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Discussion Starter #1
There must be a device that goes between mp3 player and radio AUX IN. What is it called? Thanks.

Problem: The mp3 player I use in the car has 5 mW output. Even at max volume, I still have to turn up radio volume. Internet search showed there are 2 conflicts, impedance mismatch and insufficient voltage. I am looking for a device (to buy, if not, to build) that will solve both conflicts. Thanks.
 

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Yeah, a line driver should do it. The problem is with the voltage. The wattage matters when driving headphones/speakers, not so much inputs. The ideal op amp draws zero current, but of course there is no such thing as an ideal op amp... Regardless, the current draw is quite low. A small signal step-up transformer might also be up to the task.
 

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Discussion Starter #4 (Edited)
Do you mean an audio transformer like this?
[I cannot post a link.]
It is a radio shack audio transformer with 1000 (center tapped) and 8 ohm coils. Should I connect headphone out to 8 ohm and 1000 ohm to radio AUX IN? Is the ratio too high?
Thanks.

If a simple audio transformer will do the job done, I may not need a line driver.

Frequency range is 300 Hz to 20 kHz. Will this cut out bass basically?
 

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There must be a device that goes between mp3 player and radio AUX IN. What is it called? Thanks.

Problem: The mp3 player I use in the car has 5 mW output. Even at max volume, I still have to turn up radio volume. Internet search showed there are 2 conflicts, impedance mismatch and insufficient voltage. I am looking for a device (to buy, if not, to build) that will solve both conflicts. Thanks.
there are things called headphone amplifiers :D

you may be able to use this:

Amazon.com: FiiO E5 Headphone Amplifier: Electronics

i actually have a different fiio amp but i guess you just need a slight boost in aux output.
 

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Do you mean an audio transformer like this?
[I cannot post a link.]
It is a radio shack audio transformer with 1000 (center tapped) and 8 ohm coils. Should I connect headphone out to 8 ohm and 1000 ohm to radio AUX IN? Is the ratio too high?
Thanks.

If a simple audio transformer will do the job done, I may not need a line driver.

Frequency range is 300 Hz to 20 kHz. Will this cut out bass basically?
I would be looking for a 1000 ohm in and 20000 ohm out. Won't load your mp3 player down. 8 ohm in will only load out down like headphones

Sent from my Motorola Electrify using Tapatalk
 

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Discussion Starter #8
Thanks.
1) I checked mouser and digikey. All audio transformers that meet the in-out resistance have 300 Hz limit. Am I going to lose bass?
2) Someone mentioned headphone amplifier. I googled and found CMOY headphone amplifier. Just one op amp, easy enough to build for anyone. It doesn't have 300 Hz limit. Is it a way to boost 500 mV max headphone out to line level output?
 
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