LM1875 and input impedance

[CJ900RR]

A friend of mine have asked if i can make an amp to replace and old one in a bus. The current broken one is based on TDA2040 but i want to go with LM1875 instead cause i have some of those.

The power supply avaible in the bus is +24V only so i was thinking of using the LM1875 in single-supply configuration.

There is some problems thoug.

1:
Interferences from generator etc. How should i filter them?

2: The current amp has 3 inputs and their specification is from datasheet:

600ohm -45dB Frequency 50-13000Hz +-3dB (Mic input)
600ohm -20dB, or 0dB Frequency 80-13000Hz +-3dB (Com radio input)
600ohm low level 0dB Frequency 50-15000Hz +-3dB (Car audio input)

I need to make a switch for each input, no problem, but how do i construct each input to meet the specifications, or better, above?

 

[CJ900RR]

To clearify, will this give a 600ohm input impedance or am i wrong?

 

[AndrewT]

Hi,
yes that will give a 600ohm impedance over the audio frequency range.

But why go so low?
1k or 2k or 10k should all work adequately well.

 

[CJ900RR]

Hi,
yes that will give a 600ohm impedance over the audio frequency range.

But why go so low?
1k or 2k or 10k should all work adequately well.

Ok thanks.

Well that is the specification from the manufacture of the old amp. I thought that it where a standard impedance when dealing with microphone inputs. Maybe im wrong?

 

[AndrewT]

Hi,
if it's a low impedance mike then it would prefer (maybe demand) a low impedance input, but more importantly does it need a balanced input?

Re Q1,
suppress the interference at source.