Possible impedance matching issue with subwoofer?

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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Hi Neil,

I recognize and agree with everything in your last post, up until the last sentence:

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.

My point is that the input impedance of the sub, which is the input impedance of the buffer stage you are referring to, is what it is. And depending on its value, and on the output impedance of the preamp, and on the variation of that output impedance as a function of frequency, it can have significant effects (when driven from a preamp or other line-level source).

In the case of Grannyring's sub, it is 20K, which is not an atypical value. The effects on the INPUT SIGNAL to the sub of the interaction of that 20K input impedance with the output impedance of the preamp will be no different than the interaction that would occur if the preamp were driving a main power amp having a 20K input impedance.

While the subset of those effects that are relevant to mid and high frequencies will obviously not significantly affect the sound produced by the sub, the subset of those effects that are relevant to deep bass frequencies certainly will affect the sound produced by the sub. And if the preamp does not separately buffer the outputs to the sub and the main power amp, mid and high frequencies can be affected by the sub's input impedance (and also by the capacitance of the cable to the sub) as well.

What goes on inside the sub has no relevance to any of that.

I would agree with your last sentence, btw, if it were limited to the speaker-level inputs of a powered sub. In that situation the input impedance of the sub is so vastly higher than the output impedance of the power amp that the effects I have described would be completely inconsequential.

Best regards,
-- Al

Al, your erudite response notwithstanding, it's unlikely ANY manufacturer these days would leave it to the user to make sure his product performed optimally ;--) I don't think Mr. Grannyring should be concerned in the slightest.

As he already said,

The system sounds very good as is, just wondering if perhaps I am onto something I have missed with possible bass performance improvement.

and so my response is, don't go looking for problems where none exist ;--)
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Thanks Nsgarch and Almarg for your comments and help.