1. Is the Preamps input cut-off freqency 30Hz?
The mic is in parallel with R1 and they make a total of about 2.8k ohms.
R2 and R3 are in parallel and make 50k ohms. The total resistance is 52.8k ohms.
The capacitor C1 is 0.1uF so calculate the cutoff frequency. (30.3Hz).
What is the Preamps output cut-off freqency?
It depends if the output capacitor is 0.33uF, 0.56uF or has a 0.1uF capacitor in series, and it depends on the resistance it drives.
If the input and output cut-off freqency are the same do they add or subtact at the preamps total output cut-off freqency?
They each cause the output level to be reduced 0.707 times at their cutoff frequency. So if they both have a cutoff frequency of 30Hz then the total cutoff frequency is 60Hz.
What is the total output cut-off freqency at the preamps output?
It depends on the value of the output capacitor and on the resistance it drives, and is affected by the input cutoff frequency.
Is the preamps 0.33 coupling capacitor there to calculate with the 50k load for the headphones input cut-off frequency? Then the load is in parallel then the 50k stereo pot becomes a 25k load?
Yes.
2. Will the Preamps total output cut-off frequency (add) to the headphones input cut-off frequency then adds to the outout cut-off frequency? If so. Then what will the total cut-off freqency be at the headphones output?
If all three cutoff frequencies are at 30Hz then the total cutoff frequency is 90Hz because they each reduce the level of low frequencies (a highpass filter).
If my headphones is 8ohms and each capacitor is at 470uf then the output cut-off freqency will be 42.33Hz.
If you calculate for the headphones input cut-off freqency with a capacitor of 0.33 and a 50k load in parallel that
50k is subtract in parallel and becomes a 25k load giving you a output cut-off freqency of 9.65Hz.
Now. If you take the preamps input and output cut-off freqency and add them to the headphones input and output cut-off freqency you will get the total cut-off freqency at the headphones output. Is this correct?
0.33uF and 25k ohms produce a cutoff frequency of 19.4Hz, not 9Hz. 19Hz and 42Hz combine to produce a total cutoff frequency of about 61Hz.
3. If the preamps output coupling capacitor of 0.33uf is in series with a 0.0051uf and in series with the
0.1uf at the decoders input would i get a cut-off freqency of 305Hz at the input of the decoder for the touch-tones? How will this affect the input of the decoder? What will be the cutoff frequency? Wouldn't a higher cut-off freqency at the decoders input be better for touch tones?
The designer of the decoder IC used a coupling capacitor of 0.1uF for a cutoff frequency of 16Hz for important reasons. Maybe for a 350Hz dial tone to be passed without attenuation. Don't use the low value capacitor.