I would just get a good stand alone phono stage and be done with it. That way you can use whatever cart you like. Also, you can just use regular IC's and not a phono cable.
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The Audible Illusions is very nice sounding. I have a Modwright 9.0 Sig Edition. And I use a Musical Surroundings Phonomena phono pre. It is adjustable for any cartridge MM/MC. It sounds Dynamic,detailed,accurate and quiet with plenty of punch and jump factor. It will be in my system for a long time.
I'm not saying that the AI is not good. It just looks like it doesn't have the options you may need and a separate phono stage, to me, is just a better/cleaner way to go about it. You can also get better synergy and save some money.
Here's an example of what I mean. I have a Scout as well. I use a Dynavector P-75 phono stage and a DV 20 low output cart. Both pieces are not that expensive but I'm still getting very good sound because of the synergistic match between the 2. I think if you focus too much on getting a head amp, you greatly limit your choices, not only for upgrades not, but in the future, as well.
Anyway, its just something to consider. Phono stages have come a long way over the 23 years since you bought you AI.
Sorry about that. From your post, it looked to me that your preamp was over 20 years old. I don't really follow the model numbers close enough to pick that up. Now I see your issue. It may very well make sense to go with a SUT. I still wouldn't lock myself into one, though. You may want to give The Cable Company a call. They should have some phono preamps and SUT's t lend you so that you can try them in your system first. I use them all the time for this type of thing.
The debate over "separates" vs full-function preamplifiers is a never-ending one, but suffice to say that there are many advantages to an all in one unit vs separate phono and linestages and vice-versa. But why would you expect "better synergy" and saving money to be advantages associated with separates over an all in one preamplifier? Most assuredly, those would be points in favor of the latter choice. Two more advantages are avoiding purchase and use of a costly, signal degrading interconnect, and the fact that a separate phono stage usually needs a buffer stage to drive said interconnect. The very short internal signal wires used in a full-function preamp to connect its phono section to its linestage can often be implemented in such a way as to avoid a buffer stage.
I tend to agree with what you are saying. As I stated in my last post, my comments were based on me being under the impression that the OP had a 23 year old preamp. With regards to the cost/synergy issue, I gave an example of the DV cart and phono pre. I just think its a good example of how to get great sound at a reasonable cost. Personally, I feel that there is probably an advantage to having 1 company make both products; definitely in terms of synergy, and possibly cost. You have to really take it on a case by case basis.
"But why would you expect "better synergy" and saving money to be advantages associated with separates over an all in one preamplifier?"
I wouldn't. Like I said above, you need to take it on a case by case basis. I probably wasn't clear about that, though. I was just referring to something like the DV combo I used as an example.
"Two more advantages are avoiding purchase and use of a costly, signal degrading interconnect, and the fact that a separate phono stage usually needs a buffer stage to drive said interconnect. The very short internal signal wires used in a full-function preamp to connect its phono section to its linestage can often be implemented in such a way as to avoid a buffer stage."
On this last part, we may have a real disagreement. I try not to lock myself in to general statements like this. For me, it all depends on what the equipment is and how it sounds. If it means doing it the above way that you suggest, that's OK. But if I think another way sounds better, that's OK too. I would venture to say that you may agree with me on that last statement, but I won't put words in your mouth.
So I'll go back to your original question: what is the difference between the 40 and 3 ohm settings?
This can get confusing because some SUT manufacturers list the impedance presented by the SUT (assuming its feeding a typical 47k load into the MM section of your pre) and others, like your Denon, use the figure corresponding to the internal impedance of the cart you're using. The DL103 has an internal impedance of 40 ohms, but, and this is the important part, the impedance it wants to "see" is several multiples of this (Art Dudley recommends 2-6x, Bob's Devices recommends 10x, YMMV). So if you buy these guidelines your SUT + MM stage should present somewhere between 80-400 ohms to the DL103 to sound its best.
Now, according to Denon your SUT has turns ratios that can be switched between 1:10 and 1:36. These correspond to impedance ratios of 100 and 1300 respectively (the squares of the turns ratios). To calculate the impedance presented to the cartridge you divide the input impedance of the MM stage (typically, but not always 47k ohm) by the impedance ratio of the SUT. In your case these equal 470 and 36 ohms respectively.
What I notice is that neither of these is really dead center "ideal" for a 40 ohm cartridge, though the higher setting *should* sound better. But supposing that the input impedance of your MM section is not 47k but 100k ohms (some are!), that would mean the the impedance presented to the DL103 is either 1000 or 77 ohms. In which case the 40 ohm setting might be too high, and the lower 3 ohm setting may be preferable.
Clear? as mud, I bet. :^)
And this is where I say "trust your ears"....
I do it both ways. I have both a full-function preamplifier (Atma-sphere MP1) and an outboard phono stage (revamped Silvaweld SWH550). I use the MP1 for LOMC and the Silvaweld for MM/MI. So I am not as dogmatic as I may have sounded. Nevertheless, given my 'druthers, I'll take the full-function preamplifier. It has to be top-notch in quality, however. I sense the Silvaweld is at a disadvantage, because it requires a cathode-follower output section in order to drive the IC that connects it to the linestage section of the MP1. Plus you've got to more connectors in the signal path. However, there is room on heaven and earth for both approaches.
JC Gold phono board in the AI is most likely the same daughter board they've been using for years. To find out, do a visual comparison or ask John Curl. I wouldn't trust any other opinion especially the manufacturer.
IMO SUTs are band-aids that ad more connections to an already delicate signal. In this case
["ZU/DL-103 PHONO PREAMP MATCHING
The nominal output voltage of the Zu/DL-103 Mk.II is 0.3mV and the nominal internal impedance per channel is about 45 ohms. We recommend running a step-up transformer for moving magnet phono preamps. A step-up transformer is not required for phono preamps that have a moving coil (MC) setting, setting your impedance between 80 200 ohms. Even so, you should consider a Bobs Devices 1131 or similar."]
What CJ says is spot on, which is why I was wondering at the sonic results. The OP writes that the sound is more "romantic and richer" and moreover that there is an apparent loss of gain, at the 40-ohm setting vs the 3-ohm setting. That does not make sense, if the 40-ohm setting corresponds to an input Z of about 470R or 1000R (seen by the cartridge). That sonic description more fits what I would expect from the 3-ohm setting. Could it be that the Denon SUT is mis-wired such that the 40 setting is actually the 3 setting and vice-versa? ("Romantic", "rich", and apparent reduced cartridge output are associated with using a phono input impedance that is lower than optimal for the cartridge internal impedance. 470R input Z is not going to sound that way with a 33-ohm cartridge internal resistance.)
And FWIW, lots of DL103 users do seem to prefer oddly low phono input Z (breaking the 2X to 6X "rule"), nearly equal to the cartridge internal resistance, for best sound, even though there is some loss of gain associated with such an impedance mismatch.