, as all caps have their own sound as well.

That is why we have a direct-coupled output on our preamps.

Here is a little tutorial on capacitive reactance. It bears directly on this conversation:

http://www.electronics-tutorials.ws/filter/filter_1.html

Now think about adding a variable resistance out the output of a capacitor (which already has reactance). The resistance of the divider network that comprises the passive volume control is in two parts- the part in series with the capacitor and the part that goes to ground. You can see that the series portion is being added to whatever reactance the capacitor already has. As you can see from the tutorial, reactance values are higher at lower frequencies (several examples are provided). What this means is that the capacitor has to be a **lot** larger in order to not experience a significant increase in effective output impedance if a passive control is used!

The tutorial does describe the effect of an added resistance- immediately it begins to talk about filters, which is what this thread is about as well.

Designers of audio electronics like to keep the coupling caps used to values that are as small as possible for several reasons. One reason is that the cap will introduce more coloration if it is made larger. Another reason is that the coupling cap should not express a timing constant that is lower than that of the power supply that is running the circuit. If this rule is broken, the circuit can exhibit low frequency instability, which can result in increased IM distortion and in severe cases, 'pumping' and 'motorboating' effects. So it is important to avoid arbitrarily larger coupling cap values- they **have** to adhere to proper design rules.

As a result, I think it incorrect to characterize a source as having an insufficient coupling cap value when what we are really talking about here is standard accepted engineering principle. This is why even a solid state source (like our Oppo CD player in our shop) can have an audible loss of bass impact when a passive volume control is used at anything other than full volume.

I regard this as a coloration effect- and is why I prefer a **properly designed** line section, as it can avoid this problem without adding coloration of its own. I do stress emphasis however on the **properly designed** caveat.