Measuring car amp


I need help with direction on how to measure my 4ch car amp for crossover point, gain and response. Channels 1&2 are bridged to drive left speakers and channels 3&4 are bridged to drive the other side. Now on with my problems:

1. Gain setting: with this setup the left and right speakers has its own gain. What is the correct way to adjust amp gain to make sure each side will have the same gain? JL audio website suggest to setting up amp gain with no load, using 1khz sine wave and just measure using DMM across the output of the amp. My friend say I should use a dummy 4 ohms load (resistor) across the output of the amp. Which one is the best way?

2. Crossover point: the amp has variable internal crossover point. The problem is I have no way to set low pass exactly 80 hz. I have TrueRTA and somehow I believe it can be used to measure this. My guess is to use Lpad to feed in the signal back to my soundcard line in but I need confirmation on this and what value of Lpad should I use?

3. Amp frequency response: in TrueRTA help file there is a way to measure a reponse of an equalizer but again, the problem is how to measure an amp response? I guess the solution will be the same with point 2 ?

4. Which one is better: using my current configuration with 2 + 2 channels bridged or just use 2 channels to drive left/right speakers and leave channel 3&4 unused? My concern is there will be a degradation in sound quality (freq response or other problems) with bridging 2 channels. Right now I have no plan to use bi-amp configuration.

1 - I'm not a big fan of JL's method, there are too many assumptions and shortcuts for it to be all that accurate. You could use an oscilloscope and a sine wave to watch for clipping, but if you don't have access to one then the next best way is to play a sine wave and listen for clipping, setting the gain right below the clipping point. Once you get the gain on one side set properly, just use a dmm to match the other side.

2 - Any idea what type of crossover it is? If it's a normal 12dB/oct Butterworth like most built in filters are, then there's a pretty easy way to do it. First go ahead and disconnect your speakers from the amp, then play a sine wave of whatever frequency you want to set the crossover at (in your case, 80hz). Turn off the amp's filters so it's just playing full range and use a dmm or scope to measure the output voltage. It doesn't really matter where the volume or gain are set at, so long as the amp isn't clipping and the voltage is high enough to get a good reading, say at least 1V. Multiply this measured voltage by .707, turn on the filter, and adjust the frequency knob until you get this derived voltage on the output.

3 - Your amp's FR should be pretty much bone flat unless you have a filter engaged, the important thing to measure is the FR of your speakers, because even if the amp is flat, the speakers won't be, and you're going to want to EQ the system so that the final result is flat. Either way, if you want to measure the amp's FR you can just put a DMM or scope on the output and play a series of sine waves ranging from 20-20k and measure the voltage. It shouldn't deviate more than 1/3dB at the most, unless of course you have a filter engaged.

4 - In my opinion, the extra power/headroom gained from bridging is vastly superior to the higher damping factor and lower THD you get from not bridging. I would leave it bridged personally.
sq_guy said:
sr20dem0n, for poin 1 of you reply, should I use dummy load when I measure with DMM ?

You mean at the end when you're matching them? It doesn't matter, as long as you're consistent with both channels.

What amp is it? I can almost guarantee you that since it's a 4ch amp it has 2nd order Butterworths, if it was a mono that would be a different story.