Subwoofer hunt

Apologies in advance to spoutjack...

@almarg ...So if planning to run from the 4 or 8 ohm output taps of an amp into the speaker level connections of a powered sub and the input impedance here, as per the mftr. spec, is 2000 ohm (SVS for their SB1000), the output impedance of the amp would be 0.002 - 0004 of the sub's input impedance depending on the tap used.  Do these values still qualify as insignificant fractions of the much-lower-than-100K-ohm SVS input impedance?  I would think so, but not certain.  Thanks for taking a look.   

@almarg - 
Al, in case of a formatting error to your name in the request above, I'm re-sending.  If you would be so kind, please see my question there.  Thanks in advance. 

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
 

@almarg
Very helpful. Thank you. I also took notes from your line level input discussion in reply to rbschauman’s "Help Wanted Bass" thread.

Regarding the true output impedance of my tube amps, unfortunately I’ve not seen info about damping factor in the mftr. specs. There’s not a lot of love for Stereophile in some quarters, but if you are fortunate enough to own a nice piece of gear they’ve reviewed, the measurements section is a great resource going forward.

BTW - I too was surprised about that speaker level impedance spec for the SVS SB1000.  Double checked it.  Best I can tell, barring a typo on their part, 2K Ohm is the value.  

Thanks again.