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Ohm's law for dummies

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Old 02-15-2012, 04:38 AM   #1
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I help people out with this alot, and I thought this might make a good sticky in here, so think about it. People never know what an ohm is or why its needed in selecting speakers, so Ill try my best to explain.To get multiple subs to an amp thats bridged sometimes requires some thought and most people skip this step and need either new subs, a new amp, or both . If this is you , stop reading, I make my hours off you and therefore my living. if you are the type of person that slaps stuff together without knowing a thing about it , never checks into it , never asks question and thinks you know it all, we love/hate you and keep it up to keep us fed.
All amps put out their most power at a certain ohm load(except some ones with the regulated power supplies that are pricey and arent around much). Most all amps are rated at 4 ohms per channel at a certain distortion ratio, say 100 watts per channel, 4 ohms driven at .8%THD ( Total Harmonic Distortion). This means that a static ohm load ( a resistor not a speaker on the amp) it will produce 100 watts. Now once a speaker is on the amp it will of course move and change the ohm load, but we wont get into that right now.
Now if we change the ohms, say from 4 to 2, most amps will double the power to 200 per channel, but your distortion will double or even quadriple from .8 to 1.6 or maybe 3.2. This is why the cleanest sounding systems always have high wattage amps even on the highs and even the subs are run in stereo or one sub per amp(but thats mainly to reduce "crosstalk" and difference in subs , plus the ultra high dollar amps to run multiple subs get $$$). Now most of us cant hear that level of distortion, some people say they can so we also wont worry about that.
Now another fact to consider in this , is that the amp will "see" a different ohm load once its bridged. An amp thats 4 ohm stable bridged is also 2 ohm stable stereo. This is because an amp that has a 4 ohm load on it bridged "sees" 2 ohms per channel, I dont know why, it just does so live with it. This makes it easy to see why people like to bridge their amps, because they put out more power. If our theoritical 100 wpc amp does 200 wpc at 2 ohms and you add the 2 channels, you get 400 watts into one channel. This will make you now have twice the amp you bought , it doesnt matter if its all going into one sub or 4.
When wiring multiple subs theres 3 ways to do it, series, parellel( I never know if Im spelling that right) and series/parellel. Series means that the subs share one terminal like Christmas tree lighting. When you do this, its a wire from one + to the next subs - and the left over + and - go to the amp. Parellel means they share 2 terminals like plugging in multiple pieces into a wall socket. When you series subs, their combined ohm load doubles everytime you double the subs and when you parellel them their combined ohm loads are cut in half everytime you double the # of subs.
Some subs also fall into the dual voice coil category and some even into the quad voice coil category (and if you need help wiring those up mail them to me in at least pairs and Ill try to maybe get them back to you as they are usually nice pricey subs I would like to own). DVCs need to be treated like 2 speakers per sub and 4 subs per every 2 subs. The amp doesnt care if you only have 2 subs cuz it sees 4. If you saw the paragraph before this is where the series/parellel wiring comes into play. If you have 2 DVC subs that are 4 ohm per coil and you have a 4 ohm stable amp, youll need this. What you do is run a wire from the + on one side of the sub, to the - on the other side. Do this to the other sub and now the subs are seried out to make each sub 8ohms. After this you run each subs left over + to the amp and each - to the amp.
I hope this little think I did helps some people out.
(# of subs)ohm of each sub , type of connection = combined ohm load
(2)4s par = 2 , (2)8s par = 4
(2)2s par = 1 , (2)1s par = 1/2
(4)4s par = 1 , (4)8s par = 2
(4)2s par = 1/2 , (4)1s par = 1/4

(2)4s ser = 8 , (2)8s ser = 16
(2)2s ser = 4 , (2)1s ser = 2
(4)4s ser = 16 , (4)8s ser = 32
(4)2s ser = 8 , (4)1s ser = 4

its easy to see the way it works like this and these will do probably every possible type of speaker install you will ever need including house speakers. The only 2 other installs that some people do are 3 8ohms on a 4 ohm stable amp (2.66 ohms) and 3 4ohms on a 2 ohm stable amp( 1.33 ohms). If you need more help on this , let me know and I hope I can answer it

Scott Spiehler Jr

One sub:

2 subs:

3 subs:

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Old 02-15-2012, 04:39 AM   #2
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and 4 sub setups:

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Old 02-15-2012, 01:26 PM   #3
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Nice write up. I do a bit of this on the side also but I generally run package deals.

1979 Monte Carlo with a 400hp+ 350ci.
1983 T10 w/2.8L getting parked till can restore and put a v8 in
1994 Blazer 4dr with the CPI(FOR SALE)
1995 Blazer 4Dr 4X4(FOR SALE)
2002 Trailblazer LT
2007 Envoy Denali

Independent AmsOil Dealer

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Old 02-16-2012, 01:23 PM   #4
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Yo Mammy

Awesome info. Thanks man! I stickied this one!

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