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.
Dfc79476 b7e2 448b 8f7b 3aed2a5e9ad4kalali
You're over thinking it. In practice it just doesn't matter. Barring some unforeseen truly mismatched gear that is.

Let me show you just how little it matters. My integrated tube amp doesn't even have a pre- or sub- out. So I made one. Disconnected one unused RCA input, re-wired to connect speaker output to it, put a couple resistors across it, hooked it up to my two Dayton sub amps. Works beautifully. Read my threads. Awesome, mind-blowing bass, articulate and tuneful and deep and 3D as can be.

A 5th sub, powered Talon Roc, runs daisy-chained off one of the Dayton bypass outs. These same components have been compared running 4 subs off one amp, 2 and 2, and 2 and 2 plus one. Stereo and mono. There is no stereo low bass. So that's another one you can dismiss.

As far as settings, the biggest most common mistake by far is worrying about phase. Phase is a total red herring. Not because it doesn't matter at all, but because the way so many imagine it works is so misleading. Phase isn't about timing, like it needs to all be time aligned, like it needs to be for midrange on up. Phase is about modes, reinforcement and cancellation. Phase is about smoothing and/or extending bass response.

So what you do, set your 4 or 5 or however many subs asymmetrically around the room- different distances from you, the room corners, and each other. Play music and listen. Mains run full range. Adjust sub crossover and level to as smooth and even transition as you can get in that range. Make very small adjustments and listen to different recordings. Not all recordings have great low bass. 

When it starts to get close you can try adjusting phase, or location, of one or more subs. A phase shift is nearly identical to a location shift. It is identical in terms of how it interacts with the other subs. It is different in how it interacts with the room itself. So not quite the same but the exact same in terms of what you hear, which is best thought of as EQ. This part is really time consuming since now you are listening for smooth even balance across low bass frequencies that just don't come up that often on many recordings.

Personally I did all this but didn't put a lot of time and effort into the phase/location steps. By the time it got to that point it really was feeling like splitting hairs. Ninety-plus percent of the improvement is simply running 4 subs. You get another 9 percent with position, level and crossover. By the time you get around to phase you're so close to perfect that if you're still at it I can only tip my hat to you.
I have three sets of outputs on my preamp.  Two channels go to each of the monoblocks for my speakers the other four go to one sub each.  In your case, I'd use two splitters.  
Thank you guys, appreciate it. I can see how I could use a couple of splitters off the second pair of RCA outputs and run the four (powered) subs in mono. Makes sense. And perhaps I'm overthinking the impedance matching side of it but I was just concerned about what the preamp would "see" in terms of the input impedance with basically 5 amplifiers hanging off its outputs while trying to maintain the recommended 1:10 ratio between the impedances.    
Kalali, the best way to do this is run your preamp output to a dedicated crossover like a dBx Drive Rack. The Drive Rack will then give you outputs to your satellite amp and subwoofers. Or your could get an Anthem Preamp which will handle the whole mess for you. If you have a lot of money go for a Trinnov Amethyst. Internalized subwoofer cross over/plate amps are problematic for the most part as their high pass filters are generally not that great. JL Audio is an exception. For that kind of set up to have a prayer of working well you need balanced ins and outs. Not using a high pass filter on the satellites is a non starter for those in the know.
I can see how I could use a couple of splitters off the second pair of RCA outputs and run the four (powered) subs in mono. Makes sense. And perhaps I’m overthinking the impedance matching side of it but I was just concerned about what the preamp would "see" in terms of the input impedance with basically 5 amplifiers hanging off its outputs while trying to maintain the recommended 1:10 ratio between the impedances.
Hi Kalali,

You are not overthinking this issue at all, and in situations like this impedance compatibility should be carefully considered in the context of the specific equipment that is being used. I say that for several reasons:

1) As you indicated, the line-level input impedance of most powered subs tends to be low, sometimes as low as 5K.

2) Most preamps providing multiple RCA output connectors simply wire those connectors directly together, inside the rear panel, meaning that the output stage providing the signal to those connectors will be simultaneously driving all of the loads that are connected to all of those connectors. Which from an electrical standpoint is essentially no different than using a splitter on a single output connector, and will result in the output stage seeing a load impedance that is substantially lower than the input impedance of any of the individual connected loads.

And even in cases where a preamp provides both RCA and XLR output connectors, the signal provided to the RCA connector is often the same signal that is provided to the non-inverted signal pin of the XLR connector. Creating the same potential issue, and possibly even a worse issue as the impedances relative to ground of the two signals in the balanced signal pair could conceivably be unbalanced to a degree that would degrade the common mode noise rejection a balanced interface can provide.

3) The majority of tube-based preamps utilize coupling capacitors at their outputs, which often means that their output impedance at deep bass frequencies is **much** higher than the specified output impedance, which is usually based on a mid-range frequency such as 1 kHz.

In this case, I suspect you are referring to the Aric Audio Unlimited preamp which is listed in your system description thread for one of your systems, since it has a specified output impedance corresponding to the 600 ohm figure you referred to. And as you alluded to, its multiple outputs are explicitly referred to on the rear panel as being "paralleled."

What I would suggest that you do is to contact Aric and ask him what the output impedance of the preamp is at 20 Hz. Or if he can’t readily provide that number, ask him to indicate the value of the output coupling capacitor (in microFarads), from which a reasonable estimate of the 20 Hz output impedance can be derived.

At the same time, try to determine the input impedance of whatever specific subs you may consider.

Assuming all the subs are identical, and given that you would be connecting four of them to both output channels of the preamp, in addition to connecting the 100K input impedance of the power amp, you can calculate the overall load impedance that would be presented to the preamp on each channel as follows:

((Sub input impedance/4) x (100K)) / ((Sub input impedance/4) + (100K))

The result should be at least 10x the output impedance of the preamp at the frequency within the audible range for which that output impedance is highest, which is usually 20 Hz especially in the case of most tube-based preamps.

Not meeting that criterion does not **necessarily** mean that there will be a problem; it depends on how the impedances that are involved vary as a function of frequency. But meeting that criterion assures that there won’t be an impedance compatibility problem.

More generally, btw, the combined impedance of any number of paralleled resistances is:

1 / (1/R1 + 1/R2 + 1/R3 + 1/R4 etc.)

Again, to assure that there won’t be an impedance compatibility problem that result should be at least 10x the output impedance of the preamp at the frequency within the audible range for which that output impedance is highest, which is usually 20 Hz especially in the case of most tube-based preamps.

Good luck. Best regards,
-- Al



I run 3 subs in my system. I run 2 off of my amp(speaker cable/hi pass) the 3rd I run off of my Unlimited preamps second set of out puts. Now,the 3rd HSU sub has a slight impedance mismatch. I run 2 rca's out of the pre to the 2 rca inputs on the sub. If I only run 1 out to 1 in,the volume on one channel is lower than the other. The outputs are run in parallel, therefore I run 2 to 2,to keep volume levels the same in both channels. I hope this helps. Any questions,feel free to pm me.

If you buy Rythmik subs, the XLR versions of the company's plate amp allows piggy-backing of one sub to another. You can use the adjustment knobs (level, phase, x/o filters, etc.) on each sub separately, or control the piggy-backed sub as a slave from the sub feeding it the signal. The input impedance is 30K ohms.

But to get the XLR connections, you give up the high-level (power amp) inputs found on the PEQ model plate amps.

In pro audio, a distribution amplifier is the answer.

@mijostyn You think not using a high pass filter is a problem even if the subs are built to receive a full range signal? My coincident pre's have sub cabinets with their own binding posts and an inductor to cross them at around 200hz. I just started running them with subwoofer amplifiers and have been bypassing the amps' crossover circuitry.
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 :)
@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
  
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.
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.
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.
"...did I not already say don’t overthink it?"

Yes sir, you did. Guilty as charged. I appreciate your input based on your practical experience.  
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....

... Please go back and read my first post. Then stop thinking about impedance. Start thinking about table saws, sand papers, and veneers.

Miller, my analysis establishes pretty conclusively that the OP's preamp should not be used to drive four powered subs having typical impedances, in addition to his power amp. That was something he was rightfully concerned about, and that he had specifically asked about.

Rather than telling him to "stop thinking about impedance," and throwing in a gratuitous insult (toward me) in the process, I suggest that it would be more constructive if you were to specifically address the question he and I were discussing, about driving all of those loads with his particular preamp.

Regards,
-- Al
 
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.

I have 4 subs connected to my Modwright LS100 tube preamp with no ill effects (that I'm aware of) 
get a high input impedance buffer and put it between your pre and subs. It works great fo me.
"...get a high input impedance buffer and put it between your pre and subs. It works great fo me."

Can you recommend a specific brand/model? If at all possible, I much prefer having the ability to use the RCA outputs since it potentially allows for the use of wireless powered subwoofers thus simplifying placement and cabling logistics. May rule out the DIY approach but might be worth the tradeoff between cost vs. logistics. 
@millercarbon (& @almarg ) Let's not confuse over-thinking it with correctly-thinking it.

Obviously Al is correct here, and without his input the OP could easily end up fighting bass issues without any idea why. This site needs more people like Al, not less. 
Thank you kindly, Chris (@cal3713 ).

@kalali, the suggestion by @koestner of a buffer stage is an excellent one. A number of possibilities were discussed in the following thread, most notably IMO the iFi micro iTube2:

https://forum.audiogon.com/discussions/impedance-matching-question

The micro iTube2 is available at B&H Photo Video for $399.

Also, a few years ago another member reported excellent results using a custom made tube-based buffer stage in conjunction with his Audio Research Ref 6 preamp to drive a powered sub. (That preamp has a 20K minimum load recommendation, which was not met when using the preamp to drive both the sub and his main power amp). It was custom made for him by Tom Tutay of Transition Audio Design in Florida. Tom does not have a website as far as I am aware, but his contact info should be findable via a Google search. I don't recall the price, but I believe it was somewhere between $500 and $1K.

Good luck. Best regards,
-- Al
 
the perfect solution is to place a good crossover behind the preamp.. the bass section from the crossover will feed a 1 input / 4 output amplifier, all output from bass amp being fed the same signal.. bass amp should be class a/b type.. in this way ….. 
- preamp only driving one component
- main amp driving the R & L speakers duties are lightened up not having to produce the lower frequencies = better performance..
- subwoofers should all still receive a good strong signal.. 

Does anyone of such an amp (for bass) ????
Not sure if I quite follow the crossover arrangement but the use of a buffer such as iFi micro iTube2 seems quite logical, especially with its 1M ohm input impedance and ~150 ohm output impedance in direct buffer mode. I’ll probably start with a two subwoofer set up and see how it sounds and add more if needed. My other challenge is to pick the right driver(s), most probably an 8" or 10" to better blend with my Omega Alnico single drivers.
Thank you all or your input.
Wouldn't the simplest approach be to daisy chain your subs from your main speaker input posts, if that is an option?
Whats hard to follow with the crossover arrangement .? You split the  frequency coming out of the preamp before it hits the amps. Say at 80Hz. The upper frequency gets fed to the amp driving the R & L speakers . And the 80Hz below gets fed to the bass amp. 
Got it. The comment about putting the crossover ‘behind’ the preamp threw me off a bit. What you described is more like the ‘front’ of the preamp. But, I might be overthinking it again 😎