pjsub

I've got a JL 8" sub which runs at 75 watts RMS. May amp however, in 3 channel mode outputs (for the sub) of 284 watts RMS. There are two 'input level' knobs on the amp (one for the front speakers and one for the sub/rear) - going from 6v to 250mV. Does this adjust the output to the sub? and if so, is this the only way to adjust the it to ensure the sub doesnt get too much power? Is there any way to know exactly the power going to the sub or limit the amps output to it?

I'm a bit concerned because the input level adjustment doesnt have any markings on it (apart from 6V [min] and 250mV [max]). Also, how does the voltage relate to watts?

Thanks

Chris Type R

Two suggestions:

- Do you not have adjustments on your head unit ?

- Can you not look in the manual (find one on the Internet if this missing) ?

pjsub

The HU output is adjustable, however this obviously only adjust the signal on its way to the amp and not direct to the sub.

As for the amp manual, all it says is to adjust the input level so that you get the max sound with out distortion. It doesnt mention controlling the power output.

mikey k

What cross over are you using for the sub?
HU
AMP
Seperate
This will determine how you set it up!

Once that is right you need to adjust the relevant input level.

If it is a three channel amp I would use one channel for each front and the third for the sub. Normally you would find the amp has a built in cross over for using the third channel as a sub channel!

That makes it a lot simpler to set up, you will just need to select the sub cross over limit and adjust it to the level you want!

Hope that helps!

pjsub

It's a 4 channel amp. The manufacturer's recommends 1 channel for each of the front speakers (to give each 104w RMS) and then bridging the other 2 for the sub (giving it 284w RMS), and using the amp's crossover.

It just seems a bit dodgy trying to find a safe output level for my sub by turning an unmarked knob and hoping I dont turn it too far.


ps: Here's my amp

mikey k

OK That makes sense!

Turn all the levels right down to start off with.
So you have a freq knob which is the internal crossover frequency and a switch for mode. You want the fronts to All and the rears to LP for low pass (sub). Wire it all up set the LP freq to ~ 100 Hz, turn it on! Turn up the HU you should then get front sound and sub working but wrong levels! Turn up the HU to ~50% volume then turn up the amp inputs until you get bit of distortion then back them off slightly. Then turn up the HU and adjust the amp levels again. It will get very LOUD but this is the easiest way to do a simple set up! You may find after you want to adjust the sub amp level and freq if it is too much!

That should do it!

Ruckus

mikey k's post is good advice but just to clear this up the 'knob' your talking about in the Line level input gain or just 'Gains'. Its what you use the match the RCA output of your head unit to the RCA input on your amp.
ie. Most head units have 2 volt rms RCA outputs so you would match it to that.
Some headunit on the other hand have up to 10volt RCA ( i have a Rockford HU with 10v and my Denon is 5v) so you amp can take input up to 6volts.
If you dont want to guess (good idea) then follow this guide for input gain matching.... btw you will need an oscilloscope and someone that know how to use it (good ice shop).
http://www.talkaudio.co.uk/vbb/showthread.php?t=53062

mikey k

to the ICE god

Totally agree if you have the equipment and help

pjsub

Thanks for the replies fellas. Just one more point, do you think there's much risk of my amp blowing up the sub (as the sub is 75w RMS)? Or would that only become a risk at very high volumes?

Thanks again

Ruckus

Its Very rare for a Sub to burn up due to over power.
Most times its badly set up Gains that kill subs (Clipped signal sending a square wave to the sub).
Just go careful with the loud knob on your Head unit