The single NPN transistor audio preamp

Hack a Week’s new project this week is a 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.

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  1. Ian I disagree with you.
    “It’s a good first time transistor project” It’s not true. I’ve been seeing this preamp since last 10 years in cassete players.

    1. I think the “first time transistor” comment was geared towards this project being the “first” transistor project someone would undertake, because of the low number of components, simplicity, functionality, etc. As opposed to a first-timer trying to make a full op-amp out of discrete transistors :)

  2. This design really needs a base bias to ground return resistor, an emitter resistor with parallel bypass cap. Also the supply to input resistor is only really necessary with the likes of a mic that requires DC bias. Don’t build this design willy-nilly. Simulate first (various forms of SPICE) and study more before heating up your soldering iron.

  3. I built this preamp with a few differences: Q1 is still 2n3904, C1 and C2 are still 0.1uf, R3 is still 10k, but I added a 3.3k resistor between the emitter and ground and changed R2 to 132k. Used with a 9v battery (that is actually 7.9v) it is a really nice little preamp – quiet, too.
    I have scads of preamps, distortion and overdrive pedals and they cost a lot of money….this one I made from parts I have laying around and I put it in a pedal and will use it next weekend in front of my Marshall for a boost when I solo. Speaking of Marshalls, I made some changes to mine (50W JMP w/master volume) and it is now absolutely ACDC….if you’d like to know what I did, I’ll put up some pictures and specs on the site soon. Mike

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