77.43.10.3
In Reply to: RE: I know, I know posted by Ralph on November 05, 2009 at 13:55:33
Dear Ralph,
I have provisionally replaced common cathode resistor with current generators (ideal CCS) of the same value of the bias currents.
The overall result is negative, that is, no major change of output spectrum.
Then I have checked the spectrum at the output of the first stage and at the output of the second stage. Here I have seen the likely culprit.
There is an unexpected (and fixed) -55 dBc third harmonic at the output of the first stage in both cases (that is, with and without CCS).
The likely problem is that computer simulations with 'simple' tools (of the level of free LTSpice), due their inherent numerical errors, do not deliver reliable results below -50 dBc (0.3% distortion).
As a consequence, as it is not easy for me to unmask the effect of the third harmonic that is unexpectedly present at the output of the first stage, and that is likely due to numerical rounding errors, it is not possible to ascertain with LTSpice the prospected effect of your recommended modifications.
We can only trust your professional experience.
I have been told that other SPICE tools, whose license cost more than several thousand dollars, could deliver improved accuracy (down to 0.1% distortion or even better), but that kind of investigation is too expensive for me.
Best Regards
Luca Cellai
ecc230
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