Of course it changes the impedance. What do you think the transformer in a direct box is for? Maybe they don't teach this stuff at Full Sail.
A passive DI box is a transformer.
Running through a transformer changes the impedance. In a DI box it is typically stepping it down from the few10K ohms of a guitar pickup to the several hundred ohms a micamp wants to see.
Running through the transformer backwards steps the impedance up by the exact same ratio, from the couple hundred or so ohms of a console line out to the several 10's of thousands of ohms a guitar amp is expecting to see.
This is literally how a Reamp box works. Not 'literally' in the figurative sense but literally, it's a step up transformer and a variable pad. Or a DI box wired in reverse.
Whats gotten to you? Thats a bit unneeded and offensive, I have never been to full sail, nor am I completely sure what it is but I'm pretty sure I get the implication. I work at a big UK studio, where I learned under my boss who has recorded and worked with some of the biggest acts and albums of the last 16 years. I have learned this stuff from a practical, working point of view, while boosting my knowledge on electronics etc in my spare time. Anything I post here is from experience, not from learning something at some uni course that I have never tried out. My posts were polite and trying to be as informative as possible, if you think I am wrong then cool, I'm all up for being proved wrong if I learn something but there are better ways to discuss this than cheap insults.
ANYWAY, back on topic, your posts have got me interested in researching this a bit more, and I think that we have both got it a bit wrong... On my part, I think the resistor is unnecessary for the DI setup. I think the trouble with using a DI in reverse is that the impedance is much HIGHER than a guitar normally is.
So a typical guitar pickup of say 15k might be stepped down all the way to 40 ohms with a DI box, fine for going into a mic pre. On the way back the 600ohm or so output on an audio device gets stepped up, impedance wise to 225kOHm! Once the signal is recorded, it doesn't rememeber the impedance it was stepped down too, when it comes back out of the console or soundcard it is the output's impedance that matters, which, as you say is normally a few hundred ohms, 600 used to be standard but I'm not sure if thats changed these days. These numbers are from a jensen transformer used in lots of good DI boxes.
Circuits for impedance matching seem to use 'overkill' quite a lot, so that they can be sure that ANY input impedance will be succesfully reduced to an output impedance suitable for most line or mic inputs.
And to be honest I'm still pretty sure this is oversimplifying, especially when you look at inductance and the way guitar pickups REALLY behave, but thats something even the best re-amp boxes don't really seem to have nailed, so I guess its not really something worth worrying about.
When you look at the schematics of re-amp boxes they use transformers with a 1:1 turns ratio, and then adjust the impedance with a switch or pot, which is a bit more flexible than using a 12:1 transformer or similar. As I say, in reamp boxes the transformer is just doing the job of an isolator and 'un-balancing' circuit.
On a practical level, I have reamped a lot, I just finished a months studio session where all the guitars and all the bass were re amped (very strange session..). For the most part they were ok, but not quite as good as using a real guitar, though to be honest a lot of that was probably how the guy recorded the tracks to re-amp in the first place!). We tried a few different methods, the just DI (no resistor) method sometimes was fine, and sometimes was a bit too skewed towards the low end. Sometimes a lead with a 10k resistor was fine (no DI), but most of the time some kind of isolation was needed, as it was a very 'high gain' kinda band and any little noises were gonna become massive after slamming into the dual rectifier!
Anyway, I'm glad this came up becuase I now know I don't need my resistor cable when using a DI to reamp, but I still think a re-amp box is a little more flexible overall, they are very similar devices but not quite the same. You can build one for £30 or so, less if you use cheaper bits, its not hard and should be quite fun, I sense a project coming on
Heres a schematic from the jensen website...
EDIT - I'm very much in the beggining stages of learning electronics so I'm kinda expecting to be put right on some things
