Adding Tone Controls?

Impedence issues between the CJ preamp and Pass amp were (and maybe still are) a concern. I raised this issue a while back on another forum and received feedback from several folks (including Kal Rubinson) confirming that the pairing meets the guideline that "the input impedance should exceed the output impedance of its source by 10x or more".

Like you, I am uncertain. The 10x factor should be applied to the worst case (maximum) output impedance of the preamp at any audible frequency. For a tube preamp that will commonly be at 20 Hz, due to the output coupling capacitor most (but not all) tube preamps use. Per Stereophile's measurements the original version of the C-J 16LS had a worst case output impedance (at 20 Hz) of 1.8K, and from their comments in the addendum to the review it sounds like the upgrades that were incorporated in the Series 2 version would not have affected that. The manual for your X250 indicates that the input impedance of its balanced inputs is 22K, while not indicating an input impedance for the unbalanced inputs which I presume are provided and you are using. Conceivably the unbalanced input impedance could be half the value of the balanced input, or 11K, which would result in a SLIGHT rolloff of the deepest bass when driven by an output impedance that rises to 1.8K at 20 Hz, while having significantly lower values at higher frequencies.

Re the use of digital equalizers: I did some further checking, and it appears that the two digital outputs of your transport are both 75 ohm coaxial outputs, on an RCA connector and a BNC connector. It also appears that neither the Behringer DEQ2496 or the DSPeaker will accept coaxial digital inputs. Also, the DEQ2496 does not provide unbalanced analog inputs or outputs, so it could not be inserted into a processor/tape loop on your preamp without converting or adapting the signals from unbalanced to balanced form, and back, which would introduce additional cost and/or possible sonic compromise depending on how it is implemented.

The DSPeaker device provides unbalanced analog i/o's, as I and others indicated earlier, and could be readily introduced into one of the processor/tape loops that is provided on your preamp. The one possible issue I see with respect to that approach is that the output impedance of the preamp's processor/tape outputs is not specified, and the input impedance of the DSPeaker's analog inputs appears not to be specified. You might want to contact the two manufacturers to ask them if they can supply those numbers. Although given that the DSPeaker would only be in the signal path when you are listening to the problematic low quality recordings, a less than ideal impedance match may not matter anyway.

Finally, in case you want to consider other kinds of digital equalizers, which could accept the signal from one of the transport's coaxial outputs, I would be cautious about assuming that both outputs can be used simultaneously, unless the manual or other literature explicitly indicates that they can be. It seems conceivable to me that the RCA and BNC connectors might simply be jumpered together inside the rear panel of the transport (rather than being driven by separate output stages), the expectation being that only one of them would be used. In that situation, using both at once would result in a severe impedance mismatch, which would undoubtedly degrade sonics for both outputs.

Hope that helps. Regards,
-- Al

Bama, Nice post. A couple of comments on speaker/listening chair location/set up. Keep an open mind on this because some of it is not intuitive and conrtary to customary set up.

1) Set the speakers up so that the speakers/listening chair form an equallateral triangle. In a room with you room's dimensions I would keep the distance from the back wall at 5'+, but I would seperate the speakers substantially more apart. Say 10' with the listening chair back to about 10 - 11'. This will give you a much larger soundstage and, in my experience contribute to reducing congestion which is not unusual when the speakers are too close together and listened for far back.

This will also place the speakers closer to the side walls but due to the nature of those speaker's radiation pattern side wall reflections are not a big issue (remember figure 8 pattern). The only thing it might affect is the bass response linearity a bit, maybe a little boost between 100 and 200 hz. But then all speaker position will do that Often its a matter of choice of options, not perfection.

2)I would toe the speakers in so the axis of the speakers pointed at the listening chair and the back wave bounced off the side wall behind them. This will reduce the need for acoustic treatment on the wall behind the speakers substantially and quite possibly bring the stereo image into greater focus.

This is going to look and sound different than your present set up substantially I think. It will also require some tweaking after initial set up so give it a chance. If it works it will save you money and grief and perhaps as I suspect enhance your listening experience. If not just put them back to where you have set them now. Invest a couple of weeks (at least in this project). I discovered all of this the hard way and it took me an embarrasing long time :-)

BTW, flat bass response set with meter or by ear is going to be impossible, close perhaps. But I do not let flat bass response dictate where I set my speakers unless it is gross. It's the mid's and highs that are essential. Usually the location of the listening chair is as, if not more, important for getting a good bass response anyway.

OK, I'm done. Sorry for the length of my posts. Good luck and let us know how it works out for you.

To add to Almarg's always helpful post above--About the DSPeaker--it looks like you can special order input impedance to whatever value you like for slight up charge (not specified). Standard is 10k ohm. I have been kicking the tires on that unit and looked at the owners manual yesterday. Hope this is helpful.

Almarg --- the transport is also equipped with and AES/EBU output, and seems to be driven along with the coax. I know since I was able to do some A/B testing between the Enkianthus and the Musical Fidelity DACs.

Newbee --- moving the speakers is always worth a try. In my initial setup I had them toed in more, pointing to the listening position. To my ear, that resulted in too much high energy, which tended to confirm the recommendations made in the Martin Logan manual to have the inner 1/3rd of the curvilinear panel pointing to the listening position. Opening up the spacing would be easy to try out.

The transport is also equipped with and AES/EBU output, and seems to be driven along with the coax. I know since I was able to do some A/B testing between the Enkianthus and the Musical Fidelity DACs.

Good! The rear panel photo I was looking at earlier must have been misidentified, but I now see some photos showing the AES/EBU output.

So that would provide you with a means of using the Behringer unit. I can't tell for sure from the photos and writeups I've found if the transport provides an optical output as well, though. If not you would still have to use the DSPeaker in an analog processor/tape loop, since its only digital input is optical.

Swanny, thanks very much for the info about the input impedance of the DSPeaker. I've been considering giving it a try in my system as well, at some point in the next few months, to deal with a room-related suckout I have in the 40 to 50 Hz area. Still a little concerned about the effects on transparency of introducing A/D/A conversions into the main signal path, however, although the very positive comments from Kal, REG at TAS, Roscoeiii here, and others, leave me very tempted.

Regards,
-- Al