Nonlinearity behavior of OTA all-pass unit

[jpcima]

Stumbled upon this paper, suggesting a digital model for OTA stages.

Huovilainen, Antti. "Enhanced digital models for analog modulation effects." Proc. Int. Conf. Digital Audio Effects (DAFx-05), Madrid, Spain. 2005.

I wrote faust code of the digital model section 3.2 (substitute the tanh later)

ota_allpass(f, x) =
  x + w
letrec {
  'w = w + (2 * R1 * Vt * g / R2) * ma.tanh(-R2 * (x + x' + w) / (2 * R1 * Vt));
}
with {
  g = 1 - exp(f * (-2 * ma.PI / ma.SR));
  // components
  R1 = 27e3;
  R2 = 10e3;
  C = 68e-9;
  // characteristics
  Vt = 25e-3;
};

Effect on phase response depending on signal level. (on the left is original, right is OTA)