Preamp connection to multiple powered subwoofers

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

I run 3 subs in my system. I run 2 off of my amp(speaker cable/hi pass) the 3rd I run off of my Unlimited preamps second set of out puts. Now,the 3rd HSU sub has a slight impedance mismatch. I run 2 rca's out of the pre to the 2 rca inputs on the sub. If I only run 1 out to 1 in,the volume on one channel is lower than the other. The outputs are run in parallel, therefore I run 2 to 2,to keep volume levels the same in both channels. I hope this helps. Any questions,feel free to pm me.