Preamp connection to multiple powered subwoofers

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
 

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
 

@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
  

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