Question about ohms and bridging...

[Justinw303 10+ year member]

I'm very new to the car audio scene, and I have a question. I just called a guy at a car audio place and asked him a question, and his response brought up this question.

Alright, let's say I have two 4 ohm subs (50-200 rms). I also have a 2 channel amp. Here are the specs:

200 watts RMS x 2 at 4 ohms

300 watts RMS x 2 at 2 ohms

600 watts RMS x 1 bridged output at 4 ohms

4-ohm stable in bridged mode

Now, I asked the guy if I could run one sub per channel, seeing as I have 2 subs and a 2 channel amp. Well, he said I would have to run it in bridged mode.

This is where I get confused. He said that since the subs were 4 ohms, it would be running 300 watts to each sub at 2 ohms.

So, does this mean when you bridge an amp with 2 subs you take the ohm rating of the subs and half it to know how many ohms the amp will encounter?

Also, How come he said I could bridge the 2 subs on the amp, and when I look at the specs on the amp the only time it mentions bridging is when it says 600 watts RMS x 1 bridged output at 4 ohms. So, what's up?

And the lowest number of ohms the amp can handle bridged (according to the specs) is 4, so how could I do what he told me to do?

I would really appreciate some help with this. I'm trying to learn as much as I can.

I realize the equipment is cheap, but please bear in mind that I'm a novice.

Click to read more...

 

[desi_daru 10+ year member]

http://www.rockfordfosgate.com/rftech/wiringwizard.asp

 

[jsloan10 10+ year member]

3.5.3 What happens when an amp is bridged?

-------------------------------------------

Basically, one channel's signal is inverted, and then the two channels

are combined to form one channel with twice the voltage of either of

the original channels.

Ohm's Law for Alternating Current states that I = V/Z where I is

current, V is voltage, and Z is impedance. We also know that P = IV,

where P is power. If we use Ohm's Law and substitute into the power

equation, we get P = V(V/Z), which can be rewritten as P = (V^2)/Z.

Therefore, power is the square of voltage divided by impedance.

Now, why do we care about all that? Because it explains precisely what

happens when an amp is bridged. I'll give a practical example and

explain the theoretical basis of that example.

Imagine you have a two-channel amp that puts out 50 watts into each

channel when driven into a load of 4 ohms per channel. Since we know P

and Z, we can plug these numbers back into our power equation and find

V. 50 = V^2/4 -> V = sqrt(200). So, we're seeing a voltage of 14.1

volts across each channel.

Now, imagine we bridge this amp, and use it to push just one of those 4

ohms loads. When the amp is bridged, the voltage is doubled. Since we

know the voltage (2*14.1 volts), and the impedance (4 ohms), we can

calculate power. Remember that P = V*V/Z. That means P = (28.2)^2/4,

which is 198.1 watts. It should be clear by now that the new power is

approximately 200 watts - quadruple the power of a single, unbridged

channel!

You can probably see that should be the case, especially if you look

back at the power equation. Since P = V*V/Z, if you double V, you

quadruple power, since V is squared in the power equation.

Now, all this assumes the amp is stable into 4 ohms mono. The mono

channel is putting out four times as much power as a single unbridged

channel, so it must be putting out twice as much as the two single

channels combined. Since the voltage on the supply side of the amp is

dependent on the car's electrical system, it doesn't change (OK, the

increased current might cause a voltage *drop*, but let's not worry

about that now). Looking at the first power equation, at the supply

side of the amp, we see P = IV. Now, when we bridged the amp, we

doubled the power, but the input voltage stayed the same. So, if we

hold V constant, the only way to double the power is to double the

current.

That means the amp is now drawing twice as much current when it's

running at a given impedance mono than it would be running two stereo

channels at the same impedance. There are only two ways the amp can do

that - it can simply draw more through it's circuits, and dissipate the

extra heat, or it can utilize a current limiter, to prevent the

increase in current. Of course, using the current limiter means you

don't get the power gains, either! So, if the amp can't handle the

extra current, and it doesn't limit the current in some way, kiss it

goodbye. For that reason, an amp is typically considered mono stable

into twice the impedance it is considered stereo stable.

3.5.4 Does bridging an amp would halve the impedance of the speakers?

----------------------------------------------------------------------

Impedance is a characteristic of the speakers. The speakers don't give

a flip how the amp is configured: they have a given impedance curve,

and that's that. It should be clear that when you bridge an amp, you

are changing *the amp*. The speaker's impedance is *not* a function of

the amp, but the amp's tolerance to a given impedance depends

completely on the way the amp is configured. If you'll remember from

section 4, an amp bridged into a given impedance draws twice as much

current as it would if it were driving two separate channels, each at

that impedance. So, a four ohm speaker stays a four ohm speaker, if

it's hooked to one channel, a bridged channel, a toaster, or the wall

socket. But, it is more stressful for the amp to drive any impedance

bridged than unbridged.

So, why do people talk about the impedance halving? Well, it's a

simple model that isn't correct but is easy to explain to people who

don't know what's really going on. It goes like this: When you bridge

the amp, each channel is "seeing" half the load presented to the amp.

So, if you bridge an amp to 4 ohms, each channel "sees" 2 ohms.

Therefore, each channel puts out twice as much power, and the combined

output is quadruple a single channel at 4 ohms.

Why is that still wrong? Because each channel isn't really used as a

single channel. You've used part of one channel, and an inverted part

of another channel to create a totally new channel, the bridged

channel. Also, there's no way for a channel to "see" only part of a

circuit. If it's "seeing" half the speaker, it's "seeing" it all.

Second, it makes it awkward if people believe that the impedance is

really, literally, changing. If you use that model, is it safe to run

a 4 ohm mono stable amp into a 4 ohm speaker? It should be, but we

just said the impedance halves, so that's now a 2 ohm speaker, and you

can't use it. That's wrong, and confusing, and it makes people think

they can't do things they really can.

 

[Justinw303 10+ year member]

That link really helped me out. The text in the second post just confused me //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif

Ok, I used the link and found the wiring option I needed, and it would be a 2 ohm load and bridged I assume because the positive and the negative for each sub is connected and put in the same connector.

So, since my amp is not stable at 2 ohms bridged, it wouldn't work. Right?

Sorry, I'm just not good at science.

 

[jsloan10 10+ year member]

If you read what I posted you wouldn't need to ask that, but appearently your Illiterate so I could just call you an ungrateful NooB and you wouldn't have a clue.

 

[Justinw303 10+ year member]

I guess this is what I get for asking nicely...

Look man, I'm not illiterate. I just can't follow what you posted because I am not familiar in the slightest with anything having to do with electrical workings. Therefore, I had no idea what most of the terms used in the paragraphs meant, so I get lost easy. I know you're probably very good with car audio, but please, have mercy on my questions. I'm just trying to learn a little bit...

 

[jsloan10 10+ year member]

Give a man a fish or teach a man to fish

Heres the fish-

wire the subs the up stereo like you would any speaker or bridge the amp and run the subs in series, you will get the same power to the subs either way.

Series is + from the amp to + on the sub then take the - of the sub and run it to the + on the next sub then run the next subs - back to the - on the amp.

Im having my rag right now and the cramps are killing me //content.invisioncic.com/y282845/emoticons/mad.gif.c18f003ab0ef8a0d9c27ca78d77a6392.gif

 

[Justin Thibert 10+ year member]

Here is what bridging an amp in simpliest terms is... you have a 4 channel amp that runs 50 watts rms per channel at 4 ohms each....

So that means you can run 4 speakers 50 watts or less because it has four channels right...

Well what you can also do is run 2 speakers at 50 watts from 2 channels and combine the other two channels to create one other channel that can handle twice the wattage.... this is used mostly to power a single sub.... and with this amp it would push out about 100 watts at half the ohms...

At half the ohms means if you are running a 4 ohm sub then make sure it can handle 2 ohms becuase that is what the amp will be running with the 2 bridged channels....

 

[SliderX 10+ year member]

You wouldn't be able to run 2 4ohm subs to that amp bridged because it doesn't support 2ohm bridged, I mean you could do it but it's not recommended. You won't be able to get more than 200x2 unless you get different subs.

You're getting the ohms wrong, the ohm all depends on the wiring of the subs. The amp rating is for what wattage it will give with a sub wired up in that ohm.