Electret Microphone Preamplifier
This is a sister project to my Dynamic Microphone circuit. It's essentially the same thing but with less gain and a filtered voltage feed to the electret mic capsule. Therefore I just used the same text with some minor tweaks! The preamp described here is based on the NE5534 low noise opamp which is still readily available, not too expensive and can run quite happily on a fairly low voltage which was a consideration for this project as I wanted to be able to power it from a standard 5 volt USB port or phone charger
Running the opamp at just 5 volts is right on it's limit. More would be better, but seeing that 5V is now available everywhere, this was my requirement. The 5 volts obtained from a standard type A USB port found on most PCs is usually swamped by electrical noise, so the 56 ohm resistor and 100u capacitor were added to form an RC filter. This works well in the dynamic mic version of the circuit but when using an electret mic some interference was still audible, so an extra 1K resistor and 100u capacitor were added which completely eliminate all the interference without dropping too much voltage to the IC. 56R was chosen as the 'sweet spot' as anything much less would be ineffective as a filter and higher values would drop too much voltage. 5 volts isn't much to play with as it is without reducing it further! As it stands, the opamp receives about 4.8 volts which is still enough to work OK. The 47K resistors and 100u capacitor also perform a filtering function as well as providing half supply biasing for the IC. The schematic is 'text book standard' showing the opamp configured as an inverting amplifier with the gain being set to about 21 by the 22K and 470K resistors (470,000 ÷ 22000 = 21.4). The 22K resistor also sets the input impedance for the mic. The circuit is constructed on a standard 9 x 25 hole strip board with JST XH type connectors for the terminations. The type of enclosure and audio/power sockets used is down to personal preference.
When using small sized stripboard with ICs there are some tricks that can be used to make more efficient use of the space available. Here, the unused pins 1, 5 and 8 of the IC are disconnected by snipping them off on the IC socket (not on the actual IC) shown greyed out on the diagram above. This, along with the 'solder blob' link between pins 7 and 8 allows the positive and ground rails to become available either side of the IC giving a more streamlined layout. Sometimes going around an IC is unavoidable, hence the short wire link from pin 6 to the lower track for the 470K feedback resistor... you can't win 'em all, though I did hide the link under the IC. Note also that some tracks need to be cut which are indicated by an X