The Single NPN Transistor Audio Preamp

Here’s a great little project that goes well with the LM386 audio amp. It’s a good first time transistor project because it’s simple and demonstrates the common emitter class A amplifier circuit with only six components in the signal path.

Here’s an excerpt from a great tutorial I found on NPN transistors:
A “Class A Amplifier” operation is one where the transistors Base terminal is biased in such a way as to forward bias the Base-emitter junction. The result is that the transistor is always operating halfway between its cut-off and saturation regions, thereby allowing the transistor amplifier to accurately reproduce the positive and negative halves of any AC input signal superimposed upon this DC biasing voltage. Without this “Bias Voltage” only one half of the input waveform would be amplified. This common emitter amplifier configuration using an NPN transistor has many applications but is commonly used in audio circuits such as pre-amplifier and power amplifier stages.

I powered this circuit with a single 3V coin battery I salvaged from an old computer motherboard. It works just fine at this low voltage because it’s just a preamp. Go build one and keep on hackin!

Parts List:
1 – 2N3904 NPN transistor
2 – 10K resistors
1 – 100 K resistor
1 – 0.1 uF ceramic capacitor
1 – 1 uF ceramic capacitor
1 – 3V – 9V battery and holder

26 thoughts on “The Single NPN Transistor Audio Preamp

  1. Hey Dino,

    Been following your site for a few weeks now, all good stuff!

    I’ve just modelled this little preamp in a program called LT spice, a free spice modeling program developed by Linear Technologies (only thing is, is that most of the component libraries are LT manufactured components only) Might want to give it a try if you don’t have the components to play with.

    I ran the circuit with a 1kHz 0.1V input and found some distortion on the troughs of the output wave-form. After messing around, I found a 2K resistor on the emitter cleared this distortion, but at a loss in gain.

    The gain of the circuit is now 12dB (Vout/Vin = 4) but the signal has less distortion (within the supply voltage range 3-9V). If you wanted more gain I suppose you could stick a intermediate stage between the preamp and the LM386.

  2. Dino,
    This is the first time I stumble upon your page. I love this simple preamp preamp. I just used an LM386 in a sound triggered flash project for high speed photography, and I believe that using this preamp would have been of a great help.

    cheers,
    Bassam

  3. Though confusion is my natural state, I’m particularly confused about the role of R1 in this circuit. If I’m not mistaken, R3 and R2 take care of biasing the 3904. But since C1 strips out any DC component of the input signal, I don’t understand how DC current through R1 contributes anything here. Feel free to color your response with derision as I’m really behind on these things.

    Many Thanks!
    Todd

      • Thanks, Dino. I hate it when I make mistakes like that–and it happens all the time. So R1′s function in your original version was to bias the electret element?

        I have another question, again rising out the shallowness of my knowledge: what would you say the input impedance of this little preamp is? I’m guessing it would be the equivalent series resistance of the base-emitter junction *and* the reactance, in ohms, of the input capacitor at 1kHz (that is the usual spec., isn’t it? At least for audio frequencies?).

        The reactance of the 0.1 and 1.0 caps would be about 1592 and 159 ohms, respectively. I don’t really know how to figure the impedance of the base-emitter junction. I can’t find that on the 3904 spec sheet. I guessing, though, that there’ll about a 0.75V drop across the junction, but I don’t know how that would factor into the input impedance.

        Any corrections/reprimands/ideas?

        Thanks!
        Todd

  4. Thank you for this project!

    I am trying to put it together as a preamp for a cheap ‘sm58′ type mic that doesn’t put out much juice. The problem is that all I get out of this thing when plugged in is loud noise and no mic signal at all.

    I appreciate the schematic you gave us, but I was wondering if we could see the actual build a little more closely to see how things are actually hooked up? For example, is the ground for the signal out attached to circuit (battery) ground? I have gone over the breadboard a few times just to make sure the transistor and other parts are wired in per spec, but am not finding anything out of the ordinary…

    Thanks again!

    Byron

    • update:
      It seems to work when both R1 AND its connection to positive are removed; don’t just get rid of R1, cut that connection entirely otherwise it doesn’t work…

  5. Hey Dino, I thought I should post on this page since this is the circuit I ended up building. I am having a little trouble with it though. As I mentioned before I am using this to boost the signal coming from a small 10-second recordable sound module. I have it setup on my breadboard but instead of amplifying the signal it is reducing it. I am not sure why. I am using a 4.5v battery power source. The sound module speaker lines are connected to the input of this circuit and the speaker is connected to the output of this circuit at C2. One thing I am not to sure about is what to do with the negative line from the speaker. Reading the diagram you drew I connected the negative input and the Emitter leg on the transistor to the ground of my 4.5v power source. The only way I could get any sound to come from the speaker is when I connected it to the ground/negative input/Emitter leg. I have tried the circuit with and with out R1, including an attempt where I removed the whole connection to the 4.5v power source as suggested above. I have tried different capacitors for C1 with no change in the volume level. I am not sure why this circuit is not working. Perhaps it has to do with the manor in which I am attempting to use it. I am not hooking it up to an MP3 player and running it through a power amp. Any suggestions?

  6. Dino,

    Just got finished building this preamp and an lm386 amp for use with a dynamic mic, plan on using it voice effects for a costume mask. Only one problem, I added a 100k variable resistor between the preamp and amp stage. It works, but there is a lot of distortion and buzz in the sound. However, if I touch the terminals for the variable resistor then the buzz and distortion go away entirely. I can’t find any solution, any ideas?

    All the best,
    Robert

  7. I was recently playing around with some transistors and managed to create a 9v battery powered mono amp, using no resistors (i didnt bother with biasing the transistor) in the preamp section, and a push pull output section, directly coupled together. It worked for amplifying my iPod, but to my disappointment was silent the second i plugged in my strat. What can be done to ammend this situation? Can I modify my existing Pre-amp? I was wondering if I replace my dodgy pre-amp with this one will it work for a guitar?

    Thanks
    -hazza

  8. hey dino, i want to build an amp that is basic like this but with a lot of power, i was thinking of using chips like toshiba 2SC5198, if i do that do i keep the resistor/capacitor values the same or do they need to be adjusted? thanks!

  9. Ok im a noob to this stuff and just want to amplify a small speaker i have in a project. Would just the preamp work or do i have to have both?

  10. Hello, Very nice post, and i am trying to build this now on a breadboard, but was wondering if i can power it with a USB plug. 5v should work nicely, but how many amps does it use? would it work, and if so….how?

  11. hi dino,
    I was wondering if it is possible to use this circuit to take voice input and then use the output into an arduino for speech recognition?
    Any tips?

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