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Old 30th June 2005, 04:53 PM   #1
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Default Difference between Bridge & Parallel mode

I want to know simply (because I'm not very good in English) what is the difference between the "Bridge" and "Parallel" mode.

(Especially for the LM4780 but maybe for other references too)

I read datasheet but I clearly did not understand.
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Old 30th June 2005, 06:05 PM   #2
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Simply


Bridge mode doubles the output voltage,usually 8ohm.

Pallalel doubles the output current,will driver lower ohm loudspeakers(like 4ohm)
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Old 30th June 2005, 06:13 PM   #3
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Default Re: Difference between Bridge & Parallel mode

Quote:
Originally posted by john_lenfr
I want to know simply (because I'm not very good in English) what is the difference between the "Bridge" and "Parallel" mode.

(Especially for the LM4780 but maybe for other references too)

I read datasheet but I clearly did not understand.

Bridge mode is designed to deliver more power into a typical 8 ohm loudspeaker. In a bridge mode configuration, two chips will deliver nearly twice the output power as a single chip because the output voltage will swing twice as far (as the output of one chip goes in the positive direction, the other chip goes in the negative direction).

Parallel mode will deliver more power into a typical 4 ohm loudspeaker. In Parallel mode, the 4 ohm load is shared between the two chips. Each chip delivers half of the total output current. The output voltage will not increase but the output current will nearly double.
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Old 30th June 2005, 07:47 PM   #4
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And if I have a 6 ohm speaker?

Should I chose Bridge mode or Parallel mode?

I search for the most powerful application to drive 4 to 8 ohm speakers.

So I want to know if it is dangerous to have 6 ohm speaker on a LM4780 in bridge mode?
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Old 30th June 2005, 08:05 PM   #5
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Quote:
In a bridge mode configuration, two chips will deliver nearly twice the output power as a single chip because the output voltage will swing twice as far (as the output of one chip goes in the positive direction, the other chip goes in the negative direction).
You are right that the output voltage is effectively doubled, but this does not result in twice the output power. It results in 4 times the output power. P = V^2 / R. If V increases by a factor of 2, P increases by 2^2, or 4. This is obvious if you consider that when the voltage doubles (for the same load), the current doubles too.

Of course you have to realise that you can't break the laws of physics and get say 240 W out of two chips each capable of 60 W each. If you try to do so, you will learn some harsh lessons. By bridging, you double the output current, which means that you effectively halve the impedance of the load (a 8 ohm speaker driven by a bridged amp draws as much current as a 4 ohm speaker driven normally). That limits the power supply voltage to what is a safe level for a 4 ohm load. With a bridged amp and a 8 ohm speaker, you can get (roughly) twice as much power as a single amp can deliver into a 4 ohm speaker.

If you take it a step further and go brigded-parallel, then you really can get 4 times the power as a single chip can deliver for a given speaker.
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Old 1st July 2005, 06:59 AM   #6
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Quote:
And if I have a 6 ohm speaker?
Ha, that's he same problem i had. I decided to test the parallel mode ampmodule made by Peter Daniel,
maybe i try the bridged mode module later on (from jackinj).

Normally the bridged module produces less distortion than the parallel one. But this is only in upper end where the amp runs to its limit.

In my opinion it's better to take the parallel version because you often have speakes with lower impedance, even if the manufacturer says they don't have lower impedance. Aso it could be that you'll change your speaker sometime...

mic
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Old 1st July 2005, 07:08 AM   #7
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Quote:
Of course you have to realise that you can't break the laws of physics and get say 240 W out of two chips each capable of 60 W each.
I think you misunderstood something! The LM4780 can deliver 120W, look into NS specs!

The total thermal loss in bridgemode is higher, the chip prduces more heat in that mode. This is always a problem in a A-class amplifier (ok, AB-class).

The problem is to handle so much heat energy with matching heatsinks. You're in need of a fan at such a design (power controlled so it only runs in highpower consumption).

mic
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Old 1st July 2005, 09:28 AM   #8
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Quote:
Originally posted by slackman


I think you misunderstood something! The LM4780 can deliver 120W, look into NS specs!

The total thermal loss in bridgemode is higher, the chip prduces more heat in that mode. This is always a problem in a A-class amplifier (ok, AB-class).

The problem is to handle so much heat energy with matching heatsinks. You're in need of a fan at such a design (power controlled so it only runs in highpower consumption).

mic
The LM4780 can in fact deliver 120W, but that is because it has 2 amps on a chip each capable of 60W. It is up to you whether you want to make it a 2 channel amp, single channel bridged, or single channel parallel.

OR

for a single channel use 2x LM4780 and use a parallel-bridge configuration for 4x the power output. Don't forget to buy a really huge transformer if you are actually planning on using those high powers. In all reality this is only really needed for driving subwoofers or some seriously large and inefficient speakers.
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Old 1st July 2005, 10:20 AM   #9
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Quote:
Originally posted by slackman


Ha, that's he same problem i had. I decided to test the parallel mode ampmodule made by Peter Daniel,
maybe i try the bridged mode module later on (from jackinj).

Normally the bridged module produces less distortion than the parallel one. But this is only in upper end where the amp runs to its limit.

In my opinion it's better to take the parallel version because you often have speakes with lower impedance, even if the manufacturer says they don't have lower impedance. Aso it could be that you'll change your speaker sometime...

mic
From a practical standpoint, I seem to get slightly less "power" out of the paralleled LM4780 than the bridged. The power dissipation (PDMax) of the LM4780 in the bridged mode is very high so I use a fan. THD of the bridged version is a tad lower than the parallel version.

Comparing Peter's version of the paralleled amp with mine http://www.tech-diy.com/paraclone.htm you should take into consideration that I have alloted space for a small aluminum electrolytic and ceramic bypass capacitors -- as close as physically possible to the chip's pins. I found that the best results were obtained when the resistors were matched to better than 0.1% -- you don't need an expensive 6.5 digit meter to do this -- you can set up a laboratory type wheatstone bridge. I haven't heard his version so can't comment on the merits.
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Old 1st July 2005, 03:56 PM   #10
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Default Putting aside the chips in question.......

Bridging: main goal is to get 4X the power without raising the rail voltages. A way of getting higher power on low supply rails. Can be used in single supply applications, as both outputs will be Vcc/2, and you will not need a coupling cap on the output.

In practical terms, it is a way to reduce modulation on the supply rails, as each chip is driven out of phase. The 2 amps should cancel each other out, in terms of current demands.

The main drawback is that the output Z is double. And you need a phase splitter ot drive it.

Parallel can drive lower Z loads. Another advantage is that each chip (in this case) will have to deliver less current. This should lower any thermal effects on distortion that might occur.

Drawback is finding the right value of series R to balance out current supplied by each one. Not necessarly as easy as you may think.

Like everything else in the world of engineering, there is no right answer which is best. Engineering is always a series of compromises. You have to decide which ones that you can live with.

Jocko
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