Passive & Tubed pre-amps to power amp at same time?

Not a good idea. The output impedance of the preamp that is not being used will be seen as a load by the preamp that is being used, and most likely at least one and probably both of them would be unable to drive that load with good sonic results.

The output impedance of the passive preamp will vary widely depending on whether it is resistance-based or transformer-based, and on the setting of its volume control, and on the output impedance of what is driving it. But the preceding paragraph is almost certain to be applicable in any case.

Regards,
-- Al

The Sys has a specified input impedance of only 10K, which will be too low for many tube preamps to drive with good results. For example, if like most tube preamps yours has a coupling capacitor at its output, its output impedance is likely to rise to high values at deep bass frequencies, which is likely to result in significant rolloff of the bottom octave or two with a 10K load. Also, the 10K input impedance of the Sys, as seen by the tube preamp, will actually be somewhat less than that depending on the input impedance of the power amp and on the setting of the Sys' volume control.

Perhaps the best thing to do is to get both a passive preamp not having a switch, and a separate line-level switchbox. The switchbox would select which of the two preamps has its outputs routed to the power amp, and would connect the outputs of the preamp that is not being used to nothing. A number of switchbox possibilities are suggested by me and others in this thread.

Also, regarding the passive preamp itself, and assuming that it is resistance-based (as opposed to the probably much more expensive transformer-based types), keep in mind that perceptible rolloff of the upper treble, and consequently sluggish transients, will result if the combination of cable length and capacitance per unit length of the cables at its outputs is not kept low. And if a switchbox is used between the preamps and the power amp, that would mean the total lengths and capacitances of the cables connecting the passive preamp to the switchbox AND those connecting the switchbox to the power amp.

Good luck! Regards,
-- Al

If what you are trying to accomplish is simply to increase the part of the volume control's range that is used for the CDP, consider trying a pair of Rothwell attenuators, inserted into the preamp's input jacks that are used for the CDP.

They are offered in a choice of 10, 15, and 20 db of attenuation, and balanced and unbalanced configurations. There have been reports of compromised dynamics resulting from their use in some systems, but many others (including me) have used them with fine results.

Regards,
-- Al

Ghosthouse, if you happen to have a multimeter it would be interesting to know the values of the two resistors that are presumably used in the Harrison Labs attenuator. The values aren't particularly critical, but knowing them may provide added confidence in their suitability for the application.

You would take two resistance measurements, one between the input center pin and the output center pin, and one between the output center pin and its ground sleeve.

I've measured those values in the Rothwell 10 db attenuator as 22K and 10K respectively, which seem like reasonable numbers for most (but not all) applications. Especially, as in your case, when used between a solid state signal source (presumably having low output impedance) and the input of a tube-based component (presumably having high input impedance).

Also, thanks very much for the nice words. Best regards,
-- Al

Those numbers are consistent with the specified 12 db of attenuation, and the measurements between the input pins and the ground sleeves of the output connectors correspond, as can be expected, to the sum of the other two measurements. The actual attenuation, btw, will be a bit greater than 12 db depending on the input impedance of the component into which the attenuators are connected, because that input impedance will be in parallel with the 2.19K/2.18K resistors, resulting in a combined impedance that is at least slightly lower than those values.

I'm puzzled, though, by the scale factor business. It shouldn't matter whether you use x10K or x20K, because the numeric reading would change accordingly. But multiplying each of the measured results by the x20K setting you used gives resistances that are much higher than I would expect, and higher than would seem to be desirable, even though they are in the right proportion to each other. If you indicate the make and model of the specific multimeter, it may allow me to shed some further light on this.

Also, I'll mention that when measuring relatively high resistances it would be a good idea to make a point of not having your fingers on the conductive part of at least one of the two multimeter leads. Otherwise the reading may be affected by the resistance of your skin and body.

Best regards,
-- Al

It looks like it is what is known as a 3-1/2 (three and a half) digit multimeter, which means that it can display 4 digits but the one on the left (the most significant digit) can be only a 0 or a 1, rather than any value from 0 to 9. So the maximum numeric reading it can display is 1999. That being one very tiny increment less than 2000, which explains why the scale choices all begin with 2. And the maximum possible indication with the 20K setting would be 19.99, corresponding to a resistance of approximately 19.99K (or approximately 19990 ohms). And, similarly, with the 200K setting the maximum possible indication would be 199.9, corresponding to a resistance of approximately 199.9K (or approximately 199990 ohms).

So as you can probably see the scale numbers are not multipliers. They are the maximum amount of resistance that can be measured on the particular scale, with the decimal point having to be adjusted by the user to get the approximate number of ohms. And as you can probably see the best resolution (and presumably the best accuracy) can be obtained by using the lowest scale number that is higher than the resistance being measured.

So it seems likely that the 6.77/6.75 and 2.19/2.18 and 8.97/8.95 numbers represent approximately 6.77K/6.75K ohms, 2.19K/2.18K ohms, and 8.97K/8.95K ohms respectively ("K" denoting thousands). Those numbers are all much less than the resistor values used in the Rothwells. That would work in the direction of making them an even better impedance match than the Rothwells with respect to the tube preamp, but a less good impedance match with the CDP. Although in this case that less good match would most likely still be good enough, given that the unspecified output impedance of this particular solid state CDP is most likely low. I would not drive those attenuators with many and probably most tube-based signal sources, though, as well as some lesser solid state components, because their higher and often frequency dependent output impedance would not do well when working into 8.97K/8.95K (and actually a bit less than that, due to the additional loading presented by the preamp or other destination component).

To be sure that the measurements are meaningful, though, and weren't taken on a scale that was lower than what was being measured, I'd suggest repeating the measurements on the 200K scale and verifying that the results are consistent. (Although I suspect that if the measurements were taken on a scale that was lower than what was being measured the meter would probably have given some sort of error or overload indication). On the 200K scale you'll probably see a reading of around 6.8 when measuring between the input and output center pins, corresponding to the 6.77/6.75 numbers on the 20K scale. And likewise for the other measurements.

Best regards,
-- Al

None of my sources are tubed so if those low resistance numbers aren't likely to harm the TEAC, I'll continue to use them.

To clarify, no harm (in the sense of damage) would be done even with the kinds of components I mentioned as being unsuitable for use with these attenuators. The issue with components having high output impedances, and/or output impedances which vary greatly as a function of frequency, would be adverse sonic effects, such as deep bass rolloff.

Best regards,
-- Al