Possible impedance matching issue with subwoofer?

Hi Granny,
Yep, this is another example of poorly executed balanced circuits that do not meet the balanced specs set by the recording industry but not implemented correctly by home audio. The seeming inability of the home audio and theater industry to conform to those standards is the cause for this problem. And yep, it sure does affect the sound quality of many fine components and has probably caused many bad reviews of products based on the reviewers who don't account for the problem. A true balanced circuit prevents these problems while single ended technology is rife with incompatabilities. Burson Audio makes a single ended buffer available in the USA that would solve the problem and is reported to work very well Unfortunately they don't make the a balanced buffer any more for reasons they only know. Krell with their CAST system is one of the few manufacturers who addressed this problem and essentially implemented a balanced system with their own proprietary connector. As for as I know Atmosphere is the only tube builder that has designed and uses a balanced circuit with tubes, in fact they own a patent for their design. It apparently is much easier to build solid state balanced circuits but the home audio industry does not design for this all the time either. There is apparently much more involved in designing a balanced circuit than just adding a balanced connector to the equipment's design. But if all the manufacturers would conform to the standards set by the music industry impedence mismatches between our equipment would be a thing of the past.

Sgr, As for balanced tube designs, BAT and Audio Research have been using true differential circuits for many years. I'm sure there are others as well as the aforementioned Atmasphere.

Thanks for the responses, but looking for an answer to my question which seems to have nothing to do with true balanced designs .

Grannyring, all of the understandings you expressed your OP are correct. Whether or not significant deep bass rolloff or other frequency response anomalies will result from the non-ideal impedance match depends on whether or not the impedances that are involved vary significantly as a function of frequency.

It is very common for a tube preamp to have a coupling capacitor in series with its output. I don't know whether or not the Dude has one, or uses some other design approach, but preamps that do use an output coupling capacitor will usually have an output impedance that rises significantly at deep bass frequencies, relative to its value at higher frequencies (which is already very high in the case of the Dude). That low frequency impedance rise will cause a deep bass rolloff in conjunction with the sub's low input impedance, due to what is known as the voltage divider effect.

Also, if the preamp does not drive the two sets of outputs from individual buffer stages, and instead simply wires together the two output jacks for each channel inside the rear panel, then the low input impedance of the sub can also adversely affect the signals going to the main speakers. It is very common for a preamp that provides two sets of output jacks to not buffer them independently.

Also, if the preamp does not individually buffer the two sets of outputs the capacitance of the RCA cable to the sub can affect the signals to the main amp and speakers similarly to how the capacitance of the preamp-to-main amp interconnects can. If the total of the lengths of the two sets of interconnects is long, and their capacitance per unit length is not low, given the high output impedance of the Dude the upper treble can be rolled off, at least slightly, as a result of the interaction between that output impedance and the TOTAL of the capacitances of the two sets of cables (if the preamp does not individually buffer the two sets of outputs).

Given that your sub has speaker-level inputs, it would make sense to try hooking them up to your power amp outputs, and comparing sonics with the present arrangement. The wires from amp to sub do NOT have to be heavy gauge high quality speaker cables, because they will be conducting an extremely small amount of current (as a result of the high 20K input impedance).

Regards,
-- Al