Actually my pink filter is more based upon the TLC2272 circuit here http://www.techlib.com/electronics/pinknoise.htm
. The evidence is in the way the capacitors and resistors are arranged in the feedback loop.
My modifications included changing the gain, so that his 20k feedback resistor became 220k, his 470 ohm resistor became a 330 ohm and I added RC pair C7 and R12 to further correct the response near 20kHz. The TLC2272 circuit was designed to flatten out after 4kHz, which was not good enough for my intents.
The MC4558 circuit does have a good pink response, with fewer FB components. But I didn't like the white response (and I intend on bringing both out). The white tapers off rapidly as you go below 60 Hz, which I felt was excessive. The white also peaks above 22kHz but this can probably be ignored (though it could introduce distortion somewhere down the pipeline if it was too high in amplitude).
The MC4558 pink response is dependent on the white response of the prior stage, so if you were to fix that, then you'd have to revisit the pink filter design.
I also didn't like the opamp used. You could sub out the MC4558 but then then you'd have to rework things somewhat. The MC4558 has a minimum slew rate of 1.5V/us, which seems low - I need to check this, but it is probably barely adequate.
The big objection was the noise rating, which is 12nV/sqrt(Hz) for the MC4558. I have read that you should use an opamp with less than about 5nV/sqrt(Hz) so that you're amplifying your noise source rather than the opamp's own noise.
So I chose to use the NE5532 opamp which has a typical noise rating of 5nV/sqrt(Hz) at 1kHz. It also has a slew rate of 9V/usec, which may or may not be significant but is considerably higher. The other factor for choosing the NE5532, is that I acquired several of these, pulling them from discarded TV station audio console cards, that I got from a friend.
When I first saw circuits and the advice for using low noise opamps in the pink filters, I thought this was nuts (high quality noise?) But there seems to be sound logic to it. I'm just not sure I would be able to tell the difference.
Every module build has been a great learning experience!Update:
I went back and calculated that SR=1.5V/usecs is good up until about an swing of 9.5 volts @ 20kHz. Given that noise waveforms may need a minimum of a 2nd harmonic (for faithful reproduction), this is good to about 4.25 swing volts. So this would seem to barely cover the application ok, if the signal output level was at or below 2.125 volts amplitude (4.25 volts swing).
On the other hand, a SR=9V/usec, is good for 71.6V @ 20kHz, or about 8.95V swing at the 4th harmonic of 20kHz.
Clearly a cleaner top end, though few of us would probably hear the difference.