I'm looking to interface the headphone jack on my laptop with an RCA jack on my TV. I already have a cable that goes from 1/8th inch stereo to stereo RCA. The problem is, the volume is terribly low and loud noises (such as explosions during a movie) cause the output to saturate and the volume drops even more. So I figured I could build a simple op-amp circuit. I've been looking everywhere for the electrical characteristics of the headphone jack and the RCA jack but I can't find anything! Apple's website lists the characteristics of their headphone jack as having a peak output of 4.5Vp-p with a 10 ohm source impedance. Of course, I have a Compaq laptop, but I figured mine is probably about the same, at least with respect to the peak output voltage (which is all that really matters with an op-amp since 10 ohms is nothing). Also mentioned that headphones have an impedance of 32 ohms so I thought I might make the input impendance of my little amp 32 ohms. But I can't find anything about the RCA jack. In particular, it'd be nice to know what the max voltage is that it expects. I wouldn't want my amp to drive it too high. Any information or other insight would be great!
Headphone to RCA
Dec 18, 2004
2 Replies
Consumer audio "line" inputs usually want about 1 volt peak-to-peak, so you're probably overdriving that input with an output intended for headphones. Try a plain old resistive pad. A 50-ohm potentiometer across the headset jack, with the slider connected to an RCA plug, should work with a little fiddling. Once you find a value that your stereo is happy with, you can build an equivalent with two fixed resistors spliced into the cable and package it in heatshrink tubing.
Yes, there are valid opamp solutions, but for less than unity gain, it's hard to beat the response and distortion specs of two resistors and some wire.
You need to properly match the output level from the laptop to the external amplifier. You need a line level output, or to reduce the level you have now.
Some of the computer accessory suppliers, audio accessory suppliers, or electronics parts suppliers should be able to find you the proper adabtor to convert this.
I believe you would need about a 20 Db reduction in level. If you have the base knowledge, skills, and tools to do the work, you can buy the necessary components, and build one yourself. This would not be as cost effective as finding one that is ready made for you.
Jerry G. ======
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