audioguru.
Theres only the one bridge Amp being use in the generator circuit..
There's 3 sparate amps for the decoders circuit the PreAmp, speaker Amp and headphone Amp, but the speaker amp is used only when listening to a radio,mp3 from the speaker when SWITCHING between the speaker amp and the headphone amp..
The Preamp and headphone amp will be connected when picking up the touch tones or listening to the radio,mp3. or vioce.
I Can calculate for the RC cut-off freqency for the bridge Amp and the headphone amp, But the Preamp that you posted I have no idea how to change the input cut-off freqency below 10 hz since you post that circuit for me but i think you set it at 32Hz. in Post #7. Help. can you change it if it needs to be changed.