Let face it, the selection of quality amplified microphones has become somewhat limited over the years. Many sound like junk and when you find one that doesn't, the switch fails and becomes "scratchy" in no time. Wouldn't it be nice to take a decent stock mic with an electret condenser type element and get quality audio with good gain and no battery to replace? Most modern radios give you this capability with a simple resistor change.
Look at what these radios use for a mic pre-amp today... Gone are the discrete transistors and in their place is a dual op-amp (NJM4558D). Boy does that ever give you some gain flexibility once you familiarize yourself with the gain controlling resistor in the negative feedback loop. For example, lets take the Galaxy DX959 and have a look at IC7. The 270k resistor (R176) is connected between the output pin 1, and the inverting input on pin 2. The more output you feed back into the inverting input, the less gain the circuit provides.
Watch what happens when you replace R176 with something larger, like a 330k resistor... Your stock mic element just reached the gain we usually see with a power mic. You may still be two steps away from perfection though. The audio quality will only be as good as the element. I've found many are using a good electret but sound a little "muddy" because they are using too high a value coupling cap and passing too much bass response in the 250 to 300 cycle range. You can't just add another cap in series at the element without blocking the needed DC. You have to get in front of the DC injection point, at a place like the mic gain control.
Because the mic will have more gain, it is also more susceptible to RFI feedback that can produce "squeals". You may find that the shielding on some stock mic cords is simply not up to the job of keeping the RF out of your AF. Replace the cord off a good old Astatic or other mic, or add some RF bypassing with a cap and choke (ferrite bead). You could do both if needed.
Look at what these radios use for a mic pre-amp today... Gone are the discrete transistors and in their place is a dual op-amp (NJM4558D). Boy does that ever give you some gain flexibility once you familiarize yourself with the gain controlling resistor in the negative feedback loop. For example, lets take the Galaxy DX959 and have a look at IC7. The 270k resistor (R176) is connected between the output pin 1, and the inverting input on pin 2. The more output you feed back into the inverting input, the less gain the circuit provides.
Watch what happens when you replace R176 with something larger, like a 330k resistor... Your stock mic element just reached the gain we usually see with a power mic. You may still be two steps away from perfection though. The audio quality will only be as good as the element. I've found many are using a good electret but sound a little "muddy" because they are using too high a value coupling cap and passing too much bass response in the 250 to 300 cycle range. You can't just add another cap in series at the element without blocking the needed DC. You have to get in front of the DC injection point, at a place like the mic gain control.
Because the mic will have more gain, it is also more susceptible to RFI feedback that can produce "squeals". You may find that the shielding on some stock mic cords is simply not up to the job of keeping the RF out of your AF. Replace the cord off a good old Astatic or other mic, or add some RF bypassing with a cap and choke (ferrite bead). You could do both if needed.
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