(This is all my opinion, not the forum’s)
If you’re primarily doing signal processing of external signals, you’ll need to get your signals up to Eurorack levels, which are 6db higher than line, and then back down. This is usually done with a ‘guitar interface’ module though your line signal can be anything. Linked search of modules is not exhaustive, but a starting point. Getting back down to line can be less of a problem if your mixing solution can trim, but if you’re recording directly into the OP-1 or Norns I believe you’ll need to bring the signal levels down first.
If you’re looking for the traditional idea of “effects”, there are some. Like drums, effects are not something that modular necessarily does better than non modular (there’s a whole thread devoting to discussing this actually). But, there are some interesting reverbs and delays. It’s worth looking for a few that sound interesting and either trying them out directly or watching YouTube videos about them to get a feel for what they sound like. A short list might be (anyone, chime in with your favorite):
- Erbe-verb
- Echophon
- Chronoblob
- Magneto
- Folktek Alter
The places where modular really shines in the realm of signal processing are (IMO)
- modulation over parameters
- ‘borderless’ signal processing
The first is pretty self-explanatory: basically, a good modular effects unit will give you control to automate changes to some or all parameters of a given effect. This does mean however that you’ll want modulation sources. I completely recommend a Maths as a first modulation source, because it’s such a useful module and also one that you have no problem selling if you decide Maths (or modular for that matter) is not for you. Nearly everyone wants a Maths at some point.
For 2, what I mean is that, modular pretty much lets you route signals however you want. Its reverbs and delays are fine, but you could also take incoming audio and use it to modulate the frequency of another signal. Or split that signal into a bunch of frequency bands and route them into different places in stereo space. Or feed that audio signal into something that will derive triggers from it, and use those triggers to fire very short envelopes so you are essentially getting a sawtooth sound that is related to your incoming audio. Or… ==insert Lines suggestions here==
You can kind of do anything you want, is the point. That sort of ‘what if I do this, oh neat, what if I try that’ is the draw, IMO. Also, the fact that audio signals and signals for controlling modulation are the same thing (at least in Eurorack), which opens up all kinds of possibilities.
I don’t want to discourage you from getting into modular! But I think if you put together a rack to do reverb and delay, you’ll be disappointed in the return on investment versus just getting a pedal. I hear the neat & tidy aspect of the rack, and if cost isn’t a concern, then have at it. I do think though that effects are one of the places where modular is not as effective as other kinds of gear, if you’re thinking of effects in the ‘traditional’ sense.