The lack of range on the volume control is due to the extremely and unusually high 30 db of line-stage gain. Impedance compatibility is a separate and unrelated issue.
The majority of active line-stages provide gains in the area between about 6 db and 15 db. Anything in that range will probably cause the volume you presently have at 9 o’clock to come out above the 12 o’clock position, and gain in the lower part of that range would probably bring it to the vicinity of 2 or 3 o’clock.
An impedance incompatibility, if present, would mainly affect frequency response flatness, especially in the deep bass region. The 1.2K spec is probably based on a mid-range frequency such as 1 kHz, and may rise to considerably higher values at deep bass frequencies due to the coupling capacitor that is used at the output of most tube-based preamps (the impedance of a capacitor rises as frequency decreases). Whether or not a problem occurs depends on the **variation** of output impedance over the frequency range.
To assure that an impedance incompatibility does not occur the commonly accepted rule of thumb, properly stated, is that the amp’s input impedance should be at least 10x the output impedance of the preamp, at the audible frequency for which the preamp’s output impedance is highest. Although non-compliance with that guideline does not mean that a problem will necessarily arise; compliance with that guideline assures that a problem won’t arise. For most tube-based preamps the highest output impedance at any audibly significant frequency is likely to be at 20 Hz.
If the 20 Hz output impedance can’t be obtained from the manufacturer or determined in some other manner, I suggest using a ratio of 75x to be safe. To assure compatibility with the Odyssey that would mean a nominal (mid-range) output impedance of less than or equal to 22K/75 = 293 ohms.
If the maximum output impedance at any audible frequency can be determined (and if Stereophile has reviewed the product it will be indicated in the measurements section of the review), then 22K/10 = 2.2K would be a safe upper limit.
Good luck. Regards,
-- Al
P.S: Per bdp24’s good suggestion, you might consider purchasing a pair of
Rothwell Attenuators in the 20 db configuration that is offered. Some people have reported compromised dynamics or other sonic issues with these and similar devices, while others (including me) have found them to provide excellent results. I’m fairly certain that the Rothwells will provide a load impedance that is high enough to be suitable for use with the AI preamp; I know that some competitive units from other manufacturers will not.
