Setting Gains on a Line Out Converter

I am using line out converters, too, so this question has also been nagging me.  Here's what I have determined:  Some amps can take up to 4 volts input, some take up to 6 volts.  A stock OEM deck will often put out more than 6 volts, unlike the aftermarket decks that put out less.  The gains on the converter will limit the voltage to the amp, so that the signal is clean for the amp to work with.

The gain is set on the converter first, while the amp gain is all the way down.  Read the voltage output at the amp's speaker outputs while adjusting the converter, with the deck at maximum clean volume.  Figuring that the amp you have may work with no more than 4 volts input, read the voltage on speaker L + and speaker R - (full bridged reading) to get to 4 AC volts.  The speakers can be unhooked while you do this.  I do not know if this is the correct procedure, though.

Then, adjust the amp gain with the meter probes first on L channel, then on R channel.  For example, if the amp is rated at 2 X 50 RMS @ 4 ohms, the voltage reading would be 14.14 AC volts.  While making both converter and amp gain adjustments, a test tone in the frequency range of the speakers has to be playing through the CD, try at 1000 Hz and 2000 Hz.

Without the test CD and the multimeter, the gains on the converter might be at  about the halfway point (as DYohn suggested) to get to 4 volts, because it adjusts from 9 volts down to 1.  You may find, as I did, that a slight adjustment of the converter gains will clean up the sound, as you are listening and doing the fine tuning of the system.

Tone generators here or here.  I used the latter to make a test tone CD.  Use the Ohm's law formula for finding voltage when you know the nominal impedance and the power in watts, E = SQ RT(P X R).  You would measure for 17.32 volts at each channel for amp gain setting, using different and various frequencies to find the highest output.  Disconnect speakers, just one terminal on each so they are easier to hook back up.  Have the deck at max clean output. 

As far as "halfway on the amp sounds safe", no it wouldn't be.  Halfway on the converter might be safe, but not the amplifier.  If you can't measure for 4 volts to set the LOC, try halfway (mine are set just slightly above the halfway point).  With the amp gain, if you can't measure with a meter, use your ear (with music) after the LOC has been set.  Use the test tones ONLY with speakers unhooked and using a meter, because sine waves will quickly heat up the voice coils, and they will get smelly real fast.