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Location:
Launceston, Tasmania
Registered:
May 2002
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Problems with running 8ohm subs from a 4ohm amp?
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Mon, 15 August 2005 08:14
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I have 2 rockford 8ohm subs that i would like to run from a jaycar 4x100wrms amp, the thing that i am concerned about it that the amp will only run at 2 or 4ohms. Is there any issues with running an 8ohm sub on a 4ohm amp? if not, how can i calculate the approx wattage that each sub will get?
Thanks in advance
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Location:
Wollongong
Registered:
May 2002
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Mon, 15 August 2005 08:41
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You can run higher impedance than what the amp is rated at. Just won't be as efficient/loud.
Same doesn't work the other way around though. Impedance too low = too much current draw and many popped fuses and possible broken amp.
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Location:
WA
Registered:
May 2005
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Mon, 15 August 2005 09:57
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You could wire ur subs in parallel, which would make the amp see a single 4ohm load, and then u could bridge one half of ur amp to run ur subs, and still have the other half left to run other speakers. . .
You cant really figure out how much power each of ur subs will recieve accurately if they are running at 8 ohms. The amplifier characteristics tend to change quite alot as impedance goes up. You could estimate (not accurately) by using some basic electricity laws - search on google for them.
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Location:
brisbane
Registered:
October 2004
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Mon, 15 August 2005 10:40
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you need to check 2 things:
amplifier capability:
- what your amplifier is capable of, ie what loads it is stable
of driving
driver configuration:
- what load your driver configuration presents to the amp
once you have choosen a driver configuration that is compatible with the amplifier, you have a setup and you can work out how much power the setup has.
thumbs up on the jaycar amps, they are awesome, i have a few, latest "4x100" model delivers 130x4 wrms.
amplifier capability :
most newer decent amps = bridgable, 2 ohm stable, trimode
bridgable - bridging can only be done with a 4 ohm load
2 ohm stable - a 2 ohm load can be run off a single channel
tri mode - 4 ohm on 1 channel, 4 ohms on adjacent channel, 4 ohms bridged across these two channels
driver configuration :
drivers = 4 ohm single voice coil common
4 ohm dual voice coil common
2 ohm, 8 ohm avaliable
driver load in series = driver 1 + driver 2 +... etc
driver load in parallel (4ohm) : 2 drivers = 2 ohm
4 drivers = 1 ohm
higher impedences (loads) will decrease power and visa-versa, a lower load will increase power.
amplifiers are designed to run certain loads and too low a load will damage the amplifier, too high a load will only decrease power.
clipping is the main destroyer of amplifiers though, having too little power for a driver and cranking the volume up, it sounds like it is distorting but it is clipping. dont do it!
your components are interesting, 8 ohm. probably more suited to a monoblock setup with many drivers to have many drivers and a stable load like 1 ohm on a monoblock. i cant see a way to use all of the power of the amplifier because of the 8 ohm impedence they have, it can all run using less power.
long post, any corrections welcome
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Location:
Launceston, Tasmania
Registered:
May 2002
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Tue, 16 August 2005 03:49
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Thanks for the input 
What would be the best way to power my subs with this amp? i could wire the subs in parallel but then half my amps power is going to waste as i don't have any other speakers than need powering.
Here are the specs of my amp:
Power / channel @ 4ohm 14.4V: 130WRMS x 4
Power / channel @ 2ohm 14.4V: 190WRMS x 4
Power / bridged @ 4ohm 14.4V: 380WRMS x 2
Signal/Noise ratio: >90dB
Input sensitivity: 200mV to 5V
Low pass filter: 50 - 500Hz @ 12dB/octave
High pass filter: 50 - 500Hz @ 12dB/octave
Pass thru RCA: Yes
Dimensions: 400(L) x 258(W) x 50mm(D)
My subs are 150wrms each and are 8ohms
Any suggestions would be great, i really don't want to purchase another amp, just want to get this one going as good as possible.
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Tue, 16 August 2005 06:16
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If you're using the entire 4 channel amp to run the subs, just run two bridged outputs, one per sub. Will have ample power and the amp will never be strained.
What is the specs on the subs? What sort of efficiency/sensitivity are they?
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Registered:
July 2005
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Tue, 16 August 2005 08:18
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if ya subs are dual voice coil its a perfect set up -- run the coils parallel (for 4 ohm) then just wire her up to ya amp 2 way 
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Location:
Launceston, Tasmania
Registered:
May 2002
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Wed, 17 August 2005 00:23
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Here are the specs of the subwoofers. i'll be running two of these:
Size 10 inch
Type Woofer
Voice coil Configuration - Single
Material - Aluminum
Description - 1.5"
Cone Material Poly/Plastic -
Parabolic Polypropylene
Frame Material Frame - Stamped
Material - Steel
Surround Material
Spider
Crossover
Specifications Power handling(RMS) - 150 Watts
Power handling(Peak) - 300 Watts
Impedance - 8 Ohms
Sensitivity - 85.5 db
Vas - 50
Fs - 31
Qts - .524
Xmax - 7.7
Dimensions Depth Bottom mount - 5.2 in.
Cutout - 9.25 in.
Displacement - 58.75 cu in.
Magnet weight - 38 oz.
Enclosure Recommended Sealed (Cubic Feet) - 0.65
Recommended Ported (Cubic feet) - 1.25 @ 40Hz
Just another quick question, what are the differences between a seeled and ported box in terms sound differences?
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Wed, 17 August 2005 00:33
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Ouch, that's some uber inefficient subs. I'd be running them as per my recommendations above An excess of power always sounds better because it's not strained.
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I Supported Toymods
Location:
Sydney
Registered:
December 2002
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Wed, 17 August 2005 01:16
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Run them in parallel and off two bridged channels of the amp.
This will leave you another two channels for the front (or rear).
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Location:
Launceston, Tasmania
Registered:
May 2002
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Thu, 18 August 2005 07:48
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I thought about doing that toobs but the rest of my system is already powered by another amp and i really don't have a use for the other 2 channels if i wire it up that way. Are my subs (specs above) really that inefficient?
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Thu, 18 August 2005 07:56
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Not terrible, but far from good. If you don't need the other channels, just bridge them and run a sub per bridge. An excess of power is handy.
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Thu, 18 August 2005 07:57
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It's not going to impinge on how they sound, just that they'll require more grunt for a given volume.
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Location:
Sydney
Registered:
January 2005
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Thu, 18 August 2005 08:07
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running them from seperate channels, will never ensure they receive the same power each punch. in some cases the subs will punch at different times, or different amounts of power from each.
from this, u can get worse vibrations or you may notice the different hits. this may or may not bother you. it may be bad or not. if u wire it from 2 channels then u'll know wether it bothers you or not.
running them in parralel is the better choice. dont worry about the other 2channels.
this may not bother sum, but may annoy the shit out of others.
good luck
jason
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Location:
Launceston, Tasmania
Registered:
May 2002
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Tue, 23 August 2005 05:24
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just realised that they are the wrong specs above, they are for the earlier model. the specs of my subs are:
10 inch
Type Woofer
Voice coil Configuration - Single
Material - Aluminum
Description - 2"
Cone Material Poly/Plastic -
Parabolic Injection Molded Polypropylene
Frame Material Frame - Stamped
Material - Steel
Surround Material
Spider
Crossover
Specifications Power handling(RMS) - 200 Watts
Power handling(Peak) - 400 Watts
Impedance - 8 Ohms
Sensitivity - 85.8 db
Vas - 42
Fs - 29
Qts - .500
Xmax - 11.0
Dimensions Depth Bottom mount - 5.6 in.
Cutout - 9.25 in.
Displacement - 69.12 cu in.
Magnet weight - 60 oz.
Enclosure Recommended Sealed (Cubic Feet) - 0.65
Recommended Ported (Cubic feet) - 1.25 @ 40Hz
does that look like much better quality??
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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Tue, 23 August 2005 05:35
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bad news for ya buddy - the rockford subs are not the best in the world!
their component and full-range speakers are the ducks nuts of affordable car audio, but speaking from experience, the subs are underpowered. my sub needs to be run at 20% more amp gain and double the head unit volume to be heard 
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: Problems with running 8ohm subs from a 4ohm amp?
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Tue, 23 August 2005 06:15
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| draven wrote on Tue, 23 August 2005 13:35 |
the subs are underpowered.
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Underpowered, not exactly. Terribly inefficient, shit yeah.
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