SUBWOOFERS: The advantage of using the power amp’s output as the subwoofer’s input are …..


My thanks to Wyn Tathem of Simply Hi Fi in Australia for sending me the great photo of a true Sub Woofer pair which I’ll include at the end of this email for those of you reading strictly by email.

Yesterday I suggested using a Y connector at the amplifier inputs if you were forced, as I am, to use long interconnects.  Running long interconnects isn’t a great idea for any preamp or DAC and adding another pair, just to feed the subwoofer, makes that situation even worse; hence the suggestion.

Several of you wrote and asked about using a high level input instead – like the way REL subs of the UK do – taking the output of the power amplifier and feeding the sub with that.  This is a method I am actually quite fond of and have done for years because of the way it sounds.

The advantage of using the power amp’s output as the subwoofer’s input are several fold: the amp’s sound quality characteristics are maintained so you get a more even integration and you never need to load your preamplifier down.  The problem with this setup is most subs today no longer offer a high level input and on many amplifiers it can be a really bad idea if the subwoofer designer doesn’t handle things correctly.

A power amplifier’s output is approximately 30 times louder than the preamp’s output so subwoofers with only a line level input cannot accept such a loud level and one would have to build a small resistor network to reduce the level to where it’s correct.  Further, many amplifiers today, especially Class D amps, have balanced outputs, and if you try and tie the negative output of such an amplifier to ground it’ll either shut off or blow up; neither result a good thing.

A properly designed high level input on a subwoofer, which is what I assume REL has, would have addressed these issues by making sure the input to the sub is always balanced and capable of reducing the amp’s output appropriately.  Certainly every sub amp and sub woofer I ever designed in the Genesis days had the ability to take high level signals from the amp and use them and it was always my preferred method of feeding a sub.

Alas, most subs today, including the subwoofer I helped design and still use, the Martin Logan Descents, do not have a high level input that is specifically designed to handle the output of the power amp.  ’Tis a shame.

So the moral of the story is yes, the high level input is best to use when feeding a sub if your sub provides one and it is designed properly.  If you’re going to try this yourself and make a small divider network, make darn sure your amp isn’t a bridged or balanced type and if it is, take care how you connect it.

Otherwise, we should all be connected and ready to go.

Paul McGowan / PS Audio