In terms of actual harmonic oscillators, Ensemble Oscillator isn’t quite that. You can modulate the balance or spread, but while that works beautifully for a sort of rolling, crossfaded arpeggio, it’s not as great as a harmonic sweep sort of thing.
I’m guessing that the Verbos Harmonic Oscillator is the top choice. There’s a pretty good harmonic oscillator custom unit for the ER-301 (I used a modified version that removed the sweep control and put each parttial under manual 16n Faderbank control), and it’s pretty easy to build something similar in Bitwig Grid. Possibly Zoia/Euroburo too, or Norns? I’m not sure.
I actually use the Akemie’s Castle this way somewhat – put it on the algorithm with three carriers and use slow LFOs or envelopes to bring them into and out of the mix.
I agree with the suggestion of bandpass filters/resonant filters though. I’m still working out what to do here myself. A couple of people have been advocating hard for the Serge Resonant EQ, and I’m sure it sounds lovely but I would rather have CVable frequency – so something like the ADDAC603 or Shelves is probably next on my list. Shelves has the advantage of being slightly smaller, much cheaper and lower power consumption, but it seems to me the 603 has more flexibility in many ways.
Also, I find Mimeophon’s Color control is quite good at emphasizing different spectral areas in its feedback.
The SynthTech E520 has (among a lot of other effects) some spectral filtering capability – one of which filters by amplitude threshold rather than frequency range. (E.g. if you filter out the strongest harmonics, there is a tendency to enhance weaker harmonics and noise in the signal, and vice versa) Sweeping those ranges with a relatively constant signal can definitely emphasize different areas in interesting ways.
