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