What happens to a 2 channel amp when you bridge it?  Does it change the impedance?

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Yes and no.  It doesn’t change the impedance of the speakers you are using, but it does change the impedance that the amplifier itself sees.  That’s why you will find multiple ratings on 2 channel amps as there are a few different ways you can run them and one of the more popular ways to run them is bridged.  Bridged is a way of mixing both left and right stereo signals from a 2 channel amp.  It allows you to run a mono signal.  This is great if you plan on using your two channel amp for a sub or subs.  How do you make sense of the ratings?  Here’s a run down of the typical types of ratings you will see:

X amount RMS power x 2 @ 4 ohms defines using a 2 channel amp to run a pair of full range car audio speakers

Higher X amount of RMS power x 2 @ 2 ohms is the other rating you will see.  This is like running two subs that have an impedance of 2 ohms each with one sub on the left channel and one on the right channel.

The last rating you will often see is the combined total of the last rating or Higher X amount of RMS power (combined) x 1 @ 4 ohms.

I’ve been inspired to write this post after receiving an unsettling email from a fellow reader.  He has a high powered 2 channel amp that puts out either:

150 x 2 @ 4 ohms
300 x 2 @ 2 ohms
600 x 1 @ 4 ohms

His understanding, as well as my understanding, is this:  The last rating is obviously a bridged rating.  To bridge a 2 channel amp, you typically  use the left positive terminal on the amp and right negative terminal on the amp.  See picture below:

To bridge a 2 channel amp you usually need to use the left positive and right negative on the amp terminals.

Some manufacturers do it a little differently so pay attention to the installation manual.  Note this Audison amp doesn’t use the outside terminals, but it is still using left positive and right negative for bridging:

Audison 2 channel amp show bridged.

The trick with bridging a 2 channel amp is to remember that the amp will always split and share that impedance between the two channels.  In other words, if you bridge a 2 channel amp to a 4 ohm load, the amp sees 1/2 that load or a 2 ohm load per channel.  You never want to bridge an amp with an impedance less than 4 ohms.  Given this information, it’s safe to assume that the reader’s amp will put out 600 watts RMS when bridged to a 4 ohm load.  These two ratings are essentially telling us the same thing:

300 x 2 @ 2 ohms

600 x 1 @ 4 ohms

In this particular instance, the reader is looking at purchasing two subwoofers.  The subs are offered in either single voice coil 4 ohm or single voice coil 2 ohm.  Well, we know that when we parallel two 4 ohm subs we get a 2 ohm load and if we series two 4 ohm subs we get an 8 ohm load.  So we can conclude the single voice coil 4 ohm subs would not be the right impedance to buy.

We know that if we parallel two 2 ohm subs we would get a 1 ohm load and if we series two 2 ohm subs we would get a 4 ohm load.  1 ohm isn’t going to work, but 4 ohm will.  So in this situation, the correct answer would be to buy two of the single voice coil 2 ohm subs, wire them in series, then bridge the amp.  The amp would put out 600 watts RMS when bridged to the 4 ohm load.

After contacting the manufacturer of the subs, the reader received the wrong information.  The manufacturer’s tech support disagreed and adamantly argued that he should buy the two single voice coil 4 ohm subs and series those to an 8 ohm load.  He feverishly argued that whenever you bridge an amp it sees half the impedance.

This is true.  The amp would technically see 4 ohms.  But at 4 ohms, the amp only puts out 300 watts RMS, do you agree?

Here are those ratings again:

  • 150 x 2 @ 4 ohms    – What we would get if we were to run two 4 ohm subs to each channel of the amp
  • 300 x 2 @ 2 ohms –  What we would get if we were to run two 2 ohm subs to each channel of the amp
  • 600 x 1 @ 4 ohms –  What we would get if we were to bridge the amp and give it a 4 ohm load

I consulted with another technician at the manufacturer as I was distraught that such a reputable, long time standing company was giving out such misguided direction.  I began to think I was losing my mind when the second technician sided with both of us saying we were both right.  How could we both be right?  The amp cannot produce the same 600 watts RMS when bridged to either a 4 ohm or an 8 ohm load.  If we understand anything about resistance and amplifiers we know that output varies with resistance.

Unless, of course, your company primarily produces amplifiers with regulated power supply’s.  An amplifier with a regulated power supply will put out the same amount of power regardless of whether you give it 2 ohms, 3 ohms or 4 ohms.  But this, my friends, is not a regulated power supply amp.  It’s not a class D mono amp.  It’s an old school, class A, high powered, two channel amp and that beast needs a 4 ohm load when bridged to supply you with 600 watts of raw power, damn it.

Even though I’ve been doing this for more than 10 years and have sold more than my fair share of bridged amp setups, I actually started to doubt my understanding when I encountered two manufacturer technicians that believed otherwise.  I consulted with my hubby on this one (who has more than 30 years experience in this industry).  So here is our conclusion to help YOU, the almighty consumer, navigate the numerous power ratings on amplifiers.

  • When you bridge a 2 channel amp (or the rear channels of a 4 channel amp) the amp sees 1/2 that impedance
  • Amplifier company’s will provide several different power ratings to help you understand the various capabilities of the amp
  • X amount of power RMS x 2 @ 4 ohms means this is what the amp will put out if you hook up two 4 ohm speakers or subs, one speaker or sub to the left channel and one speaker or sub to the right channel.  The amp will play a stereo signal in this scenario.
  • X amount of power RMS x 2 @ 2 ohms means this is what the amp will put out if you hook up two 2 ohm speakers or subs, one speaker or sub to the left channel and one speaker or sub to the right channel.  The amp will play a stereo signal in this scenario.
  • X amount of power RMS x 1 @ 4 ohms means this is what the amp will put out if you bridge the amp to a final 4 ohm load.  It doesn’t matter if it’s one speaker or two speakers as long as the final impedance at the speaker wire going into the amp is 4 ohms.  The amp will play a mixed stereo or mono signal in this scenario.

Let’s take a look at this screen shot from JL Audio:

Various different ratings on a 2 channel as described by the manufacturer.

We can conclude from these ratings that this amp will do the following:

  • If you hook up two full range 4 ohm car audio speakers to each channel (left and right), the amp would supply you with 110 watts RMS from each channel.
  • If you were to hook up two 2 ohm subwoofers (or two full range 2 ohm car audio speakers) to each channel (left and right), the amp would supply you with 180 watts RMS from each channel.
  • If you were to hook up one single voice coil 4 ohm subwoofer and bridge the amp, the amp would supply you with 360 watts RMS total to the one subwoofer.

This last rating demonstrates bridging the amp.  The amp is essentially seeing half that final load when bridged.  We are mixing left and right stereo and combining both channels to basically create a mono amp.  You can see by the ratings that the amp is obviously seeing 2 ohms per channel as the total RMS output at 4 ohms bridged equals that of the two channels run separately with a 2 ohm load on each.

I sincerely hope that helps to clear up confusion regarding what happens to an amp when bridged and what kind of impedance you can give an amp that is being bridged.  As a rule of thumb, never bridge an amp with anything less than a 4 ohm load.  It will be unstable and heat up and shut down on you.

This experience has taught me there’s even more misinformation out there than I otherwise thought. Do you guys agree with me?  Disagree with me?  I’d love to know.  I really hope this helps everyone out there who plans on bridging their 2 channel or 4 channel amp.

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  1. I don’t know alot about what amps an subs go together. Also I’m lost on most of the terminology. I bought a set of pioneer 10 subs in a sealed box that came with a amp. I had it installed by a good place but the amp is old an keeps shutting down so I bought two new amps one for this setup an one for when I change out the subs down the road. I just what to change out amps. The subs are two 10″ pioneer ts-w 254dvc dual ohm voice coil 500 max. An the amp is a pioneer 900 watt bridgeable 2 channel GM-A5602. I just want to change the amp out to this one an get the most out of the system without hurting anything. The amp that’s hooked up is a four channel. Can someone please help me change them out I’m lost I can send pics or whatever u need to walk me thru this. I think I can just put the wires in the same spot as the other but the speaker hook up is not the same on the amps. I would pay to have it done but got laid off today a few days before my B-day so need to save money an want to jam out. Thanks sooo much for any help

    • Hi Chris,
      Well here’s what you can do. First off, whoever sold you a 4 channel amp to power your subs should be fired. Hang on to the 4 channel maybe for your interior speakers or sell it on eBay for some extra cash. I looked up the specs on your subs. FYI they’re 250 watts RMS. That’s always the number that matters, RMS.

      Here’s how I would do it. You may need to take the subs out of the box to insure they’re wired correctly. They’re dual 4 ohm voice coil which means that each sub has TWO positive and negatives and each one is rated at 4 ohms. I would wire each subwoofer in series as shown here.

      When you series, you connect the positive from one voice coil to the negative of the other voice coil. Then you connect the remaining opening positive and negative to your subwoofer box terminal cup. You end up with an 8 ohm load on each subwoofer.

      Then outside the box you can tie both the positives together and both the negatives together. That leaves you with a final 4 ohm load for the entire woofer box.

      Then you can bridge your amp. Connect your subwoofer positive wires to left positive on the amp. Connect your subwoofer negative wires to right negative on the amp. Your amp will see “4 ohms bridged” and put out it’s rated 450 watts RMS. Hope that helps.

      • Hi i have a pioneer TS-W3003D4 sub dvc 2000max 600rms. I know my amp is to small but what’s the best way to wire My one sub to a pioneer 900w 2ch. amp Gm-a5602 to safely get the most out of it. Thanks for any help an if u could send me a diagram that would be great

  2. Yes I get it .on a newer amp 2x2ohm is = to 1x4ohm on a two channel amp.

  3. So I have a 4 channel amp, that is stable down to 1 ohm on each channel.
    Therefore it has a bridged at 4ohm rating, and a bridged at 2ohm rating.
    Does this mean I can connect a DVC 4ohm sub wired to give 2 ohms, bridged across 2 channels?
    Just unsure does the bridged Xrms rating @ 2ohm or Xrms @ 4ohm refer to the load connected, or what the amp actually ‘sees’?

    • Hi Dave,
      I have never seen, in my 12 years experience, a 4 channel amp that claims it is 1 ohm stable per channel let alone have I seen a 4 channel actually work with a 1 ohm load for more than 10 minutes before shutting down (bridged or not). But there is always a first for everything, right? Just curious, can you tell me what make and model the amp is?

      To answer your question though, the rating of “X amount of power bridged @ 4 ohms” means the load of the speaker(s) connected. So if you have a dual 4 ohm sub wired in parallel down to a 2 ohm load you should be looking at what the amp says it’s capable of doing at “X amount of power bridged @ 2 ohms”. You’d essentially be sharing the impedance of 2 ohms between 2 channels OR think of it as a 1 ohm load per channel.

      Whenever you see the rating x 4 channels, then you know what the amp is capable per channel. So a lot of new amps these days give ratings for like 50 x 4 RMS @ 4 ohms OR 75 x 4 RMS @ 2 ohms. This means you could run two 4 ohm speakers off the front channel (say your front speakers) and bridge the rear two channels to a 4 ohm sub. This would mean in that configuration the amp is pushing out 50 watts a channel to the front speakers and a combined 150 watts to the sub that is being bridged (because the 4 ohm load is being shared with each rear channel or 2 ohms per channel).

      I hope that helps clear things up, if not, please feel free to ask away!

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