I would like to set the gain on my amp but I am confused. I would be using a multimeter and will be using the E=(P*R)^.5 formula BUT I am confused which number goes where because I will be bridging the amp. In JL's tutorial, it says that I have to half the load and then double the voltage at the end.

I have a Rockford Fosgate 500a2 and 2-HE2 10" 4ohms dvc subs wired for total load of 4 ohms. The amp is bridged for mono operation. The amp is rated at 125 W X 2 @ 4 Ohms RMS,250 W X 2 @ 2 Ohms RMS, and 500 W X 1 @ 4 Ohms Bridged RMS

If I use P=500 and R=2 (half of 4) I get 31.62 which doubled is 63.34 Volts. Seems kind of high? Thanks.

also, anyone have a test cd i can use?


Modified by GSRrR at 5:24 AM 5/3/2004


Modified by GSRrR at 4:55 PM 5/3/2004

 

Only thing is the rating of the amps is never correct.

The best thing to do is turn all the adjustments down to 0 (gain and bass boost) and the freq (filter) to around 80 or so. Turn your radio up to the loudest you listen to it, and then maybe up 1 or 2 more. Now turn the gain up until the subs start to distort, and then turn it down some. Do the same with the bass boost and adjust the filter to where you like it (IE> no vocals in the subs)

Otherwise you'll have to measure the actual output off the amp if you really want to use that formula.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by GSRrR &raquo;</TD></TR><TR><TD CLASS="quote">I would like to set the gain on my amp but I am confused. I would be using a multimeter and will be using the E=(P*R)^.5 formula BUT I am confused which number goes where because I will be bridging the amp. In JL's tutorial, it says that I have to half the load and then double the voltage at the end.

I have a Rockford Fosgate 500a2 and 2-HE2 10" subs wired for 4 ohms. The amp is rated at 125 W X 2 @ 4 Ohms RMS,250 W X 2 @ 2 Ohms RMS, and 500 W X 1 @ 4 Ohms Bridged RMS

If I use P=500 and R=2 (half of 4) I get 31.62 which doubled is 63.34 Volts. Seems kind of high? Thanks.

also, anyone have a test cd i can use?


Modified by GSRrR at 5:24 AM 5/3/2004</TD></TR></TABLE>

 

The multimeter way is pretty darn effective.

You use

V = square_root(P*R)

So

V = square_root(500*4)

V = 44.72


I am not sure why you are halving the impedance then doubling result.


Do not turn the radio up the loudest you listen to it, turn it up about 3/4 volume. Most heads will start clipping after that. If you have a good head you can go higher, my last Eclipse would go full tilt without clipping, as will my current Alpine...depends on head. Also...make sure to disconnect the speakers before you start.

 

head is pioneer.

i do the half and doubling because on JL's site:

"when bridging two channels, the impedance each channel works at will be 1/2 of the load impedance. Therefore, it is necessay to divide the acutal load impedance in half and use this impedance on the chart when bridging two channels. Also, the voltage found in the chart should be doubled....."

maybe that is only for JL' amps?

 

When you bridge an amp each channel sees half the load. Say you have a 2 channel amp and you bridge it into a 4 ohm load. Each channel of the amp is going to see 2 ohms, but it is still a 4 ohm total load....and since it is one channel now (bridged) you will measure for 4 ohm on that single channel. I have never done this on a JL amp so it could be different, but I do not see how. I will take a look at their site and see what is up in full context.

 

their site is down...I will try later, or maybe someone else can clear it up

 

i think this is it. someone from another site helped me.

500 X 2 = 1000
sq rt 1000=31.6

but since i had to set left and right gain independently (2 adjustment screws), i just did each to 15.

looks right?

 

I looked at the JL site and am baffled...dunno what is up with their amps.


If your final load at the amp is 2ohm then that is right. You should not have to adjust both gains though. I had a Fosgate 500a2 that I gave to my sister, and when bridged only one gain worked...I think the Left.

 

Here is a thread on this topic on sound domain forums

http://forum.sounddomain.com/f...9;p=1

 

ya i have read that already. i have searched many audio forums and am still confused. here is what someone over at the12volt.com told me:

"Assuming the load you are placing on the amp is 4-ohms TOTAL, your correct target is 31.62. The JL tutorial is refering to using their charts which do not include the bridged power rating of the amp. Since you know yours is 500 watts @ 4 ohms, 500 X 2 = 1000, sq. rt. of 1000 = 31.62 volts. Remember to use a test tone of about 50Hz and to be sure your head unit is set below it's clipping threshold.

Your amp cannot handle anything lower than 4-ohms in bridged mode."

i thought he typed it wrong and put 2 instead of 4 so I asked again and he replied:

"No, since it is 4-ohms total each amplifier channel will see 2-ohms of load...."

my final load is 4ohms. and yes you have to set both gains equally to balance the subs. checked with the manual.

can anyone add/confirm anything? thought this was gonna be a piece of cake.

 

What that guy said goes by what that other thread says...and that is how I set my amp, which ended up just a hair higher than I had it by ear...not even really noticiable.

 

adjust it by ear. turn the gain to zero, turn your deck to the loudest you'd listen to it, then adjust the gain to that volume, increase the gain until the speaker begins to distort, then turn it back just a little bit.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by georgelb &raquo;</TD></TR><TR><TD CLASS="quote">The multimeter way is pretty darn effective.

You use

V = square_root(P*R)

So

V = square_root(500*4)

V = 44.72
</TD></TR></TABLE>

Dumb question but...

shouldn't the formula be

V=square_root(P*Z) seeing as we are dealing with impedance, not resistance??

 

turn it all the way up and listen to the radio at volue 10