Making My Fm92 Sound Better: (Transmit Audio) - Philips FM92E Manual

35: Making my FM92 sound better: (transmit audio)

OK, after some fiddling about I think I now have a nice (well OK at least) sounding FM92.
I modelled the preamp in Electronics Workbench and ran some bode plots with and without the 4.7uF electrolytic
capacitor (c3) in the microphone preamp.
With the electrolytic, the preamp boosts treble and cuts base. ASCII art time...
|
| /
| __-
| _----
| /
|/
+-----+------+
100 1kHz 10kHz
at 100Hz, the gain is about 8db
at 1kHz, the gain is about 12db
at 10kHz, the gain is about 21db
the -3db point (9db gain) is at about 110Hz
the +3db point (15db gain) is at about 3.6kHz
This explains the 'tinny' sound
Without the electrolytic, you get a reasonably flat response with 12db gain.
|
| ___------
| /
|/
|
+-----+------+
100 1kHz 10kHz
at 100Hz, the gain is 8.7db
at 1kHz, the gain is 12db
at 10kHz, the gain is 12db
the -3db point (9db) is at about 110Hz
So to make an FM92 sound better...
1. Remove the 4.7uF electrolytic capacitor from the microphone preamp.
2. Adjust R365 (microphone sensitivity) on the synthesiser board about 90 degrees, or until it sounds about right
anticlockwise with the PCB in place, or clockwise from the component side.
Philips FM92E Conversion to 6 Meters:
[by Mal VK2TMF]
Version 3.9 Page 68