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Controversy regarding Bridged Amplifier power output

RishiGuru

Dec 24, 2010
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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
 

(*steve*)

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Jan 21, 2010
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As is noted, that page is in search of someone who is an expert.

4x is not a myth.

An amplifier driving an 8 ohm load in bridged configuration effectively sees a 4 ohm load. So the simple answer is to determine the power the amplifier can produce into half the output impedance, and double it.

the 4x rule for bridging is more of a rule for designers, since a built up amplifier designed for (say) and 8 ohm load is unlikely to be able to deliver twice the power to a 4 ohm load.

Sure, as is pointed out, the effective resistance of the driving elements needs to be considered, but it is way more complex than the article makes out. Look up "damping factor" if you want an indication of what a good output impedance might be (with respect to the load).

Of more importance is the current (and power) rating of the output devices and the current capability of the power supply.

Now for your question. If the amplifier module is not rated for 4 ohms, then don't use it in bridged mode for an 8 ohm load. Simple. Find one that is rated for use with a 4 ohm load.
 

jerryg50

Apr 18, 2010
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When amplifier outputs are bridged there is an increase of distortions due to the small differences between the characteristics of the devices. The devices would have to be exactly matched, which would be very difficult to accomplish.

With well matched devices the distortions should not be audible, buy will most likely be measurable.

Jerry G.
 

RishiGuru

Dec 24, 2010
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As is noted, that page is in search of someone who is an expert.

4x is not a myth.

An amplifier driving an 8 ohm load in bridged configuration effectively sees a 4 ohm load. So the simple answer is to determine the power the amplifier can produce into half the output impedance, and double it.

the 4x rule for bridging is more of a rule for designers, since a built up amplifier designed for (say) and 8 ohm load is unlikely to be able to deliver twice the power to a 4 ohm load.

Sure, as is pointed out, the effective resistance of the driving elements needs to be considered, but it is way more complex than the article makes out. Look up "damping factor" if you want an indication of what a good output impedance might be (with respect to the load).

Of more importance is the current (and power) rating of the output devices and the current capability of the power supply.

Now for your question. If the amplifier module is not rated for 4 ohms, then don't use it in bridged mode for an 8 ohm load. Simple. Find one that is rated for use with a 4 ohm load.

Thanks (*steve*).

I WANT TO USE A PAIR OF TDA7295 OR TDA7296 BRIDGED TOGETHER

Power supply: +/-26VDC, 3.75A
My sub woofer driver is rated at 8 ohms


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

Now one techguru told me this is not possible with TDA7296 since: "each tda7296 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.) "

---------------------------------------------------------------

Now coming to the chip TDA7295, its spec sheet says:

The Output Peak Current of TDA7295 is 6A.

VS = ± 34V, RL = 8Ω --> 80W MAX
(***) VS = ± 26V, RL = 4Ω --> 80W MAX

Note (***): Limited by the max. allowable out current

It seems I can used a pair of TDA7295 in bridged mode @26VDC @ 3.75A @ 8 ohms as this chip can handle 26VDC at 4 ohms.

Which one in bridged mode should I use with the @26VDC @ 3.75A power supply? The TDA7295 or TDA7296?

Also theoretically is it impossible for a pair of bridged TDA7296 to handle +/-26VDC?


Kindly help me.

Regards, RishiGuru
 
Last edited:

(*steve*)

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If you want 120W into 8 ohms, bridge 2 of them with a supply of +/- 22V. You could use a greater supply rail, but you risk damaging the output transistors if the peak current is exceeded.

Or use the 7295 and get 160W with +/-26V.

With a 26V supply, I'd go for the 7295 if I was bridging them.

edit: you seem to have already been advised the same thing. Would you like a different answer?
 

RishiGuru

Dec 24, 2010
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If you want 120W into 8 ohms, bridge 2 of them with a supply of +/- 22V. You could use a greater supply rail, but you risk damaging the output transistors if the peak current is exceeded.

Or use the 7295 and get 160W with +/-26V.

With a 26V supply, I'd go for the 7295 if I was bridging them.

edit: you seem to have already been advised the same thing. Would you like a different answer?

(*steve*) i just wanted the green signal from you. Now I will buy a pair of TDA7295 since I will be using th 26VDC supply.

Thanks for your help.
 

(*steve*)

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The only caveat is that with a +/-26V 3.75A supply, you'll be limited to something like 110W RMS continuous power. (presuming that the power supply is for a single channel).

Your peak power should approach 160W.
 

RishiGuru

Dec 24, 2010
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The only caveat is that with a +/-26V 3.75A supply, you'll be limited to something like 110W RMS continuous power. (presuming that the power supply is for a single channel).

Your peak power should approach 160W.

Yes, I know that the toroid rating is quite strange, actually it is a manufacture specific model and comes from my Logitech Z-2300.

Toroid Rating: 150.9VA --> 20.1V X 2, 3.75A

I am using a pair of 10,000uF, 35V capacitors as ripple filters.

So power transformer is rated to have an output of two times 20.1VAC when it has its rated load. So its resistance has already dropped the open circuit voltage and its peak will be 1.414 times higher which is 28.4V peak. The single rectifier drops it to 27.4V and the ripple drops it to 26VDC.

Since this is my first project I want to start with whatever I have in hand. I want the amp to make sound first. Later I will go max power.

Thanks again (*steve*). You guys are just awesome.:)
 
Last edited:

(*steve*)

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Remember that a 20.1VAC 3.75A transformer can not be expected to deliver 3.75A from the bridge rectified output and maintain anything like 26V.

This is because current will only be drawn from the transformer when the transformer voltage exceeds the DC voltage (plus a bit for the rectifiers). Ripple will increase and the voltage will sag.
 
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