Setting gains on 4-channel bridged to 2...gain voltage not high enough.

[bikinpunk 10+ year member]

I've been getting clipping lately, which I believe is attributed to my amp getting hotter since I've recently put back in my rear seat. Changed nothing electrically, just put the seat in. I figure it might also be related to the gain settings since when I set them I used my ear (not always the best method) so I thought I would take the 20 minutes and do them right tonight...wound up running into a brick wall:

I'm using an alpine mrp-f240 4-channel amp, bridged to @ 100w rms x 2 @ 4 ohms.

http://www.crutchfield.com/S-3UitsnSCpg7/cgi-bin/prodview.asp?I=500mrpf240&search=mrpf240

By the method used here, I should be getting a gain voltage for each set of channels (1 & 2, 3 & 4) of 20v each set ---> Sqrt(100*4)=20. However, when I use my DMM on AC settings, the MAX gain goes to only 5v for the channel 1&2 setting. I didn't bother with 3 & 4 after having this issue, btw. I unplugged all other RCA's except for the ones I was actually using, once again 1 & 2. Am I missing something here? I have all 4 RCA's running since it seems to make sense because I'm using each channels' input, whether it be power or ground.

Recap: should be using a gain voltage of 20v, but maximum gain per side is only 5v. What am I missing here?

Edit: volume was set on 24/33.

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[helotaxi 5,000+ posts]

Possible that the amp regulates itself using a feedback circuit and requires a load on the output to produce power. If that is the case, you can't set the gains in the method mentioned in the tutorial. You would need a load of some kind (not necessarily 4 ohm) that can handle the power to allow you to measure the gains. Also make sure that you have an RMS voltmeter.

 

[bikinpunk 10+ year member]

How could I apply a load to the amp then? Leave the speaker plugged in?

 

[helotaxi 5,000+ posts]

A better opion would be to get a bank of resistors that will come out to a 4-8 ohm load and will be able to handle 100W. The other option is to just set the gain conservatively by ear and don't worry about it. Set it by ear with music an measure what you are getting for voltage with the speakers connected.