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New Member
As the topic says, I'm confused with bridging and ohm ratings.

I read in a previous post that someone could 'bridge' their amp into mono mode at 8 ohms to power 1 sub, or bridge it with 4 subs and output in 5 ohms.

IM SO LOST...can someone please give me a quick rundown of how these 2 things work? BRIDGING & OHMS


Ken Heath

Super Moderator...da-ta-daaa!!!
Staff member

Here's a easy-to-understand impedance chart that has helped me immensly over the years...

As for bridging your amps, you'd have to refer to the manufacturers instructions for your particular piece of equipment. Basically, bridging is when you combine both channels of a stereo amplifier to be one channel of monophonic power...which means that to run stereo with bridged amps you'd need two amps. Not all amps are capable of this feature...once again I'd check the manufacturer's instructions for exact details.


Active Member

Have a look at this link and read the part "bridging principles". It's a bit technical for the non-technical mind, but it may help. Basically, when you bridge an amp, you share the speaker load between both channels of your amp, so a 100w/ch@8ohms amp may put out 400 bridged mono into 8ohms. The catch is that because you are sharing the load, each channel "sees" only half this load, so running an amp bridged at 4ohms is the same as stereo at 2 ohms. If your amp cannot run a 2ohm load, you will not be able to bridge at 4ohms. And, the current the amp has to pass increases as the load impedance(that's your ohms) decreases, this is why an amp will get hot driving low impedance loads, and when bridged. Hope this helps.


Gear and Equipment Moderator
Speakers have a nominal impedance rating - some ar 4 ohms some are 8 ohms. Nominal means that it changes with frequency...i.e. with a signal of 300 hz, it may be 6.3 ohms and at 1500 hz it may be 10.2 ohms. Speakers wired in series, the impedance values are summed. When wired in parallel, they are halved. Most cabinets are wired in parallel as the norm so two 8's would be a total 4 ohm load on the amp.

Now bridging the amp means this .... basically you'll use the left channel of the amp to amplify the positive part of the audio sine waves and the right channel to amplify the negative part. To do this you'll connect to both positive output terminals(the two red binding posts) and also in most cases there are some switches in the amp that have to be enabled to put the amp into this 'bridged mono' mode.

That's as basic as I can get without going into a lot of electronic hoop-la that would probably go over your head.



New Member
Ok, let me see if I can understand this with a real world example you guys can relate it with...correct me if im wrong.

2 Cerwin Vega AB36 Subs, 8 ohms each
QSC RMX 4050 Amp:
-800 watts per channel in stereo mode @ 8 OHMS
-2600 watts bridged mono @ 8 OHMS
-4000 watts bridged mono @ 4 OHMS


1. Put this amp into bridged mode, and I run a split signal from ONE CHANNEL of the amp to 2 subs?

2. Put this amp into bridged mode, and I run a 1 sub per channel?

3. How much power would go to the subs in each of the above situations?

Please ignore the fact that the subs may blow with certain power ratings, this is only an example so I can relate to the pieces of info to help me understand.

Thanks for the help.


Active Member
Just to clarify, the 4050hd will give you..
800w/ch 8ohm strereo
1300w/ch 4ohm stereo
2600w 8ohm bridged
4000w max 4ohm bridged

So you have lots of options...
1 sub/side stereo, 800w/cab, good but not much headroom.
2 subs/side stereo, 650w/cab, definitely will be louder, but even less headroom. 4 cabs should give nice lowend extension, however.
1 sub bridged mono, 2600w/cab, loads of headroom, overkill and driver will have a short life. 1 cab won't go very low all by itself. Waste of an amp for 1 sub.
2 subs bridged mono, 2000w/cab, still loads of headroom, but you only have 2 cabs so you won't get as much low end extension. You need 2 amps to power 4 cabs, and you need 4 cabs to go decent low. $$$

The RMX 5050 may be a better option. it gives you..
1050w/ch at 8ohms.. better headroom.
1800w/ch 4ohm stereo, 900w/cab, but you can drive 4 cabs. Best option if you do want 4 subs eventually.
Bridged mono.. don't bother way overkill.

Another option(the right one IMO) is a pair of 1850HD
1200w bridged at 8 ohms, that's almost 2 times program power for the subs, a good match IMO.
1800w bridged at 4 ohms, back to 900w/cab, not really enough headroom if you only get 1 amp.

As you can see, I'm leaning towards using a little less power here, versus more. I think if you give a guy a big motor he's gonna floor it sooner or later, just to see what it can do. It's good to have loads of headroom, but you can't use it all without blowing drivers usually. So any more than 1200w per cab and I think you're asking for trouble.

P.S. I was looking thru the spec sheets, and the PLX3002 looks like a good match as well. A single copy of one of these will give you 3000w bridged at 4 ohms, or 1500w/cab. This has the added benefit of being way lighter than any of the other options.
P.P.S. If you go this route, have a look at the Yamaha P7000s as well. They're are half the price of the PLX line here in Ottawa, and all report say they are solid performers. I love mine and will be getting another P5000 or P7000 shortly. You should be able to demo the PLX and the Yamaha at a local rental shop... it's always good to try before you buy.


New Member
When you guys say 2 subs with a bridged mono amp, does that mean 2 subs on CHANNEL A leaving channel B alone?


does it mean 1 sub on channel A and 1 sub on channel B, same as stereo mode but just in bridged mono?

I think its scenario A.

Eventually I will be moving up to 2 more AB36s so I might as well get the 5050. For some reason my rental guy said "dont worry about bridging, just go with the 5050"



Active Member
Ahh.. it's neither of those. I guess you are still not fully understanding what bridging an amp means. Let me try to get to the skinny...

See, when you bridge you amp you no longer have 2 channels, you only have 1, and that is across both hot(red) speaker terminals. If you hook it up any other way you will get very sad results.

So to connect 2 subs to an amp in bridged mono, stack the banana connectors on the red terminals for both speakers, get it? Both speakers are connected to exactly the same terminals on the amp.

I think 2 more subs would definitely help in the long run. Cluster all of them together in a group for best results. And be sure to use low cut filters with the bigger amp, whatever that may be. The drivers are unloaded below the horn cuttoff frequency, so over excursion and damage is really easy when you have big power.


Active Member
When you bridge an amp, an iteresting thing happens. The channel A amp produces the positive part of the single wave while the channel B holds ground, then the channel B amp takes over to produce the negative part of the wave while the channel A holds ground.

think of it like the channel amp moves the speaker out and the channel B amp moves the speaker in.


New Member
Bridging made easy, hopefully

Bridging is more like if you hook up two 12 volt automobile batteries, the positive of one is directly connected permanently to the negative of the other leaving one terminal of each battery unconnected. Like this:
Battery 1 + - Battery 2 + -

Now place these next to each other and connect one terminal in common:
+ -+ -

The - (neg) of one is connected to the + (pos) of another. We will call that connection common (or ground).

If you measure with a voltmeter from the common to either end you will read 12 volts in our example. At this point please ignore polarity of the voltmeter.

If you now move your volt meter leads to the plus of one battery and the neg of the other battery you will read 24 volts across both batteries. What you have done is bridged over the common (or ground) terminal. Bridging is another way of saying ignore the common connection.

That is bridging in its simplist.

You must know that in bridging mode, the amplifiers are only connected to one input of your amp and the channel one (left) amp is amplifying the signal in phase (with that input) while channel two (right) is being fed the same signal but in the opposite polarity. Like our battery example you ignore or "bridge" over the common and get a amplified signal that is twice the power of a single amplifier.

Understanding the maximum number of speakers that can be put across the bridged amp is another level deeper in theory. Until you completely grasp the above explaination feel free to tell us what your goals are, what amp and speakers you have and I am sure you will get good guidance here.

Good Luck


New Member
Paul O,

I read his post as he only had two subs. Did I miss him saying two more subs were being purchased?

If the 4050 amp is dedicated for sub use I would advise one of two ways.

1- If he has a limiter, bridge the amp and hook both across that amp.

2- If no limiter, set the amp to parallel mode, hook one speaker per channel amd watch the meters on the front of the 4050 amp and keep it out of clipping.

FYI: I run 4 JBL MP418S subs (4 ohms, 2400 watts peak music level) each powered by a QSC 2400 watt amp all protected by a DBX DriveRack 260. 9600 watts of pure thump. The mid/hi freq are JBL MP415s powered by QSC 3400 watt amp.
Didn't read the entire post because I'm sort of on my way out the door to work, but thought I would throw one more web address out there: . While it's targeted for car audio, it does a really good job of explaining a lot of the technical concepts such as impedance, bridging, clipping, etc.
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