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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??

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  • \$\begingroup\$ why is using an op-amp buffer too complicated? i think you will struggle to find a solution that is simpler than that. \$\endgroup\$ – jme Nov 1 '13 at 2:49
  • \$\begingroup\$ 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??? \$\endgroup\$ – Remixed123 Nov 1 '13 at 3:28
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    \$\begingroup\$ 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. \$\endgroup\$ – alex.forencich Nov 1 '13 at 4:32
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    \$\begingroup\$ @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. \$\endgroup\$ – Passerby Nov 1 '13 at 5:38
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    \$\begingroup\$ 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. \$\endgroup\$ – alex.forencich Nov 1 '13 at 6:09
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An op-amp would be the best choice. Simpler than that is a single transistor pre-amp.

enter image description here

Should be powered by the same input range you need, and it will be biased Vin/2.

Alternatively, you can move R1 after the input dc-blocking cap C1, and lose C2.

enter image description here

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  • \$\begingroup\$ An NPN should be cheaper than an op amp, but I am sure there must be a way to do this with just a few resistors and capacitors, making the solution very cheap and simple (though not as simple as an op amp or transistor solution) \$\endgroup\$ – Remixed123 Nov 1 '13 at 3:36
  • \$\begingroup\$ @Remixed123 you mean you need less than 1 npn, 3 resistors and 1 cap? \$\endgroup\$ – Passerby Nov 1 '13 at 3:41
  • \$\begingroup\$ Not so much a need, I will use what ever provides the cheapest solution and still performs the task required. I just feel that it is possible to do without the transistor....but haven't been able to work out how. \$\endgroup\$ – Remixed123 Nov 1 '13 at 3:46
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    \$\begingroup\$ @Remixed123 It is not possible to amplify a signal with just resistors and capacitors. It is mathematically provable. \$\endgroup\$ – apalopohapa Nov 1 '13 at 5:37

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