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

Would any harm be done by using a Y-splitter to simultaneously connect a passive pre-amp and tubed pre-amp to the same power amp? The idea would be to use the passive for volume control and run signal from a CDP to the power amp. The tubed pre would be used for sending other inputs (e.g., phono) to the power amp. All devices would be "powered up" at the same time but only one input at a time would be used. Thanks in advance.
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.

-- Al
I'm sure someone here can give you a better technical explanation, buy I wouldn't do it. With everything powered up at once, the both sources would interact with each other. You need to keep them separate. A better solution would be to get a passive that accepts 2 or more inputs, and use that to switch between the CD player and pre amp. The downside would be that the passive will be in the signal path of the preamp, and you'll have to set the volume on the passive whenever you need to use your preamp. You would have to do some experimenting to see what setting works best.
Al, Zd - THANK YOU both very much.

Al, I was concerned about a potential interaction and absolutely hadn't a clue what the nature of that interaction might be. Appreciate your explanation.

Zd - I think you have given me a good alternative. I'm thinking about getting the Schiit SYS. This does have 2 inputs. What I've read so far seems to indicate the Schiit is pretty transparent so might be OK even if it's in the tubed pre signal path all the time. BUT given what Al has said, not sure what effect if any the passive might have on the tubed pre-amp. If I got it, would have couple of weeks to figure out if it's OK given Schiit's return policy; maybe more if I got it through Amazon.

Al - I'd certainly appreciate your comments about Zd's alternative.
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
Al - That's a lot for a non-EE to digest. Thank you for the detailed additional input. Much to ponder. Might be simpler to just live with things as they are; i.e., limited usable volume control range on pre-amp with CDP (but things do sound great).
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.

-- Al
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Al - Yes. Looking to extend the range (and "fineness") of volume control which most of the time now is limited to between 7 & 9 o'clock on my pre-amp with TEAC CD3000 as input. I was aware of the Rothwell attenuators. They were my first inclination but I was put off by reports of negative effects on sonics. The fact you found them satisfactory carries some weight with me. I understand the product option you are suggesting. The attenuators would certainly seem to be a more elegant solution than another box or two and more interconnects. As others have stated, you are a real asset to this "community". Thanks once again.
If you use your CD player with your preamp, why not try lowering the volume on the CD payer. That should give you more range when using the preamps volume control. volume control on the CDP. It's 2.6 Vms out unbalanced and 4.6 Vms balanced. That's why I had been thinking to skip the tubed pre-amp and go passive with the Schiit SYS.

If I get this right with what you want to do.

What I would do is put all your sources into the inputs of the active preamp so you can switch between them, then come out of the tape output of the active preamp into input of the passive preamp and then out of the output of it to the poweramp/s input.

Basically this way your using the active preamp as a source selector.

Cheers George
Thanks for the idea George. What I am trying to do is introduce volume control
for the CDP without any addition boost in gain while leaving other sources
unaffected that run through the tubed pre-amp. Your idea does seems like
something to explore. No tape out...but sub out. However, I am going with Al's
suggestion of attenuators. Ordered a couple of Harrison Labs -12dB RCA plugs
from Parts Express. Much less expensive than those from Rothwell and Parts
Express allows returns if they don't work out. Others on A'gon do report
satisfactory results using these. Thanks again to all for the suggestions.
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
PlanB: place the passive between the CDP and the preamp input.
Adjust to suit, but keep your cables short as passives have been shown to be quite sensitive to cables!
Al - Have multimeter. Will measure.

Atmasphere - Good one. That would allow me to do what Zd suggested 6 posts above yours. He was thinking my CDP had a volume control (at least, that's how I read it). Re passives' sensitivity to ICs, I assume this would be to ICs on the output side of the pre-amp?

Everyone - thanks. Lots of stuff to try.
More so on the output side than the input, but they are sensitive to the input side as well. As a general rule of thumb keep the cables short!
"Atmasphere - Good one. That would allow me to do what Zd suggested 6 posts above yours. He was thinking my CDP had a volume control (at least, that's how I read it). Re passives' sensitivity to ICs, I assume this would be to ICs on the output side of the pre-amp?"

Sorry. I wasn't mistaken, just confused. I was commenting on a few different threads about digital sources with volume controls. The brands were Esoteric and Wadia. And since they both use Teac transports, I somehow allowed my fingers to get the best of me.

The IC's and the passive would be on the input side of the preamp, not the output. Depending on how much room you have on your shelf, you may be able to get away 1 pair of IC's and 1 pair of jumpers. You'll only need to set the volume on the passive once. After that, you'll just use the one on your preamp. Just put the passive behind the CD player close enough to use jumpers. Also, if you don't have really deep shelves that extend too far behind the components, you can probably get away with a 1/2 meter IC. That will keep the lengths down to a minimum, as well as cost.
Al - Harrison Labs -12dB attenuators arrived today. Running them at the input to my tube pre-amp with CDP as source. They seem to work as advertised. I've picked up (depending on how loud the CD is) 30-60 degrees of additional volume knob rotation to get to my preferred listening level; almost 90 on an older CD that has always seemed "lower volume" to me. So far, I don't notice any negative sonic effects. Thanks very much for the suggestion. I haven't yet made the resistance measurements but will.
Al - Results (all as read) of the measurements you suggested making for the Harrison Labs 12dB attenuators:
Across input/output pins
#1 attenuator 6.77 ohms
#2 attenuator 6.75 ohms
Across ground sleeve & output pin
#1 2.19
#2 2.18
all measurements made using 20K scale setting
FWIW - resistance across input pin to ground sleeve is ~9 (as read, 8.97 & 8.95 respectively).

Al - not sure what multiplier to use for this scale...x10K or x20K?. Let me know if you will. Thanks for your time.
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
Al - thanks for the reply. I meant to specify the meter used in my earlier note but it slipped my mind. It is a Craftsman Model 82015. I'm thinking the 20K setting I used is good for measuring resistances in the range of 20-200K ohms. The next lower setting is for 2000 ohms. The next higher for 200K. Not sure why things are "denominated" in 2s vs 1s. Sorry if I introduced some confusion - there is no 'x' symbol associated with the scale labels for the meter dial. What multiplier to use was my question only. Let me know if you find something helpful - or other thoughts. Thanks as always. (I well might repeat the measurements taking care not to hold the leads while doing so...I hadn't taken this precaution earlier - but am hopeful they won't change things significantly if current values are sort of "as expected).
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
Al - thanks for the follow up. I did some on-line reading after posting and found similar info as in your explanations for interpreting the measured impedance values and scale upper limits being multiples of 2. I can indeed take some further readings using the 200K scale setting. As noted previously, putting the Harrison Lab attenuators on the TEAC CDP outputs did give more usable volume control "range" on my pre-amp. 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.
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
A follow up on use of the -12 dB attenuators. The Harrison Labs attenuators have indeed been beneficial in the system. They provided extra control range on the pre-amp volume control when the CDP was in use as noted earlier. I did NOT think I heard a sonic benefit in this application however, just easier fine control of volume...but no negative effects, either. Recently, I had been bothered by what seemed like upper frequency distortion when playing at higher volumes (mid to upper 80 dB at the listening position) CDs ripped to hard drive. ICs from CDP to pre are Cardas Parsecs. ICs from the Gungnir DAC to pre are Morrow Audio MA-4s. Today after switching the attenuators to the DAC ICs at the pre-amp, I enjoyed a very satisfying listening session with no trace of harshness or distortion at sound levels similar to what had been annoying 2 days ago. I'm wondering if the attenuators are doing a little "tone control" - rolling off the top end just a very little bit (by increasing the impedance of the MA-4s?). Not 100% certain the cause but did order another pair of the attenuators from Parts Express so I won't have to swap them out between CDP & DAC. We'll see if listening continues to be as pleasant and free of harshness going forward. (BTW - Al, I did come across similar values as what I measured being reported on Amazon for the Harrison Labs attenuators).