Single transistor electret mic preamp

[Blueteeth]

Hi,

Over the years I've made several op-amp based electret mic preamps, running from 9v batteries, boosted AA batteries or 5V. These are used for general voice, such as skype/MSN/gaming, and sometimes for recording acoustic instruments like guitar and piano. I've found it straight forward to vary the gain and frequency response of these amps, but for a cheaper desk mic I would like to try a single transistor amp - with some form of gain control.

Although Google has many examples, many have quite a high gain. Considering the mic input for your average soundcard is pretty poor, I'd rather not use the on-board '20dB boost' but use an external one instead. Often even this gain is not enough, so a controllable amp from say 16 to 30dB would be ideal. It should be powered by USB, for which I have made a transistor-based filter to give around 4.3V thats pretty damn clean. (one could also use an LDO set to 4-4.5V, they generally do quite well at filtering power supplies).

After tinkering with LTspice to work out exactly whats going on in a single transistor amp, it seems that changing/varying the emitter resistor in this circuit does the job, albeit in a non-linear way. I've also attempted to get LTspice to calculate the THD of the circuit, as well as the noise.

I've also noticed many circuits have a cap on the emitter, is this simply to boost the gain? Because I'm not after a massive gain, just has to be variable. So, any improvements to this are welcome!! Its a combination of application and a learning exercise:

 

[MrAl]

Hi,

Yes the emitter resistor capacitor is there to boost gain without upsetting the DC operating point. It increases the AC gain of the circuit. Distortion will increase too though so you have to watch that.

 

[Blueteeth]

Ahh nice, so I can omit the emitter capacitor to keep the gain at a reasonable level? I see how it works, effectively giving AC a direct path to ground, rather than through the resistor.

As you can see by the spice directives I tried to get a half decent idea of the THD, it says its about 0.09% with that circuit, which is more than good enough for voice, but that figure increases depending on how long the simulation runs for Bah I should just bread board it and see what gives. Cheers!