# Simple circuit to boost line-level audio

I am looking for suggestions or pointers to a simple circuit that boosts line-level audio, so that it is strong enough to use with the ADC peripheral on my MCU.

I am aware and have seen many that use an op amp, but I am looking for something cheaper (but still simple), that does not introduce any other artefacts except amplitude boost.

I believe line-level audio is centered at ground and has about a 1.6v swing, where as my ADC input requires a swing of 3.3v

EDIT: My preference is to get this done with a few resistors and capacitors, if that is possible??

• why is using an op-amp buffer too complicated? i think you will struggle to find a solution that is simpler than that. – jme Nov 1 '13 at 2:49
• Yes of course, an op amp is the simplest solution. I have edited my question to focus more on my desire to find the cheapest possible solution (that is still relatively simple). One that preferably uses a few resistors and capacitors if that is possible??? – Remixed123 Nov 1 '13 at 3:28
• It is impossible to amplify a signal without an active component like an op amp or transistor. I suppose that is not entirely correct - you can build a mag amp that amplifies with only transformers. But the bottom line is that resistors and capacitors are not sufficient - you need a transistor or an op amp. – alex.forencich Nov 1 '13 at 4:32
• @Remixed123 as the page that schematic came from says: In order to get the Launchpad to be able to read line level audio, it is necessary to add a DC bias to the audio signal. ... I use a simple circuit (a capacitor and two resistors) to get the signal centered at 1.65 V with a 1.6 V swing. That is not an amplifier, that's just dc biasing. If the signal is too weak, biasing will not provide much help. You asked for a boost, not just biasing. – Passerby Nov 1 '13 at 5:38
• Actually, I would use a combination of the two. Build the NPN transistor circuit below, then add a voltage divider after the output DC blocking cap to shift the DC component to 2.5 volts. – alex.forencich Nov 1 '13 at 6:09