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.

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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.

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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.

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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.

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

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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

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