System Cable Capacitance Question from a New Audiophile

I came across a 6 moons review that claimed that it was 50 k ohms which is why I began experimenting with a passive pre. The pascal class D module has a low input impedance natively, however red dragon has a custom input stage that mediates this. I think that should be a high enough value to interact properly with the phono pre.

Good!

I read somewhere that a TVC with a low setting on the volume knob results in lower output impedance.

True.

This would be reversed for a resistor based passive.

Well, not exactly. The impedance "looking back" into the output of a passive preamp will depend on the output impedance of the source, as well as on the impedance of the preamp itself and the setting of its volume control. And as I mentioned in my previous post the output impedance of a source component that has a coupling capacitor at its output, such as most tube-based source components (and tube-based preamps) may rise to considerably higher values at deep bass frequencies than at the mid-range frequencies at which output impedance is usually specified.

And in the case of a resistance-based passive preamp output impedance will tend to be highest when its volume control is set somewhere in the middle part of its resistance range, and lower than that when it is set closer to either of its extremes.  Exactly where the highest impedance occurs will depend on the output impedance of the source.

Also, that output impedance will be lowest when the control is set for max volume **if** the output impedance of the source is small in comparison with the overall resistance of the passive preamp. If the output impedance of the source is particularly high compared to the overall resistance of the passive preamp (which would be particularly undesirable if the output impedance of the source varies significantly over the frequency range), the output impedance of the passive preamp may actually be lower near the bottom of its volume control range than near the top.

If you may be interested in pursuing all of that further a good place to start would be reading up on Thevenin’s Theorem.

Would it make sense to choose a type of preamp based on the interaction between your source and power amp (will the volume knob be in the low or high regions) or are other considerations more important?

Assuming the input impedance of your amp is in fact 50K or thereabouts, and if you are satisfied that your phono source can provide enough voltage to utilize enough of the power capability of the amp without additional gain, I would say that the main criterion would be the quality of the particular passive preamp. I’ve never used a passive preamp myself, but I know that some here have been happy with Promitheus TVCs, among others. And as you realize there are a goodly number of suppliers of well-regarded resistance-based passive preamps.

Finally, if you opt for the resistance-based approach, before choosing a particular resistance value it would probably be a good idea to contact Pro-Ject and see if they can tell you what the worst case (highest) output impedance of the phono stage is at any audible frequency, which will probably be at 20 Hz.

Good luck. Regards,
-- Al

Al, thanks again for the explanation. I find the impedance interactions fascinating even if I do not completely understand them. I’ll read up on that link. I really like the clarity of passives but have concerns about the impedance swings having audible affects on the system.

Last night I was doing some critical listening with my Roomate and we both agreed that while using the sys, the most dynamic sound was when the volume was set under 35-40% on the knob. The system played much louder but micro dynamics in the music seemed to slowly roll off.

That being said, I’ve been trying to figure out which kind of passive volume control I want to implement as an upgrade to the sys. I came across the Tortuga L3.V25 which is an LDR device and lets the user calibrate the preamp and set specific output impedance to test for a value that works best in ones system. This seems like an interesting option.

I know this thread has deviated from the initial post, I should have perhaps clarified my initial point but sometimes you never know where these threads will go! Appreciate the time and thoughts Al.

Blue jeans LC 1 interconnect ++1, I use expensive ic but this cable is almost 90% of what my highend Cables and it’s only $37.

AJ, based on comments I’ve seen in the past and on its description and specs I suspect the Tortuga L3.V25 would be an excellent choice. And as you mentioned its user-configurable impedance is a great feature, especially given the uncertainty about the output impedance of your phono stage. It wouldn’t surprise me, though, if the Tortuga’s default setting of 20K turned out to be optimal.

Regarding your finding that volume control settings on the Sys that were below 35-40% provided best dynamics, I don’t find that to be surprising. In addition to the subjective effects of volume changes on perceived dynamics, that setting will result in a significantly lower output impedance than the maximum value the impedance would reach at a much higher setting. And hence less effect on high frequencies due to interaction of that output impedance with the capacitance of the preamp-to-power amp cable than at that higher setting.

In that regard, I’ll add to my earlier statement that...

In the case of a resistance-based passive preamp output impedance will tend to be highest when its volume control is set somewhere in the middle part of its resistance range, and lower than that when it is set closer to either of its extremes. Exactly where the highest impedance occurs will depend on the output impedance of the source.

... "the middle part of its resistance range," corresponds to a volume level that is only about 6 db less than max. Which for a typical rotary volume control probably corresponds to only around 45 degrees of rotation below the maximum setting. So the relatively low output impedance of the Sys that presumably occurs at the max volume setting (assuming low source impedance) will increase very rapidly to its max value as the control is turned down from that position, but will progressively decrease from there at progressively lower settings, and will be significantly less than its maximum value at or below 35-40%.

Finally, regarding BlueJeans LC-1 which two of the others suggested, note that it has particularly low capacitance (only 12.2 pf/foot), much lower than the capacitance of many cables that are vastly more expensive! Signal Cable also makes modestly priced good quality interconnect cables having capacitances that are almost as low.

Regards,
-- Al