Possible impedance matching issue with subwoofer?

Al, you are the MAN! thanks so much for your help.

BTW, my pre to amp ic is only 3 feet long. Yes, I use the LC-1 sub cable.

Not sure what kind of sub you have, but with the ones that have both line level and speaker level inputs, the sub just 'samples' the input signal (yes, the RCA inputs too). So your preamp isn't really 'driving' the sub's amp(s) per se; so impedance mismatching that might possibly normally occur just isn't a factor. If you're still concerned, contact the sub manufacturer and find out for sure.

02-07-12: Nsgarch
Not sure what kind of sub you have, but with the ones that have both line level and speaker level inputs, the sub just 'samples' the input signal (yes, the RCA inputs too). So your preamp isn't really 'driving' the sub's amp(s) per se; so impedance mismatching that might possibly normally occur just isn't a factor.

Hi Neil,

Not sure what you mean by the word "samples" in this context. I think you might mean that the voltage level of the signal being input to the sub is divided down to a lower level which is then processed internally. But that, and any other "sampling" process you may be referring to, are irrelevant.

If the sub's line-level inputs are being used, and their input impedance is 20K for example, then the preamp has to drive that 20K load, regardless of whatever "sampling" may occur. If, as in this case, the preamp's two sets of outputs are not individually buffered, then a single output stage in the preamp has to drive the parallel combination of that 20K and the input impedance of the main power amp. That combined impedance will be less than 20K, and will affect the signals seen by both the main amp and the sub. The preamp also has to drive the interconnect cables to both destinations, and the capacitance of both cables will affect the signals seen by the main amp as well as by the sub.

Impedance matching considerations are very much a factor in that situation, or in any other situation in which a powered sub is being driven from a line-level signal source having significant output impedance. And the importance of those considerations is increased by the relatively low input impedance that subs commonly have (as Grannyring pointed out), and by the fact that both the main amp and the sub, and their interconnect cables, are driven by a common output stage if the preamp does not individually buffer the two sets of output jacks that it may provide.

Best regards,
-- Al

No Al, that's not the way it works (anymore ;--) Yes, in the old days, 'plate amps' were basically full range ss amps that were put on a 'plate' chassis so they could be mounted on the back of a speaker -- quite often a full range monito speaker (not a sub) which (self-powered) was more convenient in studio environments (no long speaker cables; using balanced signal lines of course.) And BTW, do you remember when many subwoofer (drivers) were made with dual voice coils, to keep the preamp's L/R outputs separated?

But today's powered subs (all the ones I know about, anyway,) including the subwoofer section of certain hybrid speakers (like the MartinLogan models that have a powered woofer section) use Class D ss amps which are fine at low frequencies, but aren't meant to be driven with a full range signal. These modern subs often include an LFE (Low Frequency Effects) input which DOES drive the plate amp directly, from a single (mono) interconnect, since it's assumed that the LFE signal comes 'pre-configured' from a surround processor and only contains mono low frequency information, already with the desired slope and cutoff determined by the processor.

The L/R speaker, and L/R line level inputs on these subs are "connected" to the sub's (Class D) amp through some kind of a buffer which also a.) combines the L/R signals (while keeping the preamp's L and R signals separate) and b.) limits the frequency range going to the subs internal amp. That's why the slope, phase, and often even the level controsl on the sub do not affect the LFE input signal, which it's assumed are configured at the processor. My point is that the "input impedance" at the buffered L/R line inputs (and L/R speaker inputs) of most subs today is 'set' by the sub's designer to accept almost any preamp's L/R main output without doing "damage" to the music signal due to accidental impedance mismatch.
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