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What is the greatest or latest? Been going through some vintage recordings that lack high frequencies and was looking to excite them, when asking this question popped to my mind. I use software, but if there's staple hardware or something that you've found to work, then would be interesting to hear experiences about those as well.
Lets keep this question strictly about "exciters" or specific distortion plug-ins/hardware, free of other techniques for adding high frequency content (EQ, MB compression, reverb ERs, layering etc.).
I'd recomend BBE Sonic Maximizers. I've used their plug-ins for some time now, and have always been happy with the result. They have a few different hardware models as well. You'd have to check which would be best for you. If I had the budget, their gear would be in my rack.
Edit: To be more specific about your needs, you might want to look at the D82 Sonic Maximizer, or the H82 Harmonic Maximizer plug-in. If you're looking at hardware the 482i, or the 382i may be best for your needs.
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I believe it's the most customizable one in the market (64 combinations of algorithms!). Quite precise as well with 11 frequency bands, but I'm not sure what to think about the processing quality. With tweaking it "works", but then again it somehow doesn't feel like it can sound as good as for example Ozone's exciter. Haven't used it much yet and definitely not gone through all the combinations, so it might just be that I'm not very fluent with it yet :)
I really love the Ozone 5 exciter. I use it all the time. The different harmonic order algorithms you can chose are all different and useful and the overall sound quality is a definite upgrade from Ozone 4. Im not a fan of OneKnob's Brighter myself (though I love Phatter, pretty useful!).
Also, thought its an EQ, the Waves SSL EQ is great for adding air around 15k and may help in certain cases.