Headphone to RCA

[trogadoror]

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!

 

[Steve Rush]

On Sat, 18 Dec 2004 12:34:23 -0800, trogadoror wrote:

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!

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.