Difference between Bridge & Parallel mode
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john_lenfr said:
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.
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.
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
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
slackman said:
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.
slackman said:
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.
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
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
slackman said:
The main difference between those two is that to use my board in bridged configuration you need a phase splitter, or a source with balanced output. Jackinj board is to be used with a single ended input.
As I can configure that board in all 3 modes of operation (using basically same components), my preference with regards to the sound is as follows:
1 bridged
2 stereo
3 parallel
Simply, Yes. It is dangerous.
The 4780 cannot safely drive a 3 ohm load, and that is what the effective load will be for the amp on each half of the bridged amp.
6 ohm is a awkward impedance to use... bridging results in a 3 ohm effective load which is too lown to be safe, and paralleling results in a 12 ohm load which is too high (it's perfectly safe, but you will not get as much power out of the chips as you would with 8 ohms, because of the limits on power supply voltages).
The ideal solution for 6 ohm speaker is a bridge-parallel amp. This is contructed as two parallel amps, bridged together. The "parallel amp" parts of this can have as many chips in parallel as you want; you are not limitted to two. If you use 4 chips total (2 in parallel each side) then the effective load to each chip is 6 ohms. You can calculate the approximate output power by figuring out (with the help of the datasheet and/or the Overture Design Guide spreadsheet from National) the output power of the chip into 6 ohms (effective load per chip) and multiplying that by 4 (total number of chips). If you used a three chips in parallel on each side (6 total), then the effective load per chip is (6*3/2) = 9 ohms. To calculate the approximate power of this, find the output power for each chip into 8 ohms and subtract 11% (due to the load being 9 ohms), and multiply by 6 (total number of chips). For more information on bridge-parallel amps, download National's app note AN-1192 and their Overture Design Guide spreadsheet
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