Which subwoofers have at least a 20k input impedance as required for ARC preamp

Thanks for your input, Lalit.  Well, the JL F212 v2 has a balanced input impedance of 10K.  Certainly not an ideal load for the LS28 (which has the same 20K minimum load impedance spec as most ARC line stages and preamps).  And the paralleled input impedance of the power amp would only make that worse. 

I would consider your statement of "without any issues" as indicating "without any apparent issues."

Best regards,
-- Al 

@4425 

After reading this document at the the ARC website (http://www.audioresearch.com/ContentsFiles/The%20Thorny%20Math%20of%20Impedance(0).pdf) and doing the math, I realized that my REF6 was unable to properly drive both my Martin-Logan CLXs (from an ARC 150SE) and the M-L BalancedForce 210 subwoofer, when using the single-end inputs (10K input), as I was doing.  I was a bit shocked, to be honest.

This was discussed with both M-L and ARC and it was determined that I would be fine using either the balanced inputs (30K input), or the high-level inputs from the ARC 150SE (20K input).  I opted for the former, and purchased a set of Sub3 balanced XLR sub cables from Morrow Audio, to replace the Audioquest sub cables I was using.  There seemed to be mild improvement, but it was not striking.  Probably more to do with my piece of mind.

Either way, the M-L BF 210 is a spectacular subwoofer.  The ability to match the crossover to the main speakers exactly for your room and listening position using the Perfect Bass Kit (PBK) that is making a custom filter for the bass to correct for room nodes and antinodes is just amazing.  The runs up and down the neck of a stand-up or electric bass are clear and distinct, with each note at approximately the same volume.  No "in and out" of the notes, or booming, that I have heard in every other system that actually has bass.  To say this enhances listenability does not do justice to the improvement.

I am smitten, as you can tell.

Good luck on your search,

-jim

Thanks for all the input. Actually the issue is very complicated after I spent about an hour on the phone w ARC yesterday. In fact we didn’t really come up with a comfortable combination of amp(ref 75se) and sub.
Running balanced into the amp and unbalanced into the sub is bad regardless of the subs impedance spec such as JL’s 50k spec.
Unless the sub is true differential balanced it’s really single ended. Don’t know about the JL.
The solution might be balanced out to a sub with true balanced inputs and a reasonably high input impedance. Good luck on that one.
To try to understand all of this a potential ARC buyer should call them. They were working the math and it was complicated with no real conclusion.
Obvious answer is ARC preamps are not really designed to drive subwoofers.
Almost forgot. He thought the REL speaker level connection might work.
Im sorry that I’m not technically astute enough to better explain but I personally won’t be buying an ARC Ref pre due to this and the fact that ARC indicated that the 6550 tubes in the power supply of a Ref preamp could be very problematic should one fail in a bad way. Then there’s the amp output tubes.
Regardless of what is posted here there is apparently is a lot of shipping back and forth to ARC for service. For me there’s too much great SS now to suffer the grief unless you’re lucky.
My apologies for the length of the post.

Thanks for the follow-up, @4425. Running balanced into the amp and unbalanced into the sub will result in a very slight imbalance between the impedances of the two signal lines in the balanced signal pair, relative to ground. That will result in a slight degradation of the ability of the amp to reject common mode noise that may be present at its inputs. I’d be surprised, though, if they would consider that effect to be great enough in degree to be audibly significant, since those impedances will be dominated by the output impedance of the preamp, which is vastly lower than the 50K unbalanced input impedance of a JL sub.

Or perhaps they were envisioning the possibility of ground loop issues. But that would be a possibility, depending on the specific designs, even if the unbalanced outputs were being used to drive a power amp and the balanced outputs weren’t used at all.

Those are the only two reasons I can think of that might lead them to recommend against driving a sub having suitably high input impedance with the preamp’s unbalanced outputs, while driving the power amp balanced.

Tketcham’s post reminds me that another member here who uses ARC Ref electronics and a sub had a tube buffer stage custom made for him some years ago by Tom Tutay of Transition Audio Design in Florida, which worked out very nicely and didn’t cost a great deal (well under $1K if I recall correctly). I don’t think Tom has a website but his contact info can be found via Google.

It sounds like you’ve settled on a course of action to which none of this applies, but I’m mentioning these things in case others find themselves in a similar situation.

Best regards,
-- Al