Mic/audio mono preamp

I have quite a low sensitivity condenser microphone so it sounds like a crap with my sound card (Creative x-fi audio) mic input because of too low signal level.
That's why I designed this quite small, simple and cheap but pretty high quality preamp. The other reason was that my portable music player can record only line level audio.

Basically the preamp consists of ordinary non-inverting op-amp circuit with single sided supply voltage (extra portability), rotary gain switch and switches for on/off, output overvoltage protection and mic bias voltage.
Output overvoltage protection is done by using 4 diodes that start to conduct when voltage exceeds 2*0.6V. Generally speaking the same thing made with only 2 diodes (1*0.6V) would be better choice to protect most sensible consumer audio devices but for my purposes 1.2V limit works better.

Original schematic. See notes below (Click to enlarge)

TL072 op-amp was chosen due its low noise, high impedance JFET inputs and cheap price.
The rotary switch and resistors connected to it could be replaced by a potentiometer but I wanted steps with fixed gain values (+6 dB, +12 dB, + 18 dB, +24 db, +32dB and + 40 dB).
I also decided to use polyester caps for signal coupling because they cause less distortion than electrolytics. After assembling the whole thing I realized that 2.2uF in C3 isn't very good idea in +40 dB mode (causes some bass attenuation). 4.7uF-10uF polyester or even bigger electrolytic cap would do the job better even in +40dB mode. Yep, the simulations confirmed my thoughts. Below there is amplitude responses with two different values of C3. The first line (orange) is input signal and lines over that is output with different gain setting of rotary switch.

Amplitude responses simulated with C3 = 2.2uF (Click to enlarge)

Same simulation but only C3 has changed to 22uF (Click to enlarge)

Protoboard compatible PCB design (red & green = jumper wires)

Testing & fitting.

A little bit of brute force and it'll be finished! :)
The final product

Written by: Petteri Liikkanen


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