Difference between bridged and parallel amp?

[No One Really]

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.

 

[Phil Allison]

"No One Really"

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.

** Only if you think that connecting two identical batteries in series or
parallel is the same thing.




........... Phil

 

[Don Pearce]

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.

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

d

 

[Arny Krueger]

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

The pedantic version would be ;-)

Bridging doubles the maximum available output voltage, which
may lead to a doubling of maximum output current. The latter
eventuality needs to be carefully considered because it is a
common pitfall of bridged operation.

Parallel operation doubles the output current that is
available for delivery to the load.

You need to choose your speaker impedance to suit either
condition.

Or, you need to choose your mode of operation to suit your
speaker load and needs for dynamic range.

 

[Pooh Bear]

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.

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

 

[Tam/WB2TT]

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.

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

 

[John Woodgate]

I read in sci.electronics.design that Pooh Bear

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.

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.

 

[John Fields]

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.

---
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.

 

[JohnR66]

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.

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

 

[Guest]

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.

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

 

[Tim Martin]

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.

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

 

[Mac]

I read in sci.electronics.design that Pooh Bear

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.

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

 

[Pooh Bear]

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.

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

 

[John Woodgate]

<[email protected]>) about 'Difference between
bridged and parallel amp?', on Sun, 4 Sep 2005:

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.

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.

 

[François Yves Le Gal]

Nevertheless,
each output must look like a near short-circuit to the other amplifier.

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

 

[Pooh Bear]

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

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

Graham

 

[Arny Krueger]

Then what about amps using multiple output devices in
parallel ?

Almost always done inside the same feedback loop.

Almost always done with current-balancing resistors.

They all must look like near short-circuits to each other,
don't them ?

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

 

[François Yves Le Gal]

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

I know. But you can parallel two amp modules, use proper current balancing
resistors and derive proper feedback (if needed).

 

[François Yves Le Gal]

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

Correct. But it's quite easy to properly parallel two amps.

 

[Phil Allison]

"François Yves Le Gal"
Some pommy dickhead

I know. But you can parallel two amp modules, use proper current balancing
resistors and derive proper feedback (if needed).

** There is at least one proper, engineering type way to do it.

The two or more power stages are made to have unity gain - ie 100% NFB.

Then, a single voltage amp drivis them all.






........... Phil