This topic has been well discussed here over the years so my first advice is perhaps do a search (here).
The category affectionately known as DIs ranges from simple passive transformers to full blown preamps. Historically DIs were all passive and used transformers to scale down a source's impedance to be a better match to mic preamps (nominally 1.5-2k ohm). Passive transformers scale impedance by the turns ratio squared. Active DIs can use phantom power to operate without batteries (I designed one at Peavey back in the 80s using a bifet op amp inside).
One consideration is what kind of signal source are you trying to interface through a DI? Keyboards or gear with active drive outputs can easily drive a passive transformer DI without problem (a 10:1x transformer scales down source impedance 1/100x while scrubbing off 20dB of level). Some instrument signal sources benefit from the even higher input impedance available from active DIs.
It sounds like you are considering more than one application, so maybe consider more than one approach.
JR
Hi, JR, I do generally try to search first. Unfortunately this was a hard topic to search for because "di box" brings up a lot of unrelated results. I found some results that were specific to passive or tube DIs but I was trying to get some general advice rather than assume I knew what would be best here (which I appreciate getting). Also, I'm not sure if this is a consequence of the forum software and you might be unaware, but search results don't actually go back that far anymore
I just checked again and the oldest hit was 2020! Like I said, though, there were a couple threads about passive projects.
These are the signals I need to D.I. (not all at once):
keyboard with dual TS outs (this, as you said, should be fine knocking the signal down with a passive, but really I just would want to be able to run a balanced signal 50 feet to the board)
an acoustic with an active pickup -- 1:1, active, or any passive is fine here but would prefer not to knock down the signal
a guitar, mandolin, and fiddle all with passive pickups (these all need active as I would expect they need ~10M)
I appreciate this advice. I am certain I can't beat the price on a Live Wire or Behringer, but I did think making a couple of them might be slightly more fun. I'm prepared to buy a couple if I don't like what I end up with.
ETA: Forgot to mention! A ton of the boxes I was looking at aren't even 1M input impedance, I was concerned about the passive instruments. Commercial DIs with very high input impedance tend to be expensive. I know I could just buy/build a preamp instead, but I'm trying to minimize the crap to keep track of on stage.
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I can at least mention what I have lying around:
Schoeps mic boards, which I could easily modify to 1M input impedance and have something running on phantom power with a balanced out. Just literally don't know if this will work for an instrument signal.
Several Edcor 600:600, they're a bit heavy and probably overkill but I could stick a FET follower in front.
THAT (1646 maybe? not sure exactly where I put them ...) chips that do balanced out, but I'm not sure I can run them on Phantom.
Some 10:1 but they're nice and I don't particularly want to use them.
Plenty of guitar pedal parts and some guitar pedal boxes that any purely active DI circuit would easily fit in.