Speaker level to line level?

I know this is probably a really easy one, but instead of going out and buying line out converters I want to figure out a way to plug a pair of powered monitors that I have in my garage into a stereo. I found this, but it seems too easy.



I used to install car stereos as a teenager, and I remember line out converters involving trimpots and xfmrs.

Depends on how much juice is coming out of your speaker out!!

But, the theory is right, as long as it's a solid state amp.  Say your stereo does 10W RMS into 8 ohms, you can figure out the max voltage out using ohms' law (YAY OHM'S LAW) and the equation for power:

P = VI and V = IR (right?)
so...
P = V x V/R
so...
P = V²/R
so...
V² = PR
so...
V = sqr( PR )
so...
V = sqr( 10 x 8 )
V = 8.944

Now you can make a resistor divider to get your target max output.  Say you want 0.77V max out (0 dbm), you need to step it down 8.94V / 0.77V = 11.61 times.

Probably best to leave that 1k where it is and adjust the top resistor to match.  In this case, 10k is pretty much just what you need!

If it's a 30W or 100W amp, just plug and play  with the numbers. 

HTH

Awesome! Thank you!

> car stereos ...I remember line out converters involving ... xfmrs.

Car-audio has very hostile ground. All the crap from generator, lamp-switch, and some spark-crap, flows in frame and contaminates "audio ground".

Most recent car-sound uses UN-grounded speakers, both sides "hot" with both audio and DC. Whereas your line-in expects DC-free and probably unbalanced.

If the existing radio drives a speaker, and you are adding an amp and speaker, you wanna trim the new speaker to blend with the old speaker.

So tapping the output of a car radio is a bit tricky.

Taking output from home stereo is usually simple. 99% of the time the Speaker outputs are grounded and DC-free. Max levels run from 7V (6 Watts) to 30V (120 Watts). Powered monitor probably wants 0.7V to 3V. So as Darren proved, a 10:1 or 11:1 pad is probably all you need, or close-enough to lead to a final plan after one trial.

I would include a capacitor at the atten. out - just as a safety measure.