Subwoofer hunt

thanks for the detailed post hifiman5. one quick question, wouldnt the current on the speakers drop if you connect subs to them in parallel? because current would decrease for parallely connected components right?

No, there would be no effect on the current delivered to the main speakers.  When a powered sub is connected to the output of a power amplifier or integrated amplifier, what the amplifier is driving is the sub's amplifier, not the sub's driver, which means it is driving a very high impedance that will draw negligible current.  The Vandersteen sub Hifiman5 referred to, for example, has a specified input impedance of more than 100,000 ohms.

A consequence of that, btw, is that the cables used to connect amp outputs to a sub that can accept speaker-level signals can be much narrower in gauge than typical speaker cables.

To address your question more generally, adding a load impedance in parallel with another load impedance will only affect the current delivered to the first impedance if the voltage across the paralleled impedances becomes different from what the voltage across the first impedance would have been in the absence of the second impedance.  And that voltage will only change as a result of the addition of the second impedance if the effective output impedance of the component providing the current is high enough to be a significant fraction of the second impedance, or if the component is not capable of supplying the total amount of current that is drawn by the two loads in response to the voltage it provides. 

Very few power amps or integrated amps have effective output impedances of more than a few ohms, and most solid state amps have effective output impedances of a tiny fraction of an ohm, those numbers obviously being insignificant fractions of 100,000 ohms.  So none of this will be an issue with a powered sub.

Good luck.  Regards,
-- Al
 

Ghosthouse, yes, putting a 2000 ohm load in parallel with the vastly lower impedance of a speaker would result in a change in the overall impedance seen by the amp that is certainly negligible.  For example, if the speaker's impedance is 8 ohms the impedance of the parallel combination would be (8 x 2000)/(8 + 2000) = 7.968 ohms.  Even if the amp has a relatively high output impedance that would result in a negligible change in the voltage it would apply to the speakers, and a negligible increase in the current and power it would have to deliver.

2000 ohms, BTW, is the lowest input impedance I can recall seeing for the speaker-level input of a powered sub.

Also BTW, in most cases the 4 and 8 ohm designations of tube amp output taps do NOT represent their output impedance, despite misleading wording in some manufacturer specs that would appear to indicate that they do.  Those numbers represent the load impedance the tap is optimized to work into.  The output impedance of the 8 ohm tap equals 8 ohms divided by the damping factor of that tap, assuming the damping factor spec is provided and is accurate, and the output impedance of the 4 ohm tap is roughly half that amount in most cases.

Most tube amps have output impedances in the area between a little under 1 ohm and perhaps 3 ohms or so, although there are some designs for which the output impedance can be higher.

Best regards,
-- Al
 

Ghosthouse, I would expect it to be perfectly fine to connect the subs to the speaker terminals instead of the amp terminals.

The only conceivable difference it might make that I can think of would be an **extremely** minor difference in the amount of "back-emf" from the speaker that might be received by sub. And if that were to have any audible significance at all (which I very much doubt), who knows, it might even be subjectively preferable.

Enjoy!

Best regards,
-- Al