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)"

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

 

[Nigel Goodwin]

There's no myth - you just need to do the maths correctly and understand what bridging is.

Basically it's all about speaker impedance and supply voltages, it gives you higher power in to a higher impedance speaker from a lower supply voltage.

There's no 'magic' involved - each amplifier is simply providing the same power in to the same impedance as before.

For an example - a standard car amp, running off 12V

Stereo - 2x4W in to 4 ohms (total 8W to two speakers

Bridged - 1x8W in to 8 ohms (total 8W in to one speaker)

Stereo - 2x8W in to 2 ohms (total 16W to two speakers

Bridged - 1x16W in to 4 ohms (total 16W in to one speaker)

So this is where the 'myth' comes from the bridged amplifier will provide 16W in to one 4 ohm speaker, but one amp will only supply 4W to the same single 4 ohm speaker (so it is 4 times the power, and how most car stereos work).

The 'snag' with that is because the amplifiers have to be capable of driving 2 ohms, and if you had a 2 ohm speaker on one amp, it would deliver 8W - exactly half of what a bridged amplifier feeding 4 ohms would. And the two amps feeding two 2 ohm speakers would give the same 16W.

If you paralleled the two amps, and fed a 1 ohm speaker, you would get the exact same 16W as bridging them with a 4 ohm speaker - like I said, it's to do with speaker impedance more than anything else.

 

[RishiGuru]

There's no myth - you just need to do the maths correctly and understand what bridging is.

Basically it's all about speaker impedance and supply voltages, it gives you higher power in to a higher impedance speaker from a lower supply voltage.

There's no 'magic' involved - each amplifier is simply providing the same power in to the same impedance as before.

For an example - a standard car amp, running off 12V

Stereo - 2x4W in to 4 ohms (total 8W to two speakers

Bridged - 1x8W in to 8 ohms (total 8W in to one speaker)

Stereo - 2x8W in to 2 ohms (total 16W to two speakers

Bridged - 1x16W in to 4 ohms (total 16W in to one speaker)

So this is where the 'myth' comes from the bridged amplifier will provide 16W in to one 4 ohm speaker, but one amp will only supply 4W to the same single 4 ohm speaker (so it is 4 times the power, and how most car stereos work).

The 'snag' with that is because the amplifiers have to be capable of driving 2 ohms, and if you had a 2 ohm speaker on one amp, it would deliver 8W - exactly half of what a bridged amplifier feeding 4 ohms would. And the two amps feeding two 2 ohm speakers would give the same 16W.

If you paralleled the two amps, and fed a 1 ohm speaker, you would get the exact same 16W as bridging them with a 4 ohm speaker - like I said, it's to do with speaker impedance more than anything else.

Thanks for the superb explanation.

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."


Since if 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

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


Kindly help.

 

[Nigel Goodwin]

Thanks for the superb explanation.

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."


Since if 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

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


Kindly help.

Is it 26.4V on load or off load?.

The permissible voltage lowers for lower speaker impedances due to heat in the chip, as it's taking more current.

Presumably if the voltage drops to 22V on full power it will be fine - but bear in mind you will require substantial heatsinking for such maximum powers.

 

[audioguru]

In your other thread we calculated that the bridged amplifier tries to draw too much power from its little transformer. The bridged amplifier produces an output of about 120W into 8 ohms at very high distortion for less duration than 1 second.
Its real continuous RMS power at clipping is about 96W with 37W of heat in the two ICs if the satellite amplifiers are playing at very low power. Since the transformer will be overloaded when the satellite amplifiers are also playing then its voltage will drop and the bridged amplifier's output power also drops.

The output impedance of some very old vacuum tube amplifiers were 4 ohms or 8 ohms. Frequently the resonances of speakers sounded boomy.
But the output impedance of modern solid-state amplifiers is extremely low, 0.016 ohms to 0.04 ohms. Then the resonances of speakers are well-damped.

 

[crutschow]

The output impedance of some very old vacuum tube amplifiers were 4 ohms or 8 ohms. Frequently the resonances of speakers sounded boomy.
But the output impedance of modern solid-state amplifiers is extremely low, 0.016 ohms to 0.04 ohms. Then the resonances of speakers are well-damped.

Way back when, I had a couple of old vacuum tube audio amps that had a knob to adjust the output damping factor. The only reason I can think of for that, is apparently some of the old speakers sounded better with a low damping factor (high amp output impedance). Seems strange since modern speakers work best with a high damping factor.

 

[audioguru]

In the olden days, they didn't have the materials to make speakers with a very low resonant frequency and self-damping so they sounded like bongo drums. When they tuned the port to a frequency too low then they had reasonable bass when the amplifier had a high output impedance.

 

[RishiGuru]

Is it 26.4V on load or off load?.

The permissible voltage lowers for lower speaker impedances due to heat in the chip, as it's taking more current.

Presumably if the voltage drops to 22V on full power it will be fine - but bear in mind you will require substantial heatsinking for such maximum powers.

@ Nigel Goodwin @ audioguru

This toroid I want to use came with my Logitech Z-2300. I want to use this toroid. I specifically contacted the maker Ten Pao to know its VA rating. I provided them with all the details labeled on the transformer. After a dozen request they replied it is a manufacture specific 150.9VA transformer.

Their answer after a dozen request :

Dear RishiGuru,

After confirming with our engineer, the VA rating for this model is 150.9, and our company didn't set any retail place in India, so please contact with your Logitech Z-2300 supplier.

Best Regards
Sam Su
Marketing Department
TEN PAO INTERNATIONAL LTD.
Tel: 86-752-2312309
Fax: 86-752-2312813
Skype: sam_su85
Email: sam@tenpao.com
MSN: sushan851028@hotmail.com
website: Ten Pao International Ltd

From: RishiGuru
Sent: Sunday, December 12, 2010 9:39 PM
To: mkt@tenpao.com
Subject: 線上查詢

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

The internal fuse of the toroid was blown and I rectified it in a local repairing shop. Now the toroid is as good as new.

The toroid is 150.9VA
So, it seems 150.9 VA --> 2 x 20.1 V, 3.75A

I have two 10,000uF, 35V capacitors that I want to use as ripple filters.

According to audioguru: "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. "

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

The Output Peak Current of TDA7296 is 5A.

I WANT TO USE A PAIR OF TDA7296 BRIDGED TOGETHER

Now one techguru told me this is not possible 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

What does the above sentence mean? Does this in any way mean that if the current is less then 6A, say 3.75A, then it can handle much higher voltage?

Also it seems I can used a pair of TDA7295 in bridged mode @26VDC @ 3.75A

Kindly help me.

Regards, RishiGuru

 

[audioguru]

+/- 26V at 3.75A is a power of 195W. The rectifiers also get hot so their heat is about 15 additional Watts. The total is 210W.
But your little transformer is rated for only 151VA continuously so it will be massively overloaded and will probably blow its fuse again.

When overloaded, the little transformer's voltage drops. Then the power output from the amplifier is less.

 

[RishiGuru]

+/- 26V at 3.75A is a power of 195W. The rectifiers also get hot so their heat is about 15 additional Watts. The total is 210W.
But your little transformer is rated for only 151VA continuously so it will be massively overloaded and will probably blow its fuse again.

When overloaded, the little transformer's voltage drops. Then the power output from the amplifier is less.

audioguru please don't mind, but another techguru is telling me

"a 230:20.1-0-20.1Vac 6% regulation transformer when fed with 230Vac +10% will have a maximum output after rectification and smoothing of ~ 230 *1.1 / 230 * 20.1 * {1+6%} -400mV ~32.7Vdc when very lightly loaded.

When quiescent current is drawn, the voltage will drop. Expect about a 1Vdc to 2Vdc reduction, i.e. ~30 to 32Vdc.

When the voltage is at a more normal 230+3% (during times of lower power demand) the expected voltage being fed to the amplifier to be between 28Vdc to 30Vdc."

Now I am in a confusion that the supply voltage will be 25VDC or 30VDC.

Kindly help.

 

[audioguru]

I have excellent voltage regulation of my mains voltage here in Canada.
But the other guru is talking about horrible voltage regulation where the 230VAC is 10% too high. In addition, he says the transformer's voltage will be 6% higher than loaded when it is lightly loaded. So he calculated that the DC voltages will be +/- 32.7V when lightly loaded so the amplifier is just sitting there and the speakers are quiet.

If your mains voltage is normal at 230V (not 233V) and the transformer has its rated 151VA load then the DC voltages will be +/-25V to 26.4VDC.
When the transformer is overloaded then the voltages will be less.

 

[RishiGuru]

I have excellent voltage regulation of my mains voltage here in Canada.
But the other guru is talking about horrible voltage regulation where the 230VAC is 10% too high. In addition, he says the transformer's voltage will be 6% higher than loaded when it is lightly loaded. So he calculated that the DC voltages will be +/- 32.7V when lightly loaded so the amplifier is just sitting there and the speakers are quiet.

If your mains voltage is normal at 230V (not 233V) and the transformer has its rated 151VA load then the DC voltages will be +/-25V to 26.4VDC.
When the transformer is overloaded then the voltages will be less.

Thats why you are my guru.

My system seeps power through a auto voltage regulator + UPS. So, I think I do not have to worry about such huge voltage fluctuations.

 

[RishiGuru]

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

Power supply: 26VDC, 3.75A

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

Also it seems I can used a pair of TDA7295 in bridged mode @26VDC @ 3.75A

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

Kindly help me.

Regards, RishiGuru

 

[Nigel Goodwin]

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

Power supply: 26VDC, 3.75A

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

Also it seems I can used a pair of TDA7295 in bridged mode @26VDC @ 3.75A

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

I would suggest the 7295 to be on the safe side - however, I'm presuming your supply is +/-26V, and not a single 26V?.

 

[RishiGuru]

i would suggest the 7295 to be on the safe side - however, i'm presuming your supply is +/-26v, and not a single 26v?.

+/- 26vdc, 3.75a

 

[audioguru]

If your supply voltage varies all over the place then the TDA7295 is safer than the TDA7296.

 

[carmusic]

hi i have an amplifier that uses a 150Va 33.8vct transformer but its rated comsuption on device is at 295W. ive seen a lot of control transformer (industrial ones) rated at 100va (for lower temps rise) but they are able to deliver 2-300va for short periods (few minutes)
my amp is from d-box audio supposed to be 200w rms. It is made form 2 lm1875 and 2 pair of npn/pnp transistor they seems to be bridged but i cant find any schematics. It runs a 4ohm subwoofer. do you think it can really gets 200w rms (for short periods)

 

[Nigel Goodwin]

hi i have an amplifier that uses a 150Va 33.8vct transformer but its rated comsuption on device is at 295W. ive seen a lot of control transformer (industrial ones) rated at 100va (for lower temps rise) but they are able to deliver 2-300va for short periods (few minutes)
my amp is from d-box audio supposed to be 200w rms. It is made form 2 lm1875 and 2 pair of npn/pnp transistor they seems to be bridged but i cant find any schematics. It runs a 4ohm subwoofer. do you think it can really gets 200w rms (for short periods)

Assuming it has large enough capacitors, and the voltage is high enough, it 'should' be able to hit peaks of 200W, but it won't be able to manage 200W RMS because that requires continuous power. For only a 150W transformer I would suggest using an 8 ohm speaker and getting a proper 100W RMS output.

 

[audioguru]

I think the 200W RMS is "music power". Music power is commonly spec'd for car amps. It is the power delivered to a speaker for only 1 second before the power supply voltage sags and the volume is turned way up because the horrible distortion can be any amount.
The power supply produces +/-24VDC when the amplifier is idling then the momentary output power is about 200 Whats.

The amplifier should be rated at 100 Watts RMS continuously from 20Hz to 20kHz with distortion not exceeding 0.05%.
Then if the amp is a class-AB type its efficiency is about 67% and the power transformer will be at its max rated 150VA of power.

 

[rogs]

I'm intrigued to read that even after all these years, the term 'RMS Watts' still appears in these audio threads?
'RMS VA' I understand, but as a Watt is a derived unit of power, measured specifically as a rate of energy conversion of 1 joule per second, I fail to see how the term 'RMS' 'music power' - or anything else - can be added.

I tend to think that perhaps this guy is possibly right? Hi Fi Writer - Meaningless RMS power

But maybe I'm missing something?
'RMS Watts' certainly seems to have been used for years and years in audio circles - and continues to be??