Running twin subs:

Amp is Infinity 7521a (100w rms stereo 200 bridged i assume)

Subs are Infinity 12" 120.3 Kappa (rated 50-300 rms)


Very weak sound when both subs run as a stereo pair or double wired in mono

What's the best, neatest way to run both or is it a separate amp for each sub??

Cheers
Trevor

 

have you got the correct size enclosure for the 2 subs, if you havn't then it will sound rubbish,

Have you also set the crossovers on the amp to run subs? you will need to put the LP filter on so it only passes the low frequencies, set it to about 90Hz and tweek from there to see what sound right.

And it may just be that you amp prob wont like being run at 1 ohm, which is what it will be doing if you correctly connect 2 x 4 ohm speakers onto a bridged set of terminals?

 

It may be weak if you have the polarity wrong on one of them (i.e. + to - rather than + to + etc.) 'cos one sub is pushing air whilst the other is pulling it.

Describe how you've got it wired etc., including HU settings and someone may be able to help.

By the way, two 4 Ohm 'speakers can either be wired in series (amp + to 1st spkr +, first spkr - to 2nd spkr +, 2nd spkr - to amp -) in which case the amp will see 8 Ohms, or in parallel (amp + to 1st and 2nd spkr +, amp - to 1st and 2nd spkr -) in which case the amp will see 2 Ohm. This assumes the spkrs are 4 Ohm.

 

If you present the Amp with a 2 ohm load, and bridge the connections with that 2 ohm load, your amp will see 1 ohm across the terminals and most prob not like it (and iirc Infinity amps are not rated at 1 ohm) it is likely to get very hot if run hard and your amp will then possibly suffer from thermal shut down. If you show it an 8 Ohm load across bridged terminals then it will see 4 ohms and not produce as much power as expected.


Wired in series the subs will be 8 ohm, assuming they are 4 ohm drivers, in parallel they will be 2 ohm, agreed

You need to state how you have connected the terminals?

 

Right- box is a double enclosure (both subs have separate compartments in the double box)

Each sub has its own terminals

checked polarity- fine

LP set too

Firstly i wired each sub as a left and right speaker, still with LP, but set to stereo- very weak

Then wired from L- and R+ a seperate cable to each sub from the same amp terminal - set to mono and LP- again very weak

1 sub in above setup- perfect sound

I'm ditching my rear speakers now so could run a further amp but this then, i assume, creates further wiring and need for caps, etc....

It would then be 2x 200W mono'ed to run the 2 subs
and 1 x100W (stereo) running my 100W front components

Woud this need a cap installed, bigger power and earth cabling?

Anything else?

Cheers all
Trevor

 

Good point; I guess you can look at it like that as it's true that most (all?) amps when bridged can only handle twice the ohm'age (lol) that they can when not bridged.

 

hey another simple way i found is if you have a double enclosure is to drill a port hole in one box and leave one box sealed this will make a deeper bass sound i have done this on mine and it has made a huge differance.

danny

 

From these statements i would hazard a guess that your amp just isn't man enough! either power both subs separately which you can do as they are in independant enclosures or get a bigger amp
You should have at least 4 gauge cable anyway in a multi-amp system, a cap isn't essential but can help if your alternator isn't very big, a bigger alternator is the better way to go tbh but try the other/bigger amp first and see how you get on, your earth cable wants to be as close to the amps as possible, good luck

JL Audio

This is worth reading.

 

Thanks again for that- i think they will have to be run 1 amp each (i have 2 identical amps) so will buy 1 more to run my front components.

I'll keep you posted

Due to the need to buy a further amp- i cant get a whole boot worth of dynamat - i can stretch to 2 wedge packs- Where's the best areas to start with until funds are available?- by build is to be made in such a way as everything can be taken out


Trevor

 

one amp per sub maybe your best way forward then, they will need to be the same amps though idealy, put another on the components when you can.

you dont need to blanket cover the boot in dynamat, you can fit it in strips, squares etc, as long as the larger bare bits have some on then it will make a significant difference as this will still dampen the resonance of the pannel covered.