MP3 player too quiet when you hook it up to your car’s Aux-In or home stereo?  Make it louder with the Opa! Amp.

The Opa! Amp is an USB powered audio pre-amplifier based on a Burr-Brown OPA2227. With 0.00001% total harmonic distortion, it’s a high quality way to make your MP3 player louder when hooked up to your home or car stereo. 

This project is suitable for beginners, you'll need a few soldering tools, but even if you've never soldered before, it's an easy build.  You can etch your own board, make it on a protoboard, or pick up a kit from Gadget Gangster.

I made the Opa! Amp because I recently got a smartphone – it sounded great with headphones, but when I hooked it up to my stereo, I had to crank up the volume to hear it and there was distortion.  Listening to spoken word stuff in the car was the worst.  The Opa! Amp increases the audio level and fixes the problem without distortion.

The next few steps discuss the design and testing process. If you just want an awesome pre-amp, skip to Step 6: Making It.

Step 1: Understanding the Problem

When I hooked my smartphone to my car stereo through the Aux-In connection, the volume was very low, even when I cranked it up.  It was especially bad with anything that has a large dynamic range like audiobooks and podcasts.

I did some research and testing and the problem was easy to pinpoint:  Your headphone jack is designed to drive headphones. The jack’s max output is 300mV RMS, which drives headphones just fine, but most Aux Inputs are expecting 1V RMS as the max level. So when your MP3 player is saying “Blast it”, your stereo interprets that as “keep it pretty quiet”.

You can mess with the equalizer on your phone or compress the dynamic range, but as long as the max output is 300mV, the volume increase will be minor. And at the max volume levels, you get distortion because both the stereo and MP3 player are operating at their maximum levels.

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