[sdiy] Weird filter in OB-Mx noise circuit

[mark verbos]

"Pink noise has equal power per hertz. It's power density drops off at 
3dB/Octave. Since an RC filter drops off at 6dB/octave, a more 
compicated filter is necessary to generate a pink spectrum from a white 
noise input."  - The Art of Electronics

The middle stage is what is called a multiple feedback path lowpass 
stage. I think the advantage over a normal lowpass stage is that you can 
control the gain and the frequency in one opamp stage. This creates the 
-6dB per octave filter that makes the +3dB (white noise) signal into a 
-3dB (red) noise one.

I hope that helps in some way.

mark





Aaron Lanterman wrote:

> 
> On Tue, 18 Apr 2006, mark verbos wrote:
> 
>> what you see there is Buchla's way of filtering down white noise into 
>> pink noise. It is similar to what he did in the 265  and 266 Source Of 
>> Uncertainties' noise sections.
> 
> 
> Oh, I know _why_ it's there - I just find it interesting that I've never 
> seen that particular combination of caps, resistors, and op amps 
> before... how did he come up with it, I wonder?
> 
> - Aaron
> 
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