What it means to bridge a car stereo amp

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

Estimated read time: 6 minutes 42 seconds.  Want to read later?

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.

2 Ohm, 4 Ohm, 1 Ohm, what’s the difference?

Parallel - 2 ohm load

Ohm. What is an ohm? How does a 2 ohm load effect an amp? How does a 1 ohm load effect an amp? Or a 4 ohm load?

The ohm load effects power output in amplifiers as well as sound quality.

Subwoofers can measure 2 ohm, 4 ohm or 1 ohm. 2 ohm, 4 ohm and 1 ohm impedance’s will make the same amplifier put out different amounts of power and can greatly affect the way your car stereo system sounds and performs.

This article discusses the various subwoofer configurations available such as single voice coil 2 ohm, single voice coil 4 ohm, dual voice coil 2 ohm, dual voice coil 4 ohm, as well as series and parallel wiring and how each will effect the measurement of ohms.

Estimated read time: 3 minutes 45 seconds.  Want to read later?

All speakers have a measurement of resistance, called impedance which is measured in ohms. Car stereo speakers are typically 4 ohm. You will find car stereo subwoofers in a variety of configurations like single voice coil 2 ohm, dual voice coil 2 ohm, single voice coil 4 ohm, and dual voice coil 4 ohm.

Dual voice coil means the subwoofer has two positives and two negatives, which give you the flexibility of wiring the subwoofer two different ways giving you two options for final impedance. These variations do not make one speaker more powerful than another despite what many consumers may think. However, the varying impedance will make a difference in what type of output an amplifier will have.

As the ohms drop, the amplifier will unleash more power, but with less control. This is an important aspect to remember. Failing to understand this concept is probably the main cause of self installed amplifiers shutting down and going into protection mode.

You will notice, when you buy an amplifier, or are looking at the specifications of an amplifier, you will typically find 2 or more ratings. One output at 4 ohms, and another output at 2 ohms. The output at 2 ohms is always higher because when the resistance is lower, you allow more output. All mono amplifiers are stable at 2 ohms, very few are stable at 1 ohm.

So what kind of speakers can we hook up to a mono amplifier? Let’s use an Alpine MRP-M500 mono amplifier as an example. This amp is rated at 300 watts at 4 ohms and 500 watts at 2 ohms. In order for us to get maximum output out this amplifier, using only one subwoofer, we would have to use either a single voice coil 2 ohm subwoofer or a dual voice coil 4 ohm subwoofer wired in parallel.

If we had two subwoofers, we would need each subwoofer to be a single voice coil 4 ohm wired in parallel to the amplifier. Or we could have two dual voice coil 2 ohm subwoofers with each woofer’s voice coil wired in series, then paralled to the amplifier.

Let’s take a look at this JL Audio 10W6 which is a Dual Voice Coil 4 ohm subwoofer as an example for the different wiring schematics on Series and Parallel:

Ohm

Parallel is when you tie two positive and negative terminals together wiring them positive to positive, negative to negative, and then wire that straight into the positive and negative terminal of the amplifier. When you wire speakers in parallel, you are cutting the impedance in half. So a dual voice coil 4 ohm subwoofer wired in parallel is now measuring a 2 ohm load:

To understand Series, visualize a dual 4 ohm voice coil. You have two terminals side by side: Positive and Negative for voice coil #1 and then Positive and Negative for voice coil #2. To series this subwoofer, you would have to connect positive from voice coil #1 to the amplifier, then connect negative from voice coil #1 to the positive of voice coil #2, and then connect negative from voice coil #2 to the amplifier. This doubles the impedance. If we do this to a dual voice coil 4 ohm subwoofer, the impedance will be 8 ohms:

The problem that many people run into, is running an amplifier with the incorrect impedance. Let’s say we were to use that same dual voice coil 4 ohm subwoofer with a 2 channel amp bridged. When you bridge a 2 channel amp, you hook up left positive and right negative. You might assume you are simply wiring it as a mono amp, which in a sense you are. The important difference that is often overlooked is when you bridge an amplifier, the amp sees half the impedance at the terminal which means that 2 channel amp would see only 1 ohm. This is extremely low for a 2 channel amplifier, it would be unleashing a lot of more power than it was designed to along with very little control which would in turn cause the amplifier to heat up very quickly and shut down going into protection mode. Over time this will damage the amplifier.

For a 2 channel amp to work properly bridged, it needs to see an 8 or 4 ohm load at the terminal, this will ensure the amplifier will work properly and not overheat and shut down.

If you own an amp and you’re in the market for subwoofers, make sure you know what final impedance your amp needs to see. And vice versa, if you own subs and you’re in the market for an amp, know what impedance your subs are so that you purchase an amplifier that will give you optimum performance. It helps to talk to your local independent car stereo shop, but if there aren’t any in your area, just email me and I’ll make a recommendation for you. If you enjoyed reading this post, please spread the love. Tweet it, like it, stumble it, or use any other social sharing weapon of choice. Thanks for reading!