Why Bridge?

[jimbo1968]

Can someone give an example using LM3886 datasheet of why you might want to bridge 2 chips instead of parallel ? Assuming you are not constrained in your choice of power supply voltage.

I see lots of questions about how to, but can't work out why. If this thread answers the question, I promise to write it up in the wiki

[lineup]

say your power sypply is +20, 0V and -20 Volt

In a normal, unbridge, amplifier
one terminal of loudspeaker is connected to middle, 0 Volt.
Amplifier can max put out +20 or -20 volt across loudspeaker.
This sets the limit of max power.

Max power with 8 Ohm LSP.
20x20 / (2x8) = 25 Watt



In a bridge amplifier you have two amplifiers working together.
And loudspeaker is connected from Amp1 output to Amp2 output.

If supply is same, +-20 Volt this will happen.
When Amp1 pulls one terminal to +20
the Amp2 pulls other terminal to -20 Volt.
And same when Amp1 pulls -20 the other pulls +20
this gives a total of +-40 Volt peaks across loudspeaker.

Max power with 8 Ohm LSP and +-20 volt supply in bridge mode.
40x40 / (2x8) = 100 Watt.


So theoretically using same voltage in power supply
you can get four (4) times more power using bridge.

But as voltage is twice (2) as high across 8 ohm,
supply has to be able to deliver two times as much current,
compared to normal.

[jimbo1968]

My question was for the case where the supply is not pre-determined.

However, another question I have is whether some amps have the option to switch from parallel to bridge to suit the speakers?

Note: I realise you get amps that switch from stereo to bridged mono which are useful only if you are re-configuring your set-up.

What does the speaker impedance switch on my AV amp do?

[jimbo1968]

Found the answer I think. It's in the first sentence of the Datasheet

Max power for THD+N < .1%
4ohm > 68W
8ohm > 38W

And bridging halves the apparent load impedance.

[jimbo1968]

or maybe not.

looking at the graphs you can up the supply voltage and get better (probably equivalent) output.

eg 8ohm Vs=35V > 61W

[jackinnj]

Bridging results in slightly better THD performance.

[Cro maniac]

I'd say that except the bridge has better distortion specs, it is easier to build. In bridge configuration, you don't have to match resistors to 0,1% to avoid big offset on the output and current flowing from one chip to another.
Also, the absence of current sharing resistors in bridge amplifier results in much lower output impedance (or higher damping factor) compared to parallel amplifier.

[Nordic]

Would someone mind dicussing the loads you can drive with parallel VS bridged mode for those of us still learning.

[jackinnj]

if you go to the National Semi website you can determine which configuration is correct for your application -- the PDMax equation will tell when you are outside the boundaries.

[tiltedhalo]

I think that for people just starting out Nationals spec sheets and the PDmax equations can be a little over wellming.
lets simplify, bridged amp 40Volt DC swing amp can safeley disspipate 1 amp current = 40watts amps times volts = watts. So in a bridge config one amp is inverted one is not, 180 degrees out of phase now youve got 80V swing, 80 times 2 amps, youve got two amplifiers now so double the current to load, 80 times 2=160 watts 4 times power gain in a bridge, but hears the catch each side of the bidge sees half the overall load an 8 ohm speaker appears as a 4ohm load ect... So for a chip amp and a lot of solid state amps this is bad because the amp has to be designed to dissipate the extra power.
Parellel amp just doubles the effective current to load, better for driveing low impeadences.
bridged amp- one amp pushes and one amp pulls
Parelleled amp- both push, 40V times 2 amps = 80watts but have to be in perfect unison or they fight.