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  #1  
Old 01-23-2012, 10:25 PM
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YetAnotherITGuy YetAnotherITGuy
running 8 ohm subs on 4 ohm amp?

I have an Alpine PDX-4 amp that is rated at 4X100W into 4 (or 2) ohms, 2X200 watts bridged into 4 ohms. I was planning on bridging it and running two 4ohm subs at 200 watts. The subs I bought were two Alpine SWS-1043D subs, which were advertised as 4 ohm subs.

What I didn't know was they are DVC (dual voice coil) subs that have TWO sets of connections per sub, 4 ohms each, that have to be run in parallel or in series at 8 ohms or 2 ohms.

So, wanting to use these subs anyway, I am thinking I could get away with running the dual voice coils in series, which on my meter measures at about 6.5 ohms actual, to my PDX-4 in bridged mode. Downside to that would be a less than optimum efficiency; reduced wattage to the subs, but not going to hurt anything otherwise.

The other alternative I am thinking I could run the amp unbridged and connect the two right outputs the the voice coil terminals on one sub, the left outputs to the other, that would show a 4 ohm load all around to the amp. Only downside to that is I have to install a new terminal block for my sub-box that contains two sets of connections for each side.

I'm thinking the first option would be okay, even if it is less that optimum (and probably wouldn't sound much better/worse than the second option). Wanted to get some input -- thanks.
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  #2  
Old 01-23-2012, 11:50 PM
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Really either way is ok.

Two coils in series ran to two bridged channels at 8 ohm will be just fine...in theory you will lose 'half' your power, but then again with music you will never see 200 watts even if you were bridged at 4 ohm.

A coil per channel is also ok. While not completley nessecary since anything in the subbass region is 'almost' always a mono signal (no left and right), being me I would still probably run a single input to the left channels and put one sub on the left, and a single input to the right and run the other sub off the right channels. If each channel on the amp has seperate gains you are should to try to match the level of the two channels that are running a sub.

There is a little more to it obviously and I can go a little more in depth about it if youd like, but it is possible and there are a few things that I think should be looked at when doing a single channel per coil.
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  #3  
Old 01-24-2012, 06:41 AM
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Quote:
Originally Posted by Bartak1 View Post
Really either way is ok.
Thank you for your reply. That makes me feel better and also confirms what I was already thinking, basic Ohm's Law; E=IR and all.

Running a higher impedance for a given voltage means reduced current through the coils which translates to less usable wattage/energy, which is not going to *hurt* anything. Where folks get in trouble is running excessively low impedance (< 2 ohms, etc.) which puts an excessive current draw on the amps, overheating them, etc.

I'm going to run the two Alpine subs with the coils in series at 8 ohms (actual measured at around 6.5 ohms) which will give me around 140 usable watts per sub off the PDX-4 in bridged mode and see how it sounds. Later, I may try running the subs with the coils independant at 4 ohms each, with the PDX-4 in non-bridged mode (4X100watts @ 4 ohms), for comparison.

Either way, I suspect it is going to sound pretty good

Last edited by SalsaNChips; 01-24-2012 at 06:44 AM.
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  #4  
Old 01-24-2012, 07:57 AM
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Paul Morck GoodOlBoy711
Yea I'd run them at 8ohms before you decided to run each coil off a separate channel, that can cause problems.
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  #5  
Old 01-24-2012, 07:26 PM
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You say the amp is rated for 100x4 @ 4 ohms? If so, I think there's a more elegant solution to your problem that will eliminate improper ohm loads and have you safely getting full rated power from the amp.

1) Run the feed from your head unit to the channel 1 and 2 inputs on the amp.**
2) Switch the amp to combined input (I think it's the 1/2 setting).
3) Connect each speaker out on the amp to a seperate voice coil. Use channel 1 and 2 for the coils on one sub, channel 3 and 4 for the other (the 1/2 and 3/4 channels each share a gain control. Wiring each sub to the outputs that share a gain control should keep output levels identical to both coils on that sub).

** This presumes a stereo sub out at the head unit. If you have a mono sub out simply use a Y connector to split the signal at the amp inputs.

The result will be each channel on the amp seeing the proper 4 ohm load, the pairs of coils on the sub seeing matching signals so there are no funky harmonics or weird stress loads on the cone, and each sub getting the full 200w the amp can provide.

*IMPORTANT NOTE* I'm working out this config in my head after a very long, demanding day so I could easily be overlooking something important. Give a couple more forum members a chance to give it a once-over for potential problems before you try it.

Brad
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Last edited by Brad Johnson; 01-24-2012 at 07:46 PM.
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  #6  
Old 01-24-2012, 10:10 PM
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Paul Morck GoodOlBoy711
Just because the two channels can be seperately controlled doesn't mean they won't put out an unbalanced signal. It can still damage the cone, wiring each voice coil to a separate channel is a bad idea, one sub should be on one channel, not spread across two.
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  #7  
Old 01-25-2012, 10:38 AM
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By the gain control config it looks like channel 1 & 2 share an amp on a single gain control, with channels 3 & 4 on a seperate shared amp and shared gain control circuit. Presuming the input signal to the channels that share an amp circuit is the same (say by splitting the input with a Y cable rather than using the internal switch/combiner on the amp), shouldn't the output signal mirror that uniformity?

In other words there are 4 low level inputs on the amp with 4 corresponding speaker outputs. However, there are only TWO gain controls. One gain controls channels 1 and 2, the other gain controls channels 3 and 4. Using the 1/2 channels as an example, the shared gain control indicates to me that those channels share an amplifier circuit. Presuming that, and presuming the signal to each of the input jacks is the same, shouldn't the output on both channels 1 and 2 be the same?

I agree that it's not the best solution. That would be getting speakers and amp that match properly. I'm just trying to think of something that would work for him using the equipment he has.

Brad
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Last edited by Brad Johnson; 01-25-2012 at 10:45 AM.
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  #8  
Old 01-25-2012, 11:24 AM
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Paul Morck GoodOlBoy711
Yes, technically it should be however no amp is perfect. No amp could send out a signal that's perfectly the same all the time, everything plays a difference in what signal the sub sees, at what ohm load the amp sees the subs etc. Everything down to length of the speaker wire can vary what ohm load the amp sees and therefore sending out a different signal. So even if it sends out a bad signal say 25% of the time thats enough to damage the sub.
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  #9  
Old 01-25-2012, 08:59 PM
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Did some checking. If the amp is 4 ohms bridged up to 2 ohms in 4 channel mode the if you hooked up 8 ohms bridged you would get half of the amps output potential. Hook up each coil to a channel. One sub to 1&2 and the other to 3&4. This will eliminate over driving the amp. If you had different ohm subs it would be able to pull more power but with dual 4 ohm coils you won't be able to safely run the parrallel to a bridged amp. Dual 2 ohm coils would of been a better way to go. Then you would of been able to bridge the amp and pull more power. Unfortunately your amp is not 2 ohm stable bridged. There would of been an other way to bridge 3&4 on sub mode but the amp does not have enough dampening to control 2 subs. So this way each sub will get 200 watts and you won't hurt anything.
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Old 01-25-2012, 08:59 PM


 
 
 
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