Subwoofer wiring question - Low pass filters?

mattyprice4004 · 15 Sep 2009 at 15:36

Hi all,

Currently I have 2x 10" JBL subwoofers, fed with a Pryamid 700w RMS amplifier.

The amp input is tapped off the 6x9s in the back shelf, and is fed into the high-level inputs. The sub on the left is then driven by the left side of the amp, and the right with the right side. All simple so far, right?

My question is simply this: I am looking at a low-pass filter to stop mid and treble getting through, but the only ones I can find are for 8ohm speakers. My subwoofers are 4ohm - will it still work or will it do damage / not work?

Thanks
Mat

iaind · 15 Sep 2009 at 15:47

Does your amp not have high/low pass filters?

Why have you not connected it to a proper pre-out?

Howard · 15 Sep 2009 at 15:49

Yeah connect it to a proper sub or speaker preout and do it properly. Does the amp not have an LPF switch?

iaind · 15 Sep 2009 at 15:51

If youre hugely bothered about impedance and cant be arsed wiring it properly, connect a single speaker to the high inputs and bridge the amp outputs to the subs wired in series.

You cant hear bass in stereo so it wont matter

mattyprice4004 · 15 Sep 2009 at 15:51

Lol, I'll be running proper pre-outs soon, and my headunit or amp doesn't have filters built in.

Back to the original question? I assume it will be fine

iaind · 15 Sep 2009 at 15:52

rover416i said:
Lol, I'll be running proper pre-outs soon, and my headunit or amp doesn't have filters built in.

Back to the original question? I assume it will be fine

Do what I suggested above then, just make sure you stay at least a 12 foot radius at all time and keep a fire extinguisher handy

mattyprice4004 · 15 Sep 2009 at 15:54

I was thinking of bridging the amp and using it as 8ohm, but apparently that drops the power down?

Not that the JBLs can take the amp on full whack - I've already destroyed one

Edit: Sorry Iain, didn't see your post before I posted above

iaind · 15 Sep 2009 at 15:57

Either way I'm sure it'll be fine - although I'd be inclined to do it properly from the offset.

Spend 100~120 quid on a decent 4 channel amp, 2 to your 6x9s (eeurgh) and the other 2 bridged with an LPF to your subs

Janesy B · 15 Sep 2009 at 16:02

In his defence, the 6x9s are standard fit.

iaind · 15 Sep 2009 at 16:04

Janesy B said:
In his defence, the 6x9s are standard fit.

Oh fair enough...I'd have got rid of them personally but never mind!

mattyprice4004 · 15 Sep 2009 at 16:10

iaind said:
Either way I'm sure it'll be fine - although I'd be inclined to do it properly from the offset.

Spend 100~120 quid on a decent 4 channel amp, 2 to your 6x9s (eeurgh) and the other 2 bridged with an LPF to your subs

They came as standard

The amp is absolutely fantastic, shame it's only 2 channel, although I could get another I suppose although might be overkill for some crappy 6x9s.

Descision descisions eh

Clarkey · 15 Sep 2009 at 16:14

Doesn't sound fantastic to me, not even a filter. I've never seen an amp without that.

mattyprice4004 · 15 Sep 2009 at 16:16

I use the term for an amp that can make your insides move for 40 quid, speaking within reason for what it cost

iaind · 15 Sep 2009 at 16:23

rover416i said:
I use the term for an amp that can make your insides move for 40 quid, speaking within reason for what it cost

Volume doesnt equal quality, any amp could do that

Thomas. PLease. · 15 Sep 2009 at 17:06

why not just use a proper pre out, and set the lpf on the head unit itself.

Dogbreath · 15 Sep 2009 at 18:28

rover416i said:
I was thinking of bridging the amp and using it as 8ohm, but apparently that drops the power down?

No, power is proportional to the square of the voltage; doubling the output voltage (by bridging) gives you a theoretical 4x the power. In practice the PSU of the amp often limits this to about about double when using 4 ohm or lower speakers. Doubling the impedance gives you half the power, and puts less load on the amp. So by bridging and moving to 8 ohms you should still see an overall increase in power.

Avoid passive crossovers for subs if at all possible. They significantly degrade the damping factor of the speaker which is absolutely not what you want in a sub. Using an 8 Ohm crossover with a 4 Ohm driver will have the effect of lowering the crossover frequency by around an octave.