During many recent Denon + Audyssey trainings a number of installers asked if it were possible to extend the included Denon/Audyssey calibration microphone cable. As you know, there are instances when the installers need additional length, such as when the receiver is in an equipment closet away from the listening room.
To see if it were possible, a pre-made 25ft 3.5mm mono extension cable (male to female) was supplied to Audyssey, and had has tested in their lab. The primary intent of the testing was to see if the extension cable would alter the delicate, low-level signal from the microphone to the extent that the receiver would arrive at an incorrect Audyssey room correction solution. Audyssey's lab tests showed that the 25ft cable does indeed change the readings the receiver gets from the microphone, but not to a degree that it would alter the Audyssey "solution" to any appreciable amount.
This is what Audyssey had to say specifically:
"The extension cable causes an approximately -1.5 dB rolloff at 20 kHz and it is slowly changing, so the difference is not just at 20 kHz (it is also affecting the >2 kHz region). There is little change at 1 kHz, so absolute level calibrations will be accurate (i.e. Dynamic EQ, Dynamic Volume). The rolloff will, of course, be compounded if this cable were extended further (by daisy chaining)."
"The difference is marginal enough where it will not ruin a calibration"
As a result of their testing, we now know that extending the microphone cable is a viable solution for use when needed. Of course, this gets us to the question: how do we get an extension cable or where do we get one? The answer is that the cable can be sourced from MCM Electronics [Link: mcmelectronics.com]
This extension cable has only been tested on Denon receivers, and therefore the effect it may have on other calibration microphones is not know. For that reason, you should not use it on any other receivers with the assumption that it will not adversely affect the calibration readings. The cable may physically work, but the cable's electrical properties may alter the signal to the point the receiver arrives at an incorrect room calibration reading.