Hello,
The 18Ω number is the cartridge internal impedance and essentially represents the DC resistance of the coils. This in effect is the source impedance driving the SUT and dictates the frequency behavior at both extremes.
The 100Ω load impedance is a number that changes meaning from manufacturer to manufacturer. Some consider this value an absolute maximum value load that can be applied and still give results that are considered acceptable. (a maximum load is a small resistive value) Other manufacturers consider this value to be the value required for optimal performance and is typically given as a range. (ie 470Ω-2000Ω). In either case this is a very soft number based on some unknown parameters and typically a final value should be determined through experimentation. In the case of the Denon, the cartridge database states >100Ω suggesting it represents the maximum load and anything between 100Ω and 47kΩ would be acceptable. In reality, if you try 50Ω and like the sound there is nothing to worry about.
The nature of a SUT transforms the (typical) 47KΩ input of the phono by the turns ratio squared and presents that load to the cartridge so a 1:10 turns ratio has a 1:100 impedance ratio so that 47kΩ appears to the cartridge as 470Ω. Moving to a 1:20 gets you a 1:400 impedance ratio and 117.5Ω which still is within your manufacturers acceptable range. Going to a 1:40 nets a 1:1600 impedance ratio or around a 30Ω load which is well below the 100Ω number but I know of a few people who use a DL-103 with its >100Ω recommended load with a SUT that can be switched between 1:18 and 1:36 and prefer the 1:36 setting even though the reflected load is actually less than the cartridge internal impedance (36Ω load on a 40Ω cart). This situation makes the number counter scream since 36Ω is 1/3 of the manufacturers maximum load. Then consider that a load that is equal to the internal impedance also results in a 6dB reduction of gain. This works out so the volume level in this situation is only about 1.5dB higher for a 1:36 vs. a 1:18. The truth of the matter is since it is easy to break the rules and flick the switch, 90% of the people will simply leave it on the setting that sounds best and if that happens to be the 1:36.... then so be it.
dave
