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Preamp connection to multiple powered subwoofers

I'm trying to understand how those of you who have multiple, say four, powered subwoofers in your 2-channel audio systems feed the signal from your preamplifier to these amplifiers in addition to driving your main speakers. In addition to the physical connectivity, I'm also wondering how is the impedance matching between the preamp output impedance and the combined input impedances of these multiple amplifiers is addressed. Using my system as example, my preamp has two parallel outputs rated at 600 Ohms with one set driving a tube amplifier rated at 100K Ohms. Would the second output be shared across four powered subwoofers, most likely Class D with generally low input impedance around 10K-20K Ohms? Or folks mostly use the high level speaker connections through the main speakers and split through to the rest of the subwoofers?  I have the same question for those running passive subwoofers. Do you use the second preamp output to drive a dedicated 4-channel amplifier and out to the subwoofers, and if so, how do you handle the phase, gain, and frequency adjustments across the four subwoofers.
Appreciate any input.
128x128kalali
kalali's avatar
kalali OP
1,977 posts
 
Thank you all, particularly Al for his detailed response, as always. Assuming a 10K input impedance for the sub plate amplifiers, the combined impedance will be about 2,500 ohms which is technically too low even for the (published) 600 ohm output impedance of the Unlimited. I suspect it'll get even worse at lower frequencies. As for the output coupling capacitors, I currently have 2.2 microfarad coupling capacitors in there now but they are relatively easy to change since I've already changed/upgraded them once before. The OEM values were 3.3 microfarad.  
rocray - I think using a combination of line level and speaker high level methods may be a reasonable option. Thanks.
I have a friend with a pretty decent woodworking shop and all kinds of tools and I'm thinking about building the subs with his help. Been itching for a good winter project having sold my bike a couple of months ago :)
almarg's avatar
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almarg
9,670 posts
 
@kalali,

The impedance ("capacitive reactance") of a 2.2 uF cap at 20 Hz is 3619 ohms!  The impedance of a 3.3 uF cap at 20 Hz is 2413 ohms.

The specified 600 ohm output impedance is presumably at 1000 Hz or thereabouts, and mainly represents the output impedance of what is "ahead" of the cap (since the impedance of the cap is low at those frequencies).  The output impedance of that circuitry is probably fairly constant within the audible frequency range (i.e., it is mostly resistive in nature). 

The overall output impedance at 20 Hz is probably just a little bit higher than the impedance of the cap, as series combinations of resistive and capacitive impedances combine as the square root of the sum of the squares, rather than adding directly.

Best regards,
-- Al
  
kalali's avatar
kalali OP
1,977 posts
 
Thank you. Those impedance values pretty much rule out the use of the second (parallel) output from the Unlimited to drive all the powered subwoofers. I also remember reading somewhere that selecting too large uF output coupling capacitors can have some undesirable sonic side effects and the designers generally strike a balance between the size (uF) and coloration. Anyway, researching some older discussions on the distributed bass array topic, it looks like a more practical approach is what AudioKinesis uses in his acclaimed Swarm where I believe he utilizes a single kilowatt class AB amplifier to drive four passive subwoofers. 
https://www.parts-express.com/dayton-audio-sa1000-subwoofer-amplifier-rack-mountable--300-811
But even with this amplifier with a published 12K Ohms input impedance, I'd get a combined ~11K Ohms which probably works well with most if not all SS preamps but borderline for most tube preamps.
I'd like to hear if anyone is driving a distributed bass array system similar to the Swarm using a tube preamp as the signal source.
Thanks again.
millercarbon's avatar
millercarbon
12,531 posts
 
kalali, did I not already say don't overthink it? And then you go and compliment and start paying attention to the world's greatest offender in overthinking it? Now it appears you have overthought things to the point you've concluded that what works for everyone else cannot possibly work for you. At least that is what you think. Because you are thinking. Instead of doing.

The Dayton amps you have figured out cannot work are the exact same Dayton amps I am using. Exact. Same. Amps. 

Please go back and read my first post. Then stop thinking about impedance. Start thinking about table saws, sand papers, and veneers.
bdp24's avatar
bdp24
9,821 posts
 
If you can build the enclosures, Rythmik offers their subs in DIY kits. The kit includes the servo-feedback woofer and associated 600w plate amp, and polyfill enclosure stuffing. Even if you can't make an enclosure, Parts Express sells some really good sub flat packs, which are a snap to assemble.
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