REL subwofer High Level vs Low Level connection

I prefer connection to the high level from my amp outputs. Rel's logic seems sound for recommending this as the best implementation.

The line-level alternative requires that impedance matching be considered. More often than not preamps providing two sets of outputs do not separately buffer those outputs (i.e., they are simply connected together inside the rear panel, and driven by the same output stage). In that situation the preamp's output stage will see a load impedance equal to the parallel combination of the input impedance of the power amp and the sub, which will be significantly lower than either of those impedances individually.

That is most likely to be a problem if the preamp is tube-based, although it can sometimes also be a problem with solid state preamps. The value of the parallel combination of the two input impedances is equal to the product (multiplication) of the two numbers divided by their sum. For instance, if the input impedance of the power amp is 50K, and the input impedance of the sub is 20K, their combined input impedance is about 14.3K. That number should ideally be 10x or more higher than the output impedance of the preamp, at the frequency for which that output impedance is highest (which is often 20Hz). If only a nominal output impedance is known (presumably at mid-frequencies), I suggest using a factor of 50x.

Also, if the length of either or both of the interconnect cables is long, and the output impedance of the preamp is high, using low capacitance cables becomes increasingly important. The capacitance of the cables to the sub will affect the signals going to the main amp, as well as the signals going to the sub. A slight but perceptible high frequency rolloff might occur if the total capacitance of the two cables is high and the preamp's output impedance is high. Again, this assumes that the preamp's two outputs are not individually buffered.

Regards,
-- Al

Thanks so much to Stanwal and Bdgregory for favoring HighLevel as I have heard that REL recommends it over Low Level input. Thanks especially to Almarg for your detailed analysis of impedance matching. I'm a Cornell EE and get it.

Why does Bobby at Merlin tell me to use my sub full range with his TSMXe speakers. Am i losing out bye doing what he says. Its his speaker, and he should know........Right.

Ryriken, I suspect that you mean that he says to use the main speakers full range, as opposed to high pass filtering their inputs (i.e., filtering out the deep bass content of the signals that are provided to them). That's a different question than what is being discussed here. Among other factors, the answer would depend on how well the main speakers can deal with deep bass frequencies. I have no doubt that his recommendation is correct for his speakers.

Elunkenheimer, you're welcome! Columbia EE here :-)

Best regards,
-- Al