I am curious. One of the purported problems with passive pre-amps is impedance mismatch between source and amplifier. There are those who say that as long as the source output impedance is low enough and the amplifier impedance is high enough that all is good. And then others will say that even with this condition satisfied that there is still a loss of "drive". The big selling point of TVCs is that they better manage the impedance mismatch.
I don't know how an LDR relates to this impedance mismatch issue. Is it simply a means of creating resistance(much like a resistor) but which more evenly passes all frequencies. Or does it somehow also improve the "drive" like a TVC is purported to do. I guess I am really wondering if this notion of lack of "drive" due to impedance is a misplaced notion of why passives may be inferior to actives. It would also imply that actives are on the wrong path to optimal sound also as they seemed to be focused on improving this "drive" issue. Could it be that the real issue is simply one of the linearity of the resistance?
I very much appreciate this comment because it highlights the central issue surrounding passives generally.
Anecdotally, you can find published user accounts on various forums etc. which excoriate passive (resistive) preamps as lacking drive and dynamics, having flabby sound, the list goes on....or to sum it up...they suck! I strongly suspect that the root of these disappointments with passives generally is the impedance mismatch issue possibly compounded by sources with low line stage voltage and/or very limited current "drive" capacity. So if you couple a source with a weak line stage and high'ish output impedance (say 1k) with an amp with a low input impedance (say 15k) and then place a passive resistive attenuator between these 2 components where the attenuator's impedance varies from 50k+ to 1k- to 50k+ over the 0-max volume range, chances are you might not like the results. These components probably need a buffer between them, i.e. an active preamp.
What I've experience subjectively (as have practically all of our customers so far) is using our LDR passive between a contemporary source (decent line stage voltage/current plus reasonably low output impedance - say 500-800R) and an amp having an input impedance of say 40-50k or higher, the impedance mismatch issue is effectively made moot. A handy reference number is if you have amp:source impedance ratio of ~50:1 or higher, you're highly likely to have a very satisfying listening experience. No lack of drive or dynamics.
Now lets take a look at the DC (0 HZ) impedance curve that is typical of our LDRx attenuators. The first pic below shows the raw test data from a build of our LDR3x. It plots the series and shunt resistances for both the left and right side LDRs. Resistance is plotted logarithmically. Now check out the 2nd pic showing this same data but with resistance plotted linearly. Notice the distinct bathtub shape of the resistance curves that are more clearly evident in the linear plot. As it turns out we only need the very high series resistance on the left side and the very high shunt resistance on the right to achieve the last few dB of attenuation on either end of the attenuation curve. Normal listening is done mostly in the middle range where the impedance is relatively low - say a few K - and relatively flat. Another view of this same data is shown in the 3rd pic which shows the series/shunt resistances as dB of attenuation.
Please note that this dB vs. step curve is NOT the dB vs. step curve of the LDRx. We pick points along this curve to build the 70 step audio taper curve that gets programmed into each LDRx with each step being close to ~1 dB. See the last pic for our typical audio taper curve.
