Controversy regarding Bridged Amplifier power output - diyAudio
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Old 20th January 2011, 08:09 AM   #1
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Default Controversy regarding Bridged Amplifier power output

Reference link: Wikipedia.org-->Bridged and Paralleled Amplifiers

I am stating it from wiki:

"The x 4 myth

It is sometimes stated, usually on internet forums, (and previously here on WikiPedia) that operating an amplifier pair in bridge mode can give four times the power (of one of the pair).

Reference may be made to the fact that power is proportional to the square of the volltage, implying that if the output voltage is doubled as it is in bridge mode then the power available increases by a factor of four.

This would only true if the current available could also double as it would if the output impedance of the amplier stages was zero. But it is not; our 4ohm output impedance amplifier pair now presents as a mono amplifier with an output impedance of 8 ohms and no more current is available from each amplifier than when working singly.

In actual use, there is just one circumstance when x4 power can be achieved. This is when each of the amplifiers are rated at full output for an impedance that is lower than the connected speaker.

eg. 8 ohm speakers are very common and many amplifiers are rated down to 4 ohms. An 8 ohm speaker connected to such an amp would only be capable of using HALF of the available power. When two such amplifiers are bridged, the new output impedance is then 8 ohms and full power can be output to a 8 ohm speaker. (if the speaker is rated to accept it)"

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I have a Logitech Z-2300 where two STMicroelectronics TDA7296 are bridged together to power the sub. The power supply of Z-2300 is capable to deliver +/- 26.4 VDC, the current being 3.75 A. It sums up to 40W from each chip.

Now my question is, Bridging this two TDA7296 will produce 40 X 4 = 160W or just 40 X 2 = 80W only.

The sub bass driver is rated at 8 ohms. Ditto the two satellites.

Principles of Amplifier Operation in Z-2300:

1) A stereo audio signal comes in through the green 3.5mm audio connector with the control pod.

2) In the control pod, the signal passes through the main volume logarithmic potentiometer for attenuation.

3) This attenuated audio signal is then send down from the control pod to the subwoofer enclosure through a D-Sub connecter where the amplifier module exists. A JRC-4565 operational amplifier first receives this audio signal. The JRC-4565 having stereo handling capability distributes the audio signal into two places:
a) To the left and right satellite TDA7296 power amplifiers and then subsequently to the 2 satellite speakers.
b) Back up to the remote through the D-Sub connector.

4) Inside the remote, the stereo audio signal is split again:
a) To the headphone jack
b) To the subwoofer bass volume potentiometer (where it is combined to mono at this point)

5) Audio signal output from the subwoofer bass potentiometer finally gets fed back into the subwoofer enclosure through the D-Sub connector to the bridged TDA7296 subwoofer power amplifiers and then to the bass driver.

I wish the TechGurus will help me.

Regards, RishiGuru
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Old 20th January 2011, 08:22 AM   #2
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Perhaps a simplification of wijipedia citation might help.

If you bridge an amplifier, the available power is the power of a single channel into half the load impedance x 2.

So if you have an amplifier that is 25 w into 8 ohms, and 50 w into 4 ohms, then bridged it will be capable of 100 w into 8 ohms.

So if you have an amplifier that is 25 w into 8 ohms, and 25 w into 4 ohms, then bridged it will be capable of 50 w into 8 ohms. (in reality one should be using minimum 16 ohm speakers with this amp bridged)

dave
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Old 20th January 2011, 08:42 AM   #3
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Quote:
Originally Posted by planet10 View Post
Perhaps a simplification of wijipedia citation might help.

If you bridge an amplifier, the available power is the power of a single channel into half the load impedance x 2.

So if you have an amplifier that is 25 w into 8 ohms, and 50 w into 4 ohms, then bridged it will be capable of 100 w into 8 ohms.

So if you have an amplifier that is 25 w into 8 ohms, and 25 w into 4 ohms, then bridged it will be capable of 50 w into 8 ohms. (in reality one should be using minimum 16 ohm speakers with this amp bridged)

dave
Then what about TDA7296 for this case? I am a noob, so kindly help.
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Old 20th January 2011, 08:53 AM   #4
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The TDA7296 is not rated for use at 4 ohm with a 26V supply.

If one assumes that the spec limits itself to the max current capability of the chip it would be capable of 100W bridged into 8 ohm with a +/- 22V supply (with sufficient current capability)

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Old 20th January 2011, 10:27 AM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by RishiGuru View Post
my question is, Bridging this two TDA7296 will produce 40 X 4 = 160W or just 40 X 2 = 80W only.
bridging a pair of amplifiers allows double the power into double the impedance.

If you start with a 30W into 4r0 amplifier and you know it will adequately drive a reactive 4ohm speaker and it does not blow up when a 2r0 test is applied then a pair of these amps in bridge mode will deliver 60W into 8r0 and adequately drive a reactive 8ohm speaker.

Now two 30W amplifiers have been combined to give 60W.
What has been gained? In Power terms, nothing. You still have a maximum output from the bridged amplifier of 60W, exactly the same as the combined power output of the two single amplifiers.

Bridging does not give any extra power !!!!!!!!


That last statement taken out of context will be argued over and trashed by every user/builder on the planet.
But in context it is absolutely true
30W+30W = 60W
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Last edited by AndrewT; 20th January 2011 at 10:29 AM.
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Old 20th January 2011, 11:05 AM   #6
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Quote:
Originally Posted by planet10 View Post
The TDA7296 is not rated for use at 4 ohm with a 26V supply.

If one assumes that the spec limits itself to the max current capability of the chip it would be capable of 100W bridged into 8 ohm with a +/- 22V supply (with sufficient current capability)

dave
planet10 & AndrewT thanks for your reply. Now I got it cleared. I downloaded the TDA7296 spec sheet.

It is specifically stated that in the BRIDGE configuration the value of the load must not be lower than 8 Ohm for dissipation and current capability reasons.

Also at Rl= 8 Ohm, Vs = 18V the maximum output power obtainable is 60W. Since the TDA7296 can handle a maximum of 5 ampere, I think

60W @ 8 ohms @ 18V @ 5A

Output Power vs Supply Voltage of a single TDA7296

Click the image to open in full size.

But, my Z-2300 power supply has a toroid of 20.1V X 2, 3.75 A -->150.9VA, which I think pumps out at around 26.4VDC, 3.75A.

According to the above graph, at +/- 26.4 VDC --> 40W @ 0.5% THD @ 8 ohms @ 5A

So, can any body give a rough estimation how much will be produced by a pair of bridged TDA7296 at 8 ohms @ 26.4V @ 3.75A ?

Will it be less than 60W or more than 60W?

I am an IT Engineer but have no prior knowledge in audio application.

Last edited by RishiGuru; 20th January 2011 at 11:20 AM.
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Old 20th January 2011, 11:25 AM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by RishiGuru View Post
So, can any body give a rough estimation how much will be produced by a pair of bridged TDA7296 at 8 ohms @ 26.4V @ 3.75A ?

Will it be less than 60W or more than 60W?
I just showed you.
a 30W into 4r0 amplifier that can adequately drive a reactive 4ohm speaker will deliver 60W into 8r0 when a pair are bridged.

if you want 62W into 8r0 then halve these figures to allow you to work backwards.

You would need a pair of 31W into 4r0 amplifiers, to give your 62W target.
Exactly the same as the two single amplifiers can deliver 31W+31W
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Old 20th January 2011, 11:36 AM   #8
AJT is offline AJT  Philippines
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Quote:
Bridging does not give any extra power !!!!!!!!
and this is because you are still using the same power supply transformer from the same amp......so if you want more power use a bigger rated traffo but with the same rails....
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Old 20th January 2011, 11:49 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AndrewT View Post
bridging a pair of amplifiers allows double the power into double the impedance.
...............................

Bridging does not give any extra power !!!!!!!!


That last statement taken out of context will be argued over and trashed by every user/builder on the planet.
Quote:
Originally Posted by Tony View Post
and this is because you are still using the same power supply transformer from the same amp......so if you want more power use a bigger rated traffo but with the same rails....
I know everyone will disagree by reading the message out of context.

The rule for bridging is
double the power into double the impedance.

Note, two amplifiers are required to obtain that doubled power, i.e. no extra power from bridging.
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Old 20th January 2011, 12:00 PM   #10
AJT is offline AJT  Philippines
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i offered an explanation as to why your statement "Bridging does not give any extra power".....what is there to disagree about?
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