Preamps with two main-outs


Question for folks with two separate stereo amps driven by a single preamp. My preamp has two main outputs feeding two separate amps - one stereo and one pair of monoblocks, driving two pairs of speakers in two different rooms. Is it normal to need to turn on all the amplifiers to play any music? In other words, I can't just turn on my stereo amp to listen to music in one room. Its all or nothing. Fortunately the mono amps (MC2200) have volume control and I can turn them all the way down but can't do that for the stereo amp for the speakers in the other room. I was wondering if this is normal for all preamps with multiple pre-outs or its dependent on a particular preamp model. If it matters, the preamp is a NAD C165BEE. Thanks.

P.S. The second main-out has a little knob for gain control but its on the back of the preamp next to the ports and not practical to use for this purpose.

Convert?fit=crop&h=128&policy=eyjlehbpcnkioje1mdmxnjy3mzmsimnhbgwiolsicmvhzcisimnvbnzlcnqixx0%3d&rotate=exif&signature=53e30959bfe4386e5b718d3f8d96cbf59e274d7f4b2f87e0dbf41c028dc63549&w=128kalali
Yes it is normal!
I'd be concerned about out of phase backflow voltages coming from the turned off amp.  The electrons travel down the interconnect and when they reach the turned offed amp they don't know what to do -- they knock on the door, ring the doorbell, go around back to check the backdoor and even throw pebbles at windows to try to wake the amp up.  But once they realize "nobody is home" they straggle back to the preamp.  Once back at the preamp some of the electrons then decide to travel to the other power amp, but by then they are out of phase.  It's actually a little more complicated, but in essence, that's OOP backflow current.  Some people can't hear the effect and other people think the effect is jaw-dropping.  Go figure!

Thank you. It would have been nice to be able to select which (or both) main-out you'd like to be active at any given time especially for scenarios like mine where the amps drive speakers in different (but adjacent) rooms. I have a Schiit Sys passive pre that I can insert in the middle to control (or turn off) the volume for my stereo amp but mainly wanted to first make sure if this is normal before resorting to a workaround.

Like onhwy61's theatrical analogy.... makes good sense, I guess.

Onhwy61, I hereby nominate you for today’s edition of the Wolf_Garcia Award for most amusing post of the day.

I must, however, respectfully disagree with my learned A’gon colleagues who posted above. Kalali, while I’m not certain if what you have described is or is not normal for your particular equipment, it is certainly **very** unusual. The only explanation I can think of is the possibility that the amps short their inputs when turned off, perhaps via a relay. Which would be very unusual, and furthermore I looked at the manual for the MC2200 (as well as the manual for the NAD preamp), and I see nothing that is supportive of that explanation. So I’m baffled.

Best regards,
-- Al

I too am baffled.  My Mac C100 has two outs which I use for biamping and if I power down one set of amps the other keeps working.  There is no separate control for individual outputs, both are always going.

Bill

I guess ignorance is a bliss as I was disappointed that this behavior was normal, albeit my intuition seemed unconvinced. I'll double check my connections to make sure I didn't miss something obvious.

Thanks gentleman for providing the counter argument. 

What I would do is disconnect the ic going to the amp/speakers not wishing to be heard, at the pre-amp end.
Interesting the stated effect...having to run both or nothing...

I run outputs from my sources (TT thru a phono eq, the 'puter, CD, tapes) into a matrix which goes directly either into the main input of an amp and/or to a receivers' tape in.  Sometimes an outboard eq gets 'looped in' via the matrix for input level control and/or eq, depending on the source.

This routine cares less whether both or either are running.. there's a slight drop in input level if both are running, but it's nominal....

Maybe I'm just lucky, or I'm not experiencing the 'feedback' that onhwy61 notes...any thoughts on how this works vs. the OP's state?

I just did an experiment. Powered everything down and disconnected the ICs going to my Vincent SP331. Turned on the preamp and the MC2200s and they sounded fine. Powered everything down and plugged the ICs back in SP331 without turning it on, and turned the MC2200s and the preamp and no sound. Did the complete opposite and I can play the Vincent without having to turn on the MC2200s. The only conclusion I can draw is something in the Vincent input stage - I'm guessing its tube input stage, is somehow "shorting" the preamp output signal on both paths. I can do what bdp24 suggested if I need to play music in one room. Now I know which one I need to unplug. I'm now curious if this is normal behavior for tube amps or amps with (at least) a tube input stage.


This is quite normal.

The input of the amplifiers are active components and need to be switched on to do their job. Obviously when one amp is switched off it is providing low resistance to ground (or frequency dependent resistance) and affecting the line level signal adversely.

The fact is that this is rather a dogs breakfast approach to hooking up audio gear. It may kind of work with some gear depending on the different ways the input circuitry behaves when switched off (due to different designs)

Honestly you need a Zone Mixer between the output and the two separate zone amplifiers as it appears currently you are just connecting the same output to two amps (dogs breakfast approach)

The dogs breakfast approach could also lead to ground loop issues and other interference/noise but on the positive side perhaps different interconnects may sound different in such a setup - providing endless entertainment going down that rabbit hole. (as I have stated many times - well designed gear that is correctly setup won’t need special cabling)
Good find!  I took a look at the manual for the SP-331, and it indicates that the amp is muted for some number of seconds after turn-on while the input tubes warm up.  That is not surprising, but what is very surprising is that based on your findings it would seem to be a good bet that the muting function apparently shorts the input to ground.  I have never before heard of that being done by any component. 

And if that is a correct interpretation of the results (as opposed, for example, to the SP-331 performing in that manner as a result of some kind of defect in the particular amp), I would have to say that it is a ridiculous design.  Consider the situation where it is the only amp being used in a system, and the user has a solid state preamp having a low output impedance and chooses to leave the preamp turned on all the time, while the amp is turned on only when it is being used.  And he or she chooses to play a CD through that preamp continuously for many hours, for burn-in purposes.  The preamp would then have the signal it is trying to put out shorted to ground, with the current drawn from its output stage limited only by its own output impedance and drive capability.  Damage in that situation being a very possible result.

Or consider the very situation you have, but with a preamp that has separate output stages to drive its its two sets of outputs (although relatively few preamps have that feature).  When listening via the MC2200 you would not know that the signal the preamp is attempting to output to the unpowered Vincent is being shorted to ground.  With damage once again being a possible result, eventually if not sooner.

On another note, I'm not sure how much seriousness there was in Onhwy61's comment.  But while it is possible that in  **some** cases having an unpowered component connected to the signal path might affect sonics, in a way that would not occur if that component were powered up, the reasons would have nothing to do with "out of phase backflow current."  I can expound on that further if anyone is interested.

Best regards,
-- Al
 
P.S: Regarding the inconsistency between Shadorne’s post and mine that was posted at almost the same instant, I guess it goes to show that great minds don’t always think alike :-)

Best regards,
-- Al

Thanks again for all the input. I think Al has put his finger on the cause for this seemingly unusual behavior. It appears that the Vincent mute feature is is not only on during warm up - roughly 30 seconds, but also active while the amp is powered down. Regarding the scenario for burning in a source with only the preamp on, I suppose one could disconnect the amp through the process. I do however suspect if this is considered a normal behavior or design for majority of the amps. Those of you who consider this normal, have you actually experienced this situation with your gear? I'm sure this is a fairly popular arrangement for the folks who bi-amp their speakers using a single preamp.
I just did an experiment. Powered everything down and disconnected the ICs going to my Vincent SP331. Turned on the preamp and the MC2200s and they sounded fine. Powered everything down and plugged the ICs back in SP331 without turning it on, and turned the MC2200s and the preamp and no sound. Did the complete opposite and I can play the Vincent without having to turn on the MC2200s. The only conclusion I can draw is something in the Vincent input stage - I'm guessing its tube input stage, is somehow "shorting" the preamp output signal on both paths.
  Take a look at the INPUT of the SP331 in the schematic!!!

https://elektrotanya.com/PREVIEWS/63463243/23432455/egyeb/vincent_sp331-pwr_sch.pdf_1.png
kalali,
Can you use a ohm meter to check the input of the SP331 when it is power down to confirm that the input is shorted?
 

I’ll take the measurement when I get home this evening. Sorry but I can’t make out much by looking at the schematic but it’s nice to have it so I appreciate you posting it.

P.S. Had to look up what "dog’s breakfast"..no clue what it meant. Cool slang and a fairly accurate description of how my cabling looks....
Yes, thanks for posting the schematic, imhififan. And yes, it is of course hard to read. But it appears that what is most likely a relay is connected directly between the signal pin and the ground sleeve of the RCA input connector on each channel. Presumably when the warmup period concludes a control voltage is applied to the relay which causes its contacts to open up, allowing the input signal to proceed to the input stage. And presumably when the amp is turned off that control voltage goes away and the contacts close, resulting in a short across the input.

Unless perhaps there is some means by which the design causes the contacts that are opened during normal operation by the application of that control voltage are somehow also kept open when the amp is off. And then immediately closed at turn-on, for the duration of the warmup period. In which case a defect in the relay might be responsible for the issue. But I can’t envision how that scenario could occur, with the amp still able to operate reliably. So most likely the issue is caused not by a flaw in the relay but by a flaw in the thinking of the designer.

Best regards,
-- Al

+1 Al

looks like you found it! Nevertheless an active amplifier works as specified with input impedance as stated when powered up - I am not sure what happens to the input buffering on all amps when switched off - probably most remain high impedance but that is not a given hence my warning that amps are designed to be used when switched on.

Similary for line level inputs for different members of a band you can’t just branch all the inputs directly to one amplifier input - their gear is all connected to a MIXER that not only allows volume adjustments/effects but as a minimum the mixer buffers the outputs of individual instruments from affecting each other....
shadorn's comment about input impedance triggered a question I've been meaning to ask: Can the generally published spec for amplifiers' input impedance, e.g., 47K ohms for the Vincent, be simply measured at the input ports, presumably with the amplifier turned on, or there's a lot more involved in taking this measurement? Thanks again for all the education.
Nevertheless an active amplifier works as specified with input impedance as stated when powered up - I am not sure what happens to the input buffering on all amps when switched off - probably most remain high impedance but that is not a given hence my warning that amps are designed to be used when switched on.
Well said, Shadorne. It may or may not be an issue from a sonic standpoint depending on the specific designs that are involved.

Also, generally speaking I'd expect that the uncontrolled and unpredictable variations in input impedance that may occur in some unpowered components would be more likely to be an issue in the case of solid state designs than in the case of tube designs, particularly if bipolar transistors are used rather than FETs. Also, it's very conceivable that in many such designs the unpowered input impedance could even vary significantly as a function of signal level, which would increase the likelihood of adverse sonic effects on the signal path that is being used. But where the active device itself has a very high input impedance even when not powered, an example being the grid of a tube, the input impedance of the unpowered circuit would usually be determined essentially by resistors. And in those cases it would presumably be little different in the powered and unpowered states.

That possibility of uncontrolled and possibly varying input impedance in some unpowered components is the main reason, BTW and IMO, that having an unpowered component connected to the signal path might in some cases have adverse effects on sonics, that wouldn't occur if the component was powered up. Not the alleged phenomenon that was referred to earlier involving "out of phase backflow current."   Simple calculations will show that for cable lengths that are typically used in a home audio system energy reflected back from a destination component to the component supplying the signal will arrive at the component supplying the signal with a phase delay in the vicinity of 0.2 degrees at 20,000 Hz, and with vastly smaller delays than even that at mid-range and bass frequencies. Not to mention that reflection effects will be essentially non-existent at frequencies below RF.

Best regards,
-- Al
   
Pick up awards at the Wolf Garcia center any time. I have had 2 to 3 things hooked up to my preamp outputs (2 RCA and one XLR) with zero apparent issues for a while now, currently using 2 (tube power amp and a SS amp used for deck speakers) as I no longer have anything that utilizes the XLR outs…the third thing was a little headphone amp…may have to get a pair of XLR adapters to get the headphone amp back in the system, although it’s rarely used. I had wondered about a related topic when Morrow noted you can break in their cables by driving them to another component that is off (preamp to unpowered power amp for example). Seems weird as I would think a component that’s off would act like something that’s not there at all and not draw any signal...but hey…maybe Morrow knows things.
Can the generally published spec for amplifiers’ input impedance, e.g., 47K ohms for the Vincent, be simply measured at the input ports, presumably with the amplifier turned on, or there’s a lot more involved in taking this measurement? Thanks again for all the education.
No, in general I don’t think that measurement could or should be performed with a simple multimeter, Kalali, if that is what you are asking. For several reasons. First, the input impedance at the zero Hz (DC) frequency that is put out by the meter might be much different than the input impedance within the audio band. The input impedance might even be essentially infinite at 0 Hz if a coupling transformer or coupling capacitor is present, while being far lower at audio frequencies. Also, depending on the specific design of the amp and the meter the amp might be over-driven. And if a resistance scale is chosen that would result in a suitably low test voltage being applied, I’d imagine that in many cases a meaningful reading would not be obtainable.

A good way to perform such a measurement, which I suspect is how John Atkinson does it for Stereophile, would be to make use of an audio frequency signal generator whose output impedance and output voltage can be varied in a controlled manner. When a suitable voltage is applied and the output impedance of the signal generator is adjusted such that the unit under test loads down the output of the generator to half the amplitude that is present when its output impedance is set close to zero, that setting of the signal generator’s output impedance would be equal to the input impedance of the device under test, at the frequency that is being generated.

Best regards,
-- Al

Post removed 
it appears that what is most likely a relay is connected directly between the signal pin and the ground sleeve of the RCA input connector on each channel. Presumably when the warmup period concludes a control voltage is applied to the relay which causes its contacts to open up, allowing the input signal to proceed to the input stage. And presumably when the amp is turned off that control voltage goes away and the contacts close, resulting in a short across the input.
A modification may solve the problem:

INPUT         NO
@-------------o
 |                   \              R2
 |                    O--------WWW------
 |                     C
 |---------------o
 |                  NC
 |
V
GND

With the amplifiers off, I measure "continuity" at the input ports on the Vincent but not on the MC2200, using my very basic millimeter. Didn't feel comfortable sticking my meter probes inside the ports with the amplifiers turned on. I think we can surmise this is NOT normal behavior and is dependent on the amplifiers' input stage design. Thanks everyone for your help in explaining this behavior.


Previous post should read "multimeter" not millimeter. Autocorrect in action...
I use to have an unusual set up, 4 monblxks driving the top and bottom ends of a Dutta Signature. Since they were identical I just bought a set of custom Y connectors. Worked fine but I doubt if few have a system like this.
My stereo pre-amp (Sachs DS-2) has two outs, with differing caps on each pair. I've no clue either as to where the coitus-interrupts electrons go if you just snaggle up to one pair, but Don Sachs' suggestion is: try both and if you're still befuddled, hook up a Y connector to each channel-out pair and see what happens.
Being of the tube world, for me, output impedance is always a concern. Are you piping into a 300K ohm amp input (tube) or a 10k ohm sand amp.
Hi,  well I have a boutique high end preamp with two sets of preamp outputs.  It is basically an analog tube preamp made from Audio by Van Alstine.  It's feeding two main mono blocks V4's and a separate pair of mono blocks Silver 60's for the rear speakers.

They are made by Quicksilver Audio.  Because my preamp does not have digital processing for surroundsound, I'm using what would be considered an old style five channel home theater system made by Dynaco. It's called the QD-1 series ll L.

I also have a sub and center channel. The V4's are wired to the main front speakers as well to the QD1. From the QD there are speaker leads to the rear amps that allow me to use the front V4s for all speakers or the rear amps wired to the surrounds as well