Arne's Electronics Series

Microphone Pre-Amplifier Project (2018)

Some year before writing this, I saw a video about condenser microphones, thinking I could really use a proper mic. However, I ended up getting a cheap one, as recording audio is not a priority for me. It worked pretty well in my laptop, which has a mic in, but I later found out that my mac wouldn't detect it (I wanted to record some videos using Quicktime).

After doing some digging, I learned that the audio port on this mac needs an amplified signal. Hey! I scratch-built an amplifier using the LM358 before! It was a lousy amp, but... maybe I could at least solve my problem. For some reason I had also bought LM386es so I wanted to try one of those for this project.

First I looked up some schematics... a lot of them seemed similar, and quite simple, so I made my own based on those, adding a power-supply section (with header for case-mounted switch), a sort of experimental low-pass filter (with on/off jumper). There are various other ways to do filtering but I didn't delve into it. I also added an adjustable gain section (with on/off jumper + potentiometer). The mistakes or poor choices I probably made don't seem to matter much for simple voice recording.


Breadboard prototype. This thing does not need to drive a big speaker, but it does sort work with one. I guess it's more of a pre-amp.


Then I tried to fit it all onto a prototype board I had (3x7cm). I had some idea here about using "cards" for the PSU and filters, but discovered that my audio-jacks have flat pins which won't fit the smaller holes of this board. Apparently the diameter varies a bit between manufacturers. Also, with this IC orientation, pin 3, mic in, ends up on the wrong side.


The audio jacks were fortunately 0.1" friendly and would fit in this 4x6cm protoboard, though I did have to snip off the front plastic nipples. I'm a bit alignment obsessed, so that took priority over other more practical layout concerns, but I did try to make it single-sided, with as few tracks as possible nearing the audio pathway. Audio is very cranky, perhaps because the un-amplified mic signal is so weak that any surrounding noise can compete for attention, so to speak

Components used (mostly because they were on my desk at the time):

2x  0.1uF   (104)     Monolithic ceramic capacitors. (Power & Mic)
1x  0.33uF  (334)     Monolithic ceramic capacitor.  (Power)
1x  1uF               Electrolytic capacitor.  (Low-pass)
2x  10uF              Electrolytic capacitors. (Anti-oscillation & Gain)
1x  100uF             Electrolytic capacitor.  (Audio out)
1x  220uF             Electrolytic capacitor.  (Power)
1x  47nF    (2A473J)  Polyester capacitor.     (High-pass)

2x  4.7KΩ   (1/4W 5%) Resistors.             (LED & Mic pull-up)
1x  10KΩ    (1/4W 5%) Resistor.              (High-pass pull-down)
1x  330Ω    (1/8W 5%) Resistor.              (Low-pass)
1x  5KΩ     (W 502)   Potentiometer set to 1K. (Gain)
1x  B10KΩ   (W.L)     Potentiometer.           (Volume)

1x  1N5817            Diode                  (Protection)
1x  Red LED (2x3x4mm) 
1x  L7805CV (ST)      5V regulator.
1x  LM386   (Generic) Low-voltage audio ampifier.
2x  Audio jacks (PJ307G?) 3.5 stereo though I wired as mono)
1x  Barrel jack (5.5?)    Mine was not 0.1" friendly (T_T)

Header strips (3+3+2) and 3 jumpers.
4x6cm double sided prototype board.
Leg scraps and heat-shrink tube.
Male-male 3.5mm stereo cable.
Knurled shaft potentiometer knob.


Most of the wiring can be made by bending the legs. Saving copper wire from broken old space heaters etc helps too.


Snapping a pic and drawing traces on soldering side helps immensely. I sometimes take a pic from top and flip digitally so I know where the components are. Dotted lines are the top-side brackets/bridges I had to resort to using.

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Photos/Art by Arne Niklas Jansson