Getting bass right

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In yesterday's post I discussed the fact that there are basically only two ways to connect a modern powered subwoofer: with a high level or a low level input. The vast majority of subwoofers today no longer offering a high level input because Audiophiles are probably ok adding another cable and with the rise of home theater applications as the primary market for subs, which require a low level input from the surround processor, the fate of the high level input has been sealed.

But what advantages did that high level input offer? Plenty and, to this day, it is still my favorite for integrating a sub with a system.

Let's review what the differences between the two inputs are first. Fact is, both inputs actually go to the same place on a powered sub - they just take somewhat different routes. The high level input is designed to come from the main power amplifier's outputs - which is typically 20 to 30 times louder than its inputs. Subwoofer designers merely take a couple of resistors and reduce this very loud signal down to match the main power amplifier's input levels. If done properly the result is that there is no loudness difference between the low levelor the high level input signal that feeds the subwoofer's internal power amplifier.

There is one big difference, however, and that's the effect the main loudspeaker's power amplifier had on the signal we reduced in level. Remembering that all power amplifiers affect the sound of the music that passes through them (phase shift at the lower extremes, tube amps with their output transformers, solid state amps, vs. class D amps etc.), we'd be much better off using the already-amplified output of the power amp to feed our sub because that output will be much closer matched, sonically, to what's being fed to our main loudspeakers.

If we use, instead, the low level input directly from the preamplifier's output we have not only lost the advantage of matching the amplifier's sound but we risk the possible degradation of adding another length of interconnect to the preamp's output which, in many cases, can do a lot of sonic damage (depending on the interconnect's length and the preamp's design).

So with all this in mind here's my advice. If you're forced to connect the subwoofer with its low level inputs, use a Y connector at the end of your interconnect cable feeding the power amplifier if the run is more than 2 meters.

If you're handy with a few tools and want a better DIY path, just make a 30X resistive divider (use a 30K resistor and a 1K resistor) across the output of your power amplifier and feed the subwoofer's low level inputs from the junction of the two resistors (the 30K going to the + output of the amp and the 1K going to the - of the amp).

Either way you go, you'll be delighted you have a sub and connected it right.

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Paul McGowan

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