I am trying to build a pink noise machine to use at work. See schematic below. If I connect the output from the MM5837N (noise generator), directly to the amp circuit. It works fine, and is nice and loud. However, when I include the passive LPF as shown from the example, no sound comes out. Using a scope, Fig. 1 shows the output from the MM5837 w/o the LPF. Voltage is around 4V. Figure 2 shows the output after the LPF. The signal is around 2V. At this point, the output from the LPF is not connected to the amp input. When I do connect the output to the amp input and probe at the same point, the signal drops to around .2V, and there is no output from the amp. Scope shows a flat-line on the amp output. I am not an EE, so don't understand what is going on here. Do I need some kind of a buffer op-amp between the two? Any help is appreciated! Also the LPF cap is just a ceramic type, if it matters. Link to the amp data sheet.
Best Answer
You show a range of values for your LPF. If R=10k and C=0.2 uF, then the cutoff frequency of your filter is about 80 Hz. Most of the energy generated by your noise source is well above that frequency. If you set R=1k and C=0.02 Uf, then the cutoff frequency rises to about 16 kHz and you should see a considerably larger signal. Also your 10k pot will load the output of the LPF and also reduce the available signal.