rogs!OK --it doesn't seem as if anyone is going to jump in here, so I'll try one last time.......
The reason your circuit is not working could be for several reasons.
The HT9170 (MT8870) is looking for two tones. One from the high group, one from the low. If it detects anything other than a valid pair of two tones, it will not update the output latches.
'Other' could mean noise, non-valid alternative tones, hum, reflections from the room ambience creating false tones....or a combination of any of those things. Because your unit is working close up , but not further away, then the one thing it probably isn't, is clipping.....
There is no way that any of us can know which of those is causing your problem, without either seeing an oscilloscope trace, or listening to a recording of the audio being received at the input of the HT9170.
Readers of this thread will know I have suggested, on several occasions, that you record and upload a sample of your received audio.
You just say you can't - and then move on to your next attempt at trying something different, without really knowing if you're even moving in the right direction.....it's called 'thrashing around in the dark'......
If you really are keen to get this working - and over 100 posts suggests you are - then post an audio sample of what you're getting at the moment.
Then we can all can hear what's going on with your prototype, and maybe offer some suggestions?.
Although this may seem like a simple project, it's actually not that easy to make a high gain, low noise audio preamp -- especially only using a 5V supply -- as a beginners project....
Post an audio sample....
Where do i connect the wire leads to the audio output of the amp or the input of the decoder.Please explain how to connect them.
I will try and record this audio signal. Thats if my PC records it.