How to connect susb to my system

Something like this can address the impedance issue - 

So what happens when you decrease the load driven by the preamp below its minimum specification? The extra current demand on the output stage softens dynamic impact and results in generally poorer sound as the vacuum tube output stage struggles to drive the low impedance of the parallel load of the two connected amplifiers. Luckily, this won't damage anything. But sound quality is why you purchased Audio Research products in the first place, right? - and we never want to hurt the sound quality!

Faced with the desire to add a subwoofer or second amplifier, what can you do? Check the specifications page of the manual of the amplifier or subwoofer you are interested in adding. (Audio Research provides the input impedance of our amplifiers in our owners' manuals and on our website.) Use the equation above to see if the gods of the high end smile upon you or smite you.

Good luck finding the input impedance specs on the speaker manufacturers' websites! Of six well-known subwoofer manufacturers' websites I visited, only one bothered to specify its sub's input impedance.

Nobody promised this was a hobby for the faint of heart...

The Thorny Math of Impedance:

Nearly all ARC vacuum tube preamps are rated to drive a connected load (read amplifiers) of not less than 20,000 ohms (20K ohms). What does this mean? The output stage of a vacuum tube preamp has a high source impedance of (usually) several hundred ohms. Vacuum tubes, because of their inherently high internal resistance (known as impedance) compared to a transistor, are limited in their ability to supply current into a connected load. Solid state preamps, on the other hand, usually have a very low source impedance of< 100 ohms.

Now let's use a (tiny) bit of math to sort this out. When you connect more than one amplifier to a preamp, the loading on the preamp's output stage is considered in parallel.

That means each amplifier's input resistance (input impedance) is less than the individual load by some amount as expressed by the parallel resistance equation 1/ Z1 + 1 / Z2 = 1/ Z total, where Z = amplifier input impedance. An easy example: 100 Kohm+ 100 Kohm. Using the 1/X button on your kid's scientific calculator converts 100 Kohm to

.00001. Add .00001 + .00001 = .00002. Hit the 1/X key again and you have 50,000 ohms (SOK ohms). This is the

parallel impedance the preamplifier's output stage must drive.

Now let's take a real world example to see why adding a subwoofer or second amplifier to a vacuum tube preamp is problematic. The REFS SE is rated to drive not less than a 20K ohm load. A connected REF75 SE power amp presents the REFS SE with a 300 Kohm input impedance. Good so far. But say we connect a typical self-powered subwoofer to the second set of main outputs on the REFS SE. Our theoretical subwoofer has an input impedance of 20 K

ohm. Well, 20 Kohm is OK, right? Not so fast, audio tweaker! We must use the parallel resistance equation above: 1/ 300K ohm+ 1/ 20K ohm, or .000003333 + .00005 =.00005333. Hit 1/X and you have 18,750 ohms. This is below our minimum recommended load for the REFS SE.

It gets even worse: Many powered subwoofers have input impedances of 10K ohms. Plug this into the example above and your net parallel load on the REFSSE drops to 9677 ohms !

@leog2015 I have a REF6 (now SE) and used to feed my Martin-Logan BL210 with a pair of single-end connectors, until I read a document that used to be up on the ARC website (that whole section has been taken done).  After I did the impedence calculations, I switched to balenced interconnects and thought I heard a mild improvement.  It was not dramatic.  I will try to post the important part of the pdf, as I can not attach it here.

I missed the part about you having 2 subs.  Same answer, just use the Main 2 outputs, connect one to Main R, the other to Main L.  This is your answer. 

I have 4 complete systems, all have subwoofers.  One system has an ARC LS 26, similar outputs (but not as nice!) as your Ref 10.  I have 4 subs in my main system, two of them are Rythmik F25s.

By the way, the subs are Rythmik 15”s with 4 cu.ft. sealed DIY boxes.

Main out 2, preferably the XLR.  Right or left, doesn't matter.

@soix ,

If he uses the preamp to connect the sub off of an additional "main out" on the preamplifier, the speakers will still receive the full range sound from the amplifier. (the other "main out" from the preamp goes to the amplifier, which powers the speakers...)

If he uses the speaker connections and chains the subs in off the amplifier, I would expect the subwoofer to filter the signal. Subs get the bass, and then anything above the crossover point on the subwoofer amplifier should be sent to the speakers. (Meaning, the sub acts as an inline crossover. I am aware that not all subs do this, but most do.)

Just trying to clarify.....

Using the RCA/XLR connections will maintain full range sound to your speakers.

If you chain them in via speaker cables, most subs (you haven’t specified what you have) will play the sub-bass and then pass the upper frequencies to your speakers.

This looks backwards to me.

Let me know when you get it rocking. You may have to turn on the output in the preamp, I don't know...

Thanks for your help and comments!!

@leog2015 , I looked up that preamp, and it appears you could use a set of the "main output" xlr’s to the subwoofers.

Very nice system, by the way.

May I plug them in any RCA/XLR output of my preamp?

Then you can either chain them in between the amplifier and the speakers (using speaker cables)...

OR...

Plug them into the back of your preamp via the RCA/XLR connections.

Using the RCA/XLR connections will maintain full range sound to your speakers.

If you chain them in via speaker cables, most subs (you haven't specified what you have) will play the sub-bass and then pass the upper frequencies to your speakers.

Yes, both.

Well, do your subs have speaker connections, as well as RCA/XLR jacks?