Controversy regarding Bridged Amplifier power output

[RishiGuru]

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

 

[planet10]

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

 

[RishiGuru]

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.

 

[planet10]

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

 

[AndrewT]

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

 

[RishiGuru]

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

An externally hosted image should be here but it was not working when we last tested it.

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.

 

[AndrewT]

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

 

[TonyTecson]

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....

 

[AndrewT]

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.

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.

 

[TonyTecson]

i offered an explanation as to why your statement "Bridging does not give any extra power".....what is there to disagree about?

 

[tangmonster]

Here is a simpler way of putting it. But I am repeating what planet10 said.
Let's take a make believe amplifier that can deliver 100W into 4 ohm and the amplifier built in power supply can handle this 4 ohm load at 100W.
This amplifier can not work into 2 ohms.

8ohm load = 50W
4ohm load = 100W

8 ohm load 2 amplifiers bridged = 200W (you have twice the amplifiers and 4 times the wattage compared to 1 amplifier and 8 ohm , each amplifier "sees" 4ohm load)

4 ohm load 2 amplifiers bridged = NOT possible!!! ( each amplifier will see 2ohm load , the amplifier and/or power supply cannot handle this!)

 

[AndrewT]

Tony,
take two amplifiers and bridge them.
You do not invent extra power output.

Take two modified amplifiers with an improved power/current/voltage delivery and bridge them. You do not invent extra power output.

The rule holds true
A bridged amplifier delivers double the power into double the impedance.

The total power delivered by the bridged pair is exactly the same as the total power of the two single amplifiers. There is no extra power.

 

[RishiGuru]

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

Now, I got what you said. Sorry for the late realization. Guys just be a little easy with me, I admit I am a noob.

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

From the power supply vs. voltage graph of TDA7296 I found: 50W @ 4 ohms @ +/-22V @ 5A

So, according to what you said, a pair of bridged TDA7296 will produce:

2 times 50W @ 4 ohms @ +/-22V @ 5A = 100W @ 8 ohms @ +/-22V @ 5A

Right!

The TDA7296 is not rated for use at 4 ohm with a 26V supply.

Does this imply that a pair of bridged TDA7296 can handle a maximum of +/-22VDC @ 8 ohms

 

[TonyTecson]

Tony,
take two amplifiers and bridge them.
You do not invent extra power output.

Take two modified amplifiers with an improved power/current/voltage delivery and bridge them. You do not invent extra power output.

The rule holds true
A bridged amplifier delivers double the power into double the impedance.

The total power delivered by the bridged pair is exactly the same as the total power of the two single amplifiers. There is no extra power.

i think i know what the problem is, i was thinking of a two channel amp, in that amp, since the power transformer stays the same, power is doubled into 8 ohms and not more.....

 

[AndrewT]

i think i know what the problem is, i was thinking of a two channel amp, in that amp, since the power transformer stays the same, power is doubled into 8 ohms and not more.....

I think we are nearly there (in agreement).
take your 2channel amplifier that is rated for 4ohm speakers and above.
It delivers a maximum of 100W+100W both channels driven into 8r0+8r0.
It also delivers 180W+180W both channels driven in to 4r0+4r0.

Now bridge those two channels.
The specification will become 360W into 8r0 and 200W into 16r0.

No extra power is magically invented.

The total output power of the 2channel amplifier is not different when bridged. The total power delivered to the speakers is not changed by bridging.

Exactly the same applies when two monoblocks are bridged.

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.

 

[RishiGuru]

Tony,
take two amplifiers and bridge them.
You do not invent extra power output.

Take two modified amplifiers with an improved power/current/voltage delivery and bridge them. You do not invent extra power output.

The rule holds true
A bridged amplifier delivers double the power into double the impedance.

The total power delivered by the bridged pair is exactly the same as the total power of the two single amplifiers. There is no extra power.

The TDA7296 is not rated for use at 4 ohm with a 26V supply.

Andrew my question is can a pair of bridged TDA7296 handle +/-26.4VDC @ 8 ohms when at 4 ohms it is rated upto +/- 22VDC.

Please answer.

 

[AndrewT]

Andrew my question is can a pair of bridged TDA7296 can handle +/-26.4VDC @ 8 ohms

look up the datasheet.
find what a single amplifier with a 4r0 load can do with +-26.4Vdc as the supply.
If the datasheet says 98W into 4r0, then expect it to blow up.
If the datasheet comes up with a more realistic figure then use the datasheet to check all the other operating conditions.

I don't have your datasheet at hand.
Let's suppose +-26.4Vdc and 4r0 does show as being capable of 51W.
A bridged pair of these amps will deliver 102W into 8r0.
That is not the end of the design exercise.
You should estimate the variation in supply voltage, I guarantee it will rarely be +-26.4Vdc.
You should check the quiescent current and from your heatsink and ambient conditions estimate the chip temperature when the amp is not delivering power.
You should estimate the chip package temperature when the amplifier is delivering 1W to the load and 10W to the load.
You should estimate the peak current into a real speaker.

I think you should copy an existing proven design exactly as the designer/builder tells you to do it.

I do not think you are ready to design your own and certainly not a bridged version.

 

[RishiGuru]

look up the datasheet.
find what a single amplifier with a 4r0 load can do with +-26.4Vdc as the supply.............................to design your own and certainly not a bridged version.

I do not think you are ready to design your own and certainly not a bridged version.

I agree with you on that part.

Anyway Andrew on the TDA7296 spec sheet I found it can handle a maximum of
VS = ± 29V, RL = 8Ω --> 60W MAX
VS = ± 24V, RL = 6Ω --> 60W MAX
VS = ± 22V, RL = 4Ω --> 60W MAX

I found at 8 ohms it can handle 29VDC, but at 4 ohms only 22VDC

Quoting from the datasheet: "Another application suggestion is the BRIDGE configuration, where two TDA7296 are used, as shown by the schematic diagram. In this application, the value of the load must not be lower than 8 Ohm for dissipation and current capability reasons. A suitable field of application includes HI-FI/TV subwoofers realizations. The main advantages offered by this solution are:

– High power performances with limited supply voltage level.
– Considerably high output power even with high load values (i.e. 16 Ohm).

The characteristics shown by figures 23 and 24, measured with loads respectively 8 Ohm and 16 Ohm.

With Rl= 8 Ohm, Vs = ±18V the maximum output power obtainable is 60W, while with Rl=16 Ohm, Vs = ±24V the maximum Pout is 60W."

But I am not quite sure that a pair of bridged TDA7296 @ 8 ohms can handle 26.4 VDC or not.

Kindly help.

 

[kevinkr]

I agree with you on that part.

<snip>
But I am not quite sure that a pair of bridged TDA7296 @ 8 ohms can handle 26.4 VDC or not.

Kindly help.

Since each chip sees effectively a 4 ohm load when bridged into an 8 ohm load the answer is: NO... You need to keep the supply rails to 22V or less to stay within the SOA capabilities of the 7296. (There can be exceptions relating to transformer/supply sag under load, but best for a newbie not to go there.)

 

[RishiGuru]

Since each chip sees effectively a 4 ohm load when bridged into an 8 ohm load the answer is: NO... You need to keep the supply rails to 22V or less to stay within the SOA capabilities of the 7296. (There can be exceptions relating to transformer/supply sag under load, but best for a newbie not to go there.)

Thanks kevinkr, this was the answer I was avidly waiting for.

Now coming to a more powerful chip TDA7295, its spec sheet says:
VS = ± 34V, RL = 8Ω --> 80W MAX
VS = ± 26V, RL = 4Ω --> 80W MAX

Quoting from the TDA7295 datasheet: "Another application suggestion is the BRIDGE configuration, where two TDA7295 are used, as shown by the schematic diagram of figure 25. In this application, the value of the load must not be lower than 8 Ohm for dissipation and current capability reasons. A suitable field of application includes HI-FI/TV subwoofers realisations.

The main advantages offered by this solution are:

High power performances with limited supply voltage level.
- Considerably high output power even with high load values (i.e. 16 Ohm).
The characteristics shown by figures 20 and 21, measured with loads respectively 8 Ohm and 16 Ohm.

With Rl= 8 Ohm, Vs = ±22V the maximum output power obtainable is 100W, while with Rl=16 Ohm, Vs = ±30V the maximum Pout is 100W. "

It seems a pair of bridged TDA7295 @ 8 ohms can handle 26 VDC.

Kindly verify.