Preamp & Subwoofer impedance mismatch?

I use a passive level control of my own design, -10db is about 6.7kΩ in series with the CD player driving a Force sub with 10kΩ nominal impedance in parallel with main amp.

I used it with a Parasound A23 which also has 33kΩ input impedance. Currently driving a NuPrime ST-10 with 23.5kΩ impedance. Measured output is flat in both cases.

See http://www.ielogical.com/Audio/#LastHurrah

However, I would not recommend using the HiPass feature of any subwoofer. IMO 4th order Linkwitz-Riley filter in the very region where the mains are beginning to fade is not the best solution and creates difficult to integrate slope and phase issues, particularly with 3rd party subs. See http://www.ielogical.com/Audio/#Basement. No mention is made of the main speaker capability, but unless they already have substantial low end capability, the lowish 130Hz E112 corner frequencies would give me pause. My experience has found that a simple passive XO for the mains and integrating a sub with higher corner, lower order filters on its own may be a better solution as there is a wider region for ’melding.’ Steep filters can be problematic with a sub and mains cut from divers sonic cloths.

See http://www.ielogical.com/Audio/SubTerrBlues.php for more.

I believe that I heard that JL has new subs with higher input impedance to address this issue.

Good point, but looking at the JL website it appears to apply to the Fathom v2 series (those models having unbalanced input impedances of 50K, although their balanced input impedances are only 10K), but does not apply to the E-Sub series the OP is considering.  And the Fathom models cost considerably more than the E-Sub models.

In the event that I decide to add a active subwoofer crossover, I assume it will essentially act like a SS buffer right? Since both crossover has high input impedance (50K / 100K) and really low output impedance (100 ohms / 150ohms).

Yes.

Good luck.  Regards,
-- Al
 

FWIW I believe that I heard that JL has new subs with higher input impedance to address this issue. I almost bought a Ref 6 before discovering that it’s a poor match with my 10k sub. Turned out to be fortuitous as I discovered an unbelievable solid state pre instead. 

Hi Al,

Thank you so much for the feedback, I did a quick search on the forum and found your past posts regarding the impedance issue and learned a lot, I will defintly try your suggesting and see if I notice any bass roll off first

In the event that I decide to add a active subwoofer crossover, I assume it will essentially act like a SS buffer right? Since both crossover has high input impedance (50K / 100K) and really low output impedance (100 ohms / 150ohms)

While the 10K load would certainly not be an ideal match for the preamp, a mitigating factor is that the **variation** in output impedance between 20 Hz and higher frequencies is not huge. In many other tube-based preamp designs the output impedance may be 3K or 4K ohms at 20 Hz, but only a few hundred ohms at higher frequencies. That kind of variation would result in a much greater deep bass rolloff than in this case, when working into a low impedance such as 10K.

And of course whatever deep bass rolloff will occur can be compensated for to at least a crude approximation via the adjustments that are provided on the sub.

So IMO it’s probably worth giving it a shot with a direct connection. If that proves to be unsatisfactory you can always add a buffer stage or crossover later. Also, while I’m not familiar with the crossovers you mentioned, a member here who has a high quality ARC-based system and faced a similar issue a few years ago resolved it by having Tom Tutay of Transition Audio Design in Florida build a custom made buffer stage for him, which provided high input impedance and low output impedance. I believe the cost was much less than $1K.

I don’t think Tom has a website, but a little Googling should provide you with his contact information.

One final point to keep in mind: If you were to ultimately decide to not utilize the high pass filter function in the sub, and drive both the sub and the power amp from the preamp using the preamp’s two pairs of output jacks, keep in mind that those two pairs of outputs are most probably driven from the same output stage, with the two output jacks for each channel simply jumpered together inside the preamp’s rear panel. That connection arrangement would therefore result in the preamp’s output stage seeing a load impedance of 10K in parallel with 33K, which is only 7.7K, an even worse situation than the one you are contemplating. Although an alternative in that situation would be to connect the sub at speaker-level, to the outputs of the amp.

Good luck. Regards,
-- Al