Electret-Mic DIY Pre-Amplifier using OP-Amp

One afternoon i was very motivated to improve the preamplifying part of my live stream installation.

For a project, i want to capture live audio from outside the window. I use an electret microphone that is very sensitive, but the signal output is very weak and needs amplification close to the mic to prevent hum from power lines and other sources. Hence the amplification module is fixed next to the mic.

My first attempt to build the pre-amp myself was to buy pcb and parts as a kit from Conrad. That worked, but it turned out that the module was very outdated. The signal was high and strong enough, but the transistors produced a thick layer of noise. I tried to circumvent this by using a low-pass filter. But with this restriction use cases were limited. In my opinion, the 19–53–75 can serve educational purposes at most. I needed a new pre-amp.

The next iteration is based on an op-amp. I found out that there is a pretty common schematic for using an op-amp for audio amplification. I settled for a low-noise TL071.

schematics single channel amp with TL071

I like to re-use things. Once i had a lot of boxes filled with electronic parts, pcbs, cables and old devices. I still have a few e-caps from the 70s and want them to start living their (second) electronic life. Since i consider this pre-amp version an iteration, they’ll do just fine. For a speedy result, i decided to solder the pre-amp on a prototype pcb. Quick and dirty alright, so here’s a low resolution image only:

The mic, signal output and the 12v power supply are connected via a header connector. The P1 potentiometer adjusts the amplification (10..1000x) while P2 is responsible for the maximum output level (as a rule, Line level should never surpass 1V).

The circuit worked very well from the start. The signal to noise ratio is significantly better than with the transistor solution. Of course, there’s always room for improvement (a third iteration is already lurking behind the corner). For my purposes, the pre-amp sufficiently reproduces the frequency spectrum of the incoming signal. And this DIY solution using a lot of spare parts was as cheap as a Döner in Berlin Neukölln.

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