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The Opa! Amp - iPod Amp for Aux-In

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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.

 
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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.
Hi,
Thank you for the post.
please post circuit diagra
jcelania1 year ago
Well done and this is really exciting for us to see especially from the Op Amp team at Texas Instruments! I love this innovation especially with audio applications. That Op Amps for Everyone book really has been a hit over the years and we think we are on to something else that going to be a game changer, search for "TI Precision Designs". Our Amp and ADC experts have simulated and built boards on popular challenges to reduce your headaches and design time. It's mostly on high precision applications but I'm sure there is a lot of commonality that you can leverage from. Thanks again and Op Amps ARE the Bomb!
sa_penguin1 year ago
You input ground goes to your "analog" ground [+2.5V point]. OK - but your output also goes to this. Which goes to a car audio, and hooks to the car ground.
Which therefore shorts your "analog" ground to car ground, zapping about half of your power supply and playing havoc with the split rail bias of the op-amp.
Did you consider a single rail op-amp?
This is the solution I have been looking for, literally, for years.

Thanks!
pangi2 years ago
Hi There, unfortunately I have found that I can't power a phone charge at the same time as powering this device. The amp works until I plug the phone charger into a 2nd cigarette lighter socket at which time amp stops working.

Hope you can help.
ivanjacob2 years ago
lol opa is grandpa in dutch XP
anuraj3 years ago
Hi,
Thank you for the post.
Well I have tried making amps and it did worked fine, but i am no good at electronics and did not understand the diagram at all i would request you to, kindly simplify the circuit for people like me.
Thank you!!

I am adding an image of wat i tried from this website, this diagram made it easier for me to make it. I am coping the same image which i got in that post. I hope you will help with this regard. XYz
amp-dia.jpg
SharpyWarpy3 years ago
The same thing can be done with a headphone amplifier. A regular Cmoy works. Using the OPA2132 chip you can use a cigarette lighter connection, 12V. With the Cmoy design there are no electrolytic caps used on the inputs or for bypass. Build your own with instructions on the Headwize.com site or buy one on ebay. Search for Cmoy. Or I'll build you one for $25.00 including shipping. Piece of cake.
Gadget Gangster (author)  SharpyWarpy3 years ago
Yeah, Cmoy will work fine with a 12V power supply. Usually, headphone amps have a coupling cap in the audio chain to prevent passing a bias current into the speaker coil. The cap causes phase distortion and acts as an RC filter, but it's pretty minor when the amp is designed correctly.
See the following URL for the design and build of the Cmoy pocket headphone amplifier: http://gilmore2.chem.northwestern.edu/projects/showfile.php?file=cmoy2_prj.htm The basic design without the volume control is the one of interest for our purposes. The RC filter formed by the + input cap and resistor does not intrude into the audible frequencies. There is also no audible phase distortion. The amp can be powered from a cigarette lighter socket when an appropriate chip is used. I have a board layout for anybody that want one for etching that uses one chip for both channels. Please don't misunderstand me, I don't mean to cut down your instructable -- it's great. Just offering an alternative.
profpat3 years ago
great project !!!!
mrx23dot3 years ago
hi, you can reduce the power consumption from:
I=5V/(150*2) = 17mA to <1mA by using >1K divider and a cheap DC opa.

THD below 1% does nothing if you don't use dedicated PSU (no digital/smps stuff on rails). At least use CLC filter, w/ f0=<20Hz.
3V3 linear regulator would be also wise to use, w/ 2,2mF caps & low esr films & tantals (only on rails).

In the signal line use only film caps or HQ electrolytes.

cheers,


Gadget Gangster (author)  mrx23dot3 years ago
I wouldn't even suggest using electrolytes in the audio chain, just for bypassing.

You can reduce current consumption by using high value resistors on the divider, I used 220's because that's what I have handy. Consumption isn't an issue here, unless you run it from batteries, but even 17mA will last pretty long with some AA's.

Also agree, the power supply quality can significantly diminish sound quality. I used bypassing to help it work with some nosier power supplies, but an inductor filter works well. Batteries are a great power supply, too.

Running a 3.3v single supply isn't an option with the Opa227, but there are rail-to-rail op amps where that can work.
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