How to bridge a 2Ch amp to two subs running at 2ohms?

 

Bronze Member
Username: Themax96se

USA

Post Number: 21
Registered: Jan-05
I guess most 2 & 4 channel amps cannot run a 2ohm load BRIDGED?

I realized that yesterday when trying to hook up 3 different types of amps, to 3 different sub combos. I blew a few fuses, and sometime it worked but it would sound real weak.

Why cant multichannel amps run 2ohm loads while bridged? How should I run 2 subs, 4ohms each to a 2 channel amp>?
 

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 7233
Registered: Dec-03
simple answer is:
when the amp is bridged, it sees half of the actual load, so it's like running at 1 ohm stereo.


if you understand electrical principles, here's a more detailed explanation:

Bridging an amplifier refers to configuring a two channel (stereo) amplifier to drive a single load with more power than the sum of the two original channels combined. For an example, a 100 watt per channel at 4 ohms amp may put out 400 watts(one channel at 4 ohms) after bridging.

There are important things to know about running an amplifier in the bridged mode:
An amplifier running in bridged mode has one output channel to which a load (speaker) can be connected. It is no longer a two channel (stereo) amp as far as input signals and loads are concerned.
If the amp you want to run in bridged mode does not have built in facilities for doing so, you should not attempt to use it in this manner (unless you are thoroughly sure of what you are doing).
If you run bridged amplifiers, you must pay close attention to speaker phasing (see next item). Otherwise, you may have "hollow" or "weak" sound.
You must pay close attention to speaker wiring. The manufacturer will state which terminal is really the "positive" connection when bridged.
The speaker output signals of a bridged amplifier are floating; such connections must never be connected to any grounded device (such as an external accessory power meter, for example). If you do make such an illegal connection, one amplifier channel is basically short circuited (worst case result is a blown amplifier!).
Amplifiers running in bridged mode are generally limited to speakers with impedance ratings of no less than 4 ohms (in other words don't use a 2 ohm speaker load unless the manufacturer specifically allows it).

Bridged amplifiers work basically as follows:
A single input signal is applied to the amplifier. Internal to the amp, the input signal is split into two signals. One is identical to the original, and the second is also identical except it is inverted (sometimes called phase-flipped). The original signal is sent to one channel of the amp, and the inverted signal is applied to the second channel. Amplification of these two signals occurs just like for any other signal. The output results in two channels which are identical except one channel is the inverse of the other. The speaker is connected between the two amplifier speaker output terminals. In other words, one channel "pulls" one way while the second channel "pulls" in the opposite direction. This allows considerably more power to be delivered to a single load.

If we had our perfect amplifier, upon bridging it we would have a single channel amplifier with exactly four times as much power as any one channel of the amplifier in "normal" stereo mode, assuming a 4 ohm speaker load. This is because the effective output voltage available to drive the speaker has doubled as a result of bridging. A doubling of voltage on a given load results in a fourfold increase of power delivered to that load. If we used a 4 ohm load on the perfect bridged amplifier, the output power would be a very substantial eight times the normal stereo single channel 4 ohm output! These numbers should give some clues as to why real world amplifiers cannot meet such expectations. Once again, we are back to limitations of the power supply. In reality, most amplifiers in bridged mode will put out about 3 times the power as any one channel of the amp in normal stereo mode. The fourfold increase cannot be achieved because the power supply is unable to provide the current required for such performance. With 2 ohm loads, the situation is compounded. The amount of current required to drive a 2 ohm load when in bridged mode will tax the amplifier's power supply to its absolute limits. Not to mention, the output stage may not be able to safely handle the extra heat that will be dissipated.
Bottom line: stay away from 2 ohm loads if you are running an amplifier in bridged mode!
 

Bronze Member
Username: Themax96se

USA

Post Number: 24
Registered: Jan-05
Interesting stuff. So if I have two 4ohm speakers in an enclosure, and a 2 channel amp, how should I wire?
 

Bronze Member
Username: Themax96se

USA

Post Number: 31
Registered: Jan-05
Ok, I am thinking you cannot use a 2ch amp for dual 4ohm subs... at least it wont be efficient
 

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 7256
Registered: Dec-03
right.
best you'll get is one sub on each channel at 4 ohms.
for two 4 ohm SVC subs, you'd want a mono 2 ohm stable amp.
 

Bronze Member
Username: Themax96se

USA

Post Number: 34
Registered: Jan-05
Thanks, thats what I figured. I have been reading up on your site alot as well. Learning alot. Thanks, Louis
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