LM1877 2.1 System Interfacing Summing Amp

[andrew.grimm3]

I need help calculating values for a 2.1 audio system using LM1877 IC's. I understand the 2w power output is not much but the design is what I need help with. I'm trying to drive a left and a right channel along and a channel to pass frequency's less than 250hz, using a summing amp to interface the left and right channels to provide the input's for the summing amp. About the only thing I know is how to get my gain value resistors for the left/right channels on the project. I picked a gain of about 20 to be pretty friendly. I need help with some of the design equations of how to solve down for my capacitor values and resistor values to only pass frequency's less than 250hz. Thanks

 

[andrew.grimm3]

Also I know the datasheet already solves out values but I'm not allowed to use them. What I need to know is what equations they used to get those values. All in all I need help with understand the design process of how the coupling caps and all electronics work in the system.