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

Most or all of the JL Audio Gotham and Fathom series subs have unbalanced input impedances of 50K. (Their balanced input impedances are only 10K). Those numbers are indicated in their manuals, which are linked to under the "Support" tab that appears in the description of each sub at jlaudio.com.

However, schematics I’ve seen for several ARC preamps all show that their unbalanced outputs are provided with the same signal that is provided to one of the two signal pins on their balanced outputs. And furthermore, if they provide dual output connectors of each type, for each channel, the connectors are wired in parallel.

Therefore what should be at least 20K is the parallel combination of the sub’s input impedance and the input impedance of your power amp. That number would be the product (multiplication) of the two impedances divided by their sum.

It can be calculated from that relationship that if the sub has an input impedance of 50K the power amp would have to have an input impedance of at least 33K to result in the preamp seeing a load impedance of 20K or more. If the input impedance of the power amp is less than 33K, a sub having an input impedance that is higher than 50K would be necessary for the preamp to see a load impedance of 20K or more.

Good luck.  Regards,
-- Al

@4425, to answer your earlier question 20K in parallel with 300K is 18.75K, and 20K in parallel with 1M is 19.6K. I doubt that there would be a noticeable sonic difference between 20K and 18.75K. However I suspect that 20K already represents a slight compromise (I recall that the specs for some older ARC preamps say "60K recommended; 20K minimum"), and given the caliber of the equipment you are contemplating I would not want to introduce any compromises, however slight they may be.

50K in parallel with 300K is 42.9K, and 50K in parallel with 1M is 47.6K. So the JL Audio Gotham or Fathom subs I referred to should be fine in terms of impedance compatibility, if (and only if) connected single-ended.

Good luck. Regards,
-- Al

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 

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