Microphone Preamp MAX4466 Modified

This a simple modification to a standard electret microphone and MAX4466 preamplifier module.

The main purpose is to stabilise the operation of the preamplifier. I have found that the modules are very prone to self oscillation preventing normal operation. I traced this to the lack of power supply decoupling of the main power supply to the MAX4466 IC as recommended in its data sheet. The modules do have two ferrites in the ground and supply lines but in my experience they are not sufficient to give stable operation.

One could probably add local decoupling very close to the pins connecting to the module but I chose to do it directly on the module itself to avoid compromising the mechanics of the module.

In addition, I decided to reconfigure the connection to the electret microphone itself. This allows using different electrets and they can be remotely connected if that is more convenient.

The modification needs a 220uF 6.3V SMD tantalum capacitor and a 100R SMD resistor.

An optional 3D printed box can be used to contain the module and 2 M2 6mm self tapping screws can be used to secure the module in this.

If the mechanical reconfiguration is done then a 2 pin socket is also required.

The onboard decoupling is achieved by removing the ferrite SMD in the VCC power line. This easily achieved by breaking the ferrite in half with a blade and then using a soldering iron to clean the pads.

The tantalum capacitor is soldered onto the ground (middle pin. The 100R SMD is soldered to the incoming VCC pin. A fine wire is then soldered to the board side of the power supply using the contact pad where the ferrite was (furthest away from the pins). This is then wrapped round the edge of the board and connected first to the free end of the 100R resistor and then carried on to the free end of the capacitor.

The main board power is now supplied via a 100R / 220uF decoupling network. This produces stability and reduces any noise on the power lines.

This change makes the microphone element pluggable and lowers the profile of the overall solution.

The first step is to remove the electret from the board. The electret is originally mounted vertically on the board and quite close to it.

One could probably help this by removing solder from its 2 pins using solder braid, but I also found it fairly easy to do by alternating melting the solder on each pin in turn and levering up each side in turn. 3 or 4 cycles are normally sufficient to remove the component.

Once free I then soldered a 2 pin socket (compatible with same type of pins used on the VCC,GND, Output.

A couple of pins can also be soldered onto the electret itself to allow it to be plugged back in, or it can be remotely connected using a couple of wires terminated in the same pins.

I designed a small 3D printed box at Thingiverse https://www.thingiverse.com/thing:7288607 . This takes the modified board and provides access to the electret socket and the power/output pins.