[sdiy] SSM2040 filter question

[Antti Huovilainen]

On Wed, 5 Nov 2003, René Schmitz wrote:

> This nonlinearity is there, but you will only see it in certain

The way I see it, the circuit has only two asymmetries:
Asymmetric supplies (+5V "ground") and the darlington buffer.

The distortion from the differential pair itself is symmetric (tanh()).

Because of local negative feedback, distortion is mostly apparent around
cutoff frequency. Below that negative feedback reduces it to very low
levels and above that the lowpass filter action takes over so output level
is very low.

I would like someone explain how SSM2040 differs from 4 OTAs. To me each
section looks identical to CA3080 + darlington follower.

Also, how is the feedback amplitude limited since there is no nonlinearity
in the feedback path in that version?

> stages. That means that each half wave of the input signal will undergo
> that "clipped integration", only in different parts of the circuit. That
> would mean that in total, both halfes will be clipped. A circuit with
> only noninverting stages, would be an interesting variation.

If the signal is not running to rails and the darlington buffer has low
output impedance, then this shouldn't make any difference at all.

There are then three possible conclusions:
- Input goes close to supply rails and thus there is asymmetric distortion
  (unlikely to sound good there).
- Darlington pair has highish output impedance, thus distorting the audio.
- SSM2040 doesn't clip asymmetrically and sounds good for other reasons.

Antti

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