Difference between bridged and parallel amp?

Take a close look to see how the speaker is hooked up in the bridge configuration. Tam

Use bridging when your amplifier is voltage limited, i.e. when it won't put out enough voltage to drive your speaker to full power.

Don't use parallel amplifiers unless the feedback loop includes both amplifiers, which it doesn't unless the amplifiers were specifically designed for parallel operation.

Norm Strong

--- What no one has commented on so far is that for a given load impedance, using paralleled outputs will do nothing as far as increasing the power into the load goes, while using bridged outputs will result in _quadrupling_ the power into the load! That is, of course, assuming the power supply can deliver the increased (doubled) current.

-- John Fields Professional Circuit Designer

No. Bridging doubles the output voltage, while parallel doubles the output current. You need to choose your speaker impedance to suit either condition.

You got your answer but I'll add that I wouldn't recommend that parallel configuration. It involves a pair of current sharing resistors that increase the output impedance.

Graham

Looking at the datasheet for a National LM4780 audio amplifier IC I was wondering: what's the difference between a parallel and a bridged amp? They sort of seem to be doing the same thing.

I read in sci.electronics.design that Pooh Bear wrote (in ) about 'Difference between bridged and parallel amp?', on Fri, 2 Sep 2005:

One UK company sold/sells sound system amplifiers with two 60 W MOSFET amp modules in parallel. It works, but it worried me a lot when I first saw it. There are no current-sharing resistors and no cross-connections of any sort. Not happy at full output at 20 kHz, though; smoke happens.

As stated, output power is quadrupled because of the doubled voltage swing and thus, doubled current in the output. However, one must pay attention to the maximum current in the output devices. For example, an audio amplifier IC may be rated to drive a 4 Ohm minimum load at rated supply voltage. When bridged, it can no longer handle the 4 Ohm load. 8 Ohm would be the new minimum. So, considering you are trying to get as much power from the amp by using the lowest output impedance possible in any case, the maximum you can gain by bridging is double the power.

John

"No One Really"

.......... Phil

They

The main difference is the input connections. In the parallel amplifier echematic, the inputs of the two channels ae the same, but in the bridged amplifier schematic, the input of channel B is inverted wrt to the input of channel A.

And so the outputs of the bridged amplifier are also inverted wrt to each other; and by connecting the speaker across the outputs for both channels, it will receive twice the voltage (one terminal at +V, one at -V, rather than one terminal at +V and the other at 0V).

Tim

Isn't this the point where someone usually jumps in and says that MOSFET's don't need current sharing resistors because resistance goes up with temperature? This person might even cite an excellent and well known electronics text.

And then someone else, possibly even one of the authors of that text jumps in and says "No, when used in the linear region, virtually all power MOSFET's DO need current sharing resistors?"

Just wondering.

--Mac

To answer your question, I've found that the audio specific lateral mosfets made by Hitachi and also similar ones from Semelab and Exicon share current very well without any ballast Rs..

Other mosfets types have very variable treshold voltages and can't be used reliably in this way.

But that's only in the application where those devices are in a single amplifier.

Paralleling entire amplifiers is wholly unwise with current sharing Rs. Mainly since they won't have exactly the same voltage gain, given normal component tolerances. The feedback loop will win over everything.

Graham

Almost always done inside the same feedback loop.

Almost always done with current-balancing resistors.

One reason why amps in parallel look more like short circuits to each other are the individual feedback loops.

These are not paralleled devices but complete 60 W amplifier cards with the outputs paralleled. The output source impedance of each is quite low, due to negative feedback, but not audiophool low. Nevertheless, each output must look like a near short-circuit to the other amplifier.

John was pointing out correctly the difference between paralleling devices inside and outside a closed feedback loop.

Graham

Then what about amps using multiple output devices in parallel ? They all must look like near short-circuits to each other, don't them ?

Not exactly a KW for $1.98 but still impressive:

The issue of price was raised: