## søndag 15. oktober 2017

### Noise filter responses

I just downloaded LTspice IV in order to simulate filter responses for the state variable filter for the XM8. To learn, I started out by simulating the various noise filters from the Andromeda A6.

Here are some nice plots to look at (continuous line is frequency response, dashed is phase response):

 The A6 noise circuit. I've used LT1001 opamps, just because they where the first in the list.
 White noise filter - an HPF. Measured Fc seems to be around 5.3Hz; each of the two filters have an Fc of 3.43Hz. Gain is around 36dB (?)  (multiplier: 72)

 Pink noise filter, a 3dB low pass filter approximation, fairly linear from 10Hz to 20kHz. Gain is around 26dB (multiplier around 19.6 measured, as opposed to 21 calculated from the normal non-inverting amplifier formula, due to the 68k resistor)

 White and pink noise filters combined

 Red noise filter, 12dB low pass with Fc at 107.6Hz. Gain is approx 10dB (multiplier: 3.16)

 White, pink and red noise filters combined

Now, after simulating the A6, of course I had to do the same to my pro-one/elliot white/pink noise combination. It turned out like this:
 Pro one modificaton white noise. This is an HPF, similar to the one from the A6. The gain is very different though, about 55dB for this one.

 The Elliot 3dB pink noise filter. Not very different from the A6, but with a very different phase change.

 White and pink noise filters combined. Fairly similar to the A6.
Now for the interesting part - how do the two circuits compare? Well, first of all, the overall gain of the A6 is significantly less than the pro one/Elliot version. Here is a combined plot:

 A6 (green) vs pro one/Elliot (blue).
I did however do a calculation of all expected gains and updated the A6 circuit with a 20k resistor in place of the first 2.43k one in the white noise circuit. It sounded very similar to the pro one/Elliot after that, but what do the plot look like?

 A6 after replacing a resistor (green) vs pro one/elliot (blue)
I'd say they are very similar! One thing to notice though, is the difference at the deep end. The pro one/elliot removes much more of the really deep frequencies. This is below what your ear should be able to hear though.