Preamp connection to multiple powered subwoofers

You're over thinking it. In practice it just doesn't matter. Barring some unforeseen truly mismatched gear that is.

Let me show you just how little it matters. My integrated tube amp doesn't even have a pre- or sub- out. So I made one. Disconnected one unused RCA input, re-wired to connect speaker output to it, put a couple resistors across it, hooked it up to my two Dayton sub amps. Works beautifully. Read my threads. Awesome, mind-blowing bass, articulate and tuneful and deep and 3D as can be.

A 5th sub, powered Talon Roc, runs daisy-chained off one of the Dayton bypass outs. These same components have been compared running 4 subs off one amp, 2 and 2, and 2 and 2 plus one. Stereo and mono. There is no stereo low bass. So that's another one you can dismiss.

As far as settings, the biggest most common mistake by far is worrying about phase. Phase is a total red herring. Not because it doesn't matter at all, but because the way so many imagine it works is so misleading. Phase isn't about timing, like it needs to all be time aligned, like it needs to be for midrange on up. Phase is about modes, reinforcement and cancellation. Phase is about smoothing and/or extending bass response.

So what you do, set your 4 or 5 or however many subs asymmetrically around the room- different distances from you, the room corners, and each other. Play music and listen. Mains run full range. Adjust sub crossover and level to as smooth and even transition as you can get in that range. Make very small adjustments and listen to different recordings. Not all recordings have great low bass. 

When it starts to get close you can try adjusting phase, or location, of one or more subs. A phase shift is nearly identical to a location shift. It is identical in terms of how it interacts with the other subs. It is different in how it interacts with the room itself. So not quite the same but the exact same in terms of what you hear, which is best thought of as EQ. This part is really time consuming since now you are listening for smooth even balance across low bass frequencies that just don't come up that often on many recordings.

Personally I did all this but didn't put a lot of time and effort into the phase/location steps. By the time it got to that point it really was feeling like splitting hairs. Ninety-plus percent of the improvement is simply running 4 subs. You get another 9 percent with position, level and crossover. By the time you get around to phase you're so close to perfect that if you're still at it I can only tip my hat to you.

I have three sets of outputs on my preamp.  Two channels go to each of the monoblocks for my speakers the other four go to one sub each.  In your case, I'd use two splitters.  

Thank you guys, appreciate it. I can see how I could use a couple of splitters off the second pair of RCA outputs and run the four (powered) subs in mono. Makes sense. And perhaps I'm overthinking the impedance matching side of it but I was just concerned about what the preamp would "see" in terms of the input impedance with basically 5 amplifiers hanging off its outputs while trying to maintain the recommended 1:10 ratio between the impedances.    

Kalali, the best way to do this is run your preamp output to a dedicated crossover like a dBx Drive Rack. The Drive Rack will then give you outputs to your satellite amp and subwoofers. Or your could get an Anthem Preamp which will handle the whole mess for you. If you have a lot of money go for a Trinnov Amethyst. Internalized subwoofer cross over/plate amps are problematic for the most part as their high pass filters are generally not that great. JL Audio is an exception. For that kind of set up to have a prayer of working well you need balanced ins and outs. Not using a high pass filter on the satellites is a non starter for those in the know.

I can see how I could use a couple of splitters off the second pair of RCA outputs and run the four (powered) subs in mono. Makes sense. And perhaps I’m overthinking the impedance matching side of it but I was just concerned about what the preamp would "see" in terms of the input impedance with basically 5 amplifiers hanging off its outputs while trying to maintain the recommended 1:10 ratio between the impedances.

Hi Kalali,

You are not overthinking this issue at all, and in situations like this impedance compatibility should be carefully considered in the context of the specific equipment that is being used. I say that for several reasons:

1) As you indicated, the line-level input impedance of most powered subs tends to be low, sometimes as low as 5K.

2) Most preamps providing multiple RCA output connectors simply wire those connectors directly together, inside the rear panel, meaning that the output stage providing the signal to those connectors will be simultaneously driving all of the loads that are connected to all of those connectors. Which from an electrical standpoint is essentially no different than using a splitter on a single output connector, and will result in the output stage seeing a load impedance that is substantially lower than the input impedance of any of the individual connected loads.

And even in cases where a preamp provides both RCA and XLR output connectors, the signal provided to the RCA connector is often the same signal that is provided to the non-inverted signal pin of the XLR connector. Creating the same potential issue, and possibly even a worse issue as the impedances relative to ground of the two signals in the balanced signal pair could conceivably be unbalanced to a degree that would degrade the common mode noise rejection a balanced interface can provide.

3) The majority of tube-based preamps utilize coupling capacitors at their outputs, which often means that their output impedance at deep bass frequencies is **much** higher than the specified output impedance, which is usually based on a mid-range frequency such as 1 kHz.

In this case, I suspect you are referring to the Aric Audio Unlimited preamp which is listed in your system description thread for one of your systems, since it has a specified output impedance corresponding to the 600 ohm figure you referred to. And as you alluded to, its multiple outputs are explicitly referred to on the rear panel as being "paralleled."

What I would suggest that you do is to contact Aric and ask him what the output impedance of the preamp is at 20 Hz. Or if he can’t readily provide that number, ask him to indicate the value of the output coupling capacitor (in microFarads), from which a reasonable estimate of the 20 Hz output impedance can be derived.

At the same time, try to determine the input impedance of whatever specific subs you may consider.

Assuming all the subs are identical, and given that you would be connecting four of them to both output channels of the preamp, in addition to connecting the 100K input impedance of the power amp, you can calculate the overall load impedance that would be presented to the preamp on each channel as follows:

((Sub input impedance/4) x (100K)) / ((Sub input impedance/4) + (100K))

The result should be at least 10x the output impedance of the preamp at the frequency within the audible range for which that output impedance is highest, which is usually 20 Hz especially in the case of most tube-based preamps.

Not meeting that criterion does not **necessarily** mean that there will be a problem; it depends on how the impedances that are involved vary as a function of frequency. But meeting that criterion assures that there won’t be an impedance compatibility problem.

More generally, btw, the combined impedance of any number of paralleled resistances is:

1 / (1/R1 + 1/R2 + 1/R3 + 1/R4 etc.)

Again, to assure that there won’t be an impedance compatibility problem that result should be at least 10x the output impedance of the preamp at the frequency within the audible range for which that output impedance is highest, which is usually 20 Hz especially in the case of most tube-based preamps.

Good luck. Best regards,
-- Al