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How does BTL double the voltage swing?
How does BTL double the voltage swing?
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Old 19th May 2012, 02:37 PM   #31
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by AndrewT View Post

Completely untrue.

A 4ohm capable amplifier will not be capable of driving a 2ohm speaker.
In a similar vein, when you bridge the two 4ohm capable amplifiers and
connect them to a 4r0 load you will not get four times the output power.

The rule is as I stated:

No matter how many times you see "4times the power" it is still wrong.
Hi,

Its not completely untrue. For car audio a single simple amp can put out about
5W into 4 ohms and a pair of simple BTL amplifiers about 20W into 4 ohms.

Of course this implies each amplifier of the BTL pair must be 2 ohm capable.
If they are then "4 times the power is true".

Of course if each amplifier is only rated down to 4 ohms and has a fixed
current limit related to 4 ohms loading, bridging the amplifiers will give
you no more power into 4 ohms, but will give you twice the power into
8 ohms, and 4 times the power into 16ohms.

What you can say is always true but not always applicable is :
"Bridging into twice the load impedance always gives you twice the power."

Bridging into the same load impedance can give you
anything between no extra power and 4 times the power.

For HiFi going bridged is usually applicable for 8 ohm speakers and 4 ohm
capable amplifiers. If the stereo amp is say 100W 8 ohm, 150W 4 ohm,
then bridging into the same same speaker will give you 3 times the power.

rgds, sreten.

Last edited by sreten; 19th May 2012 at 02:50 PM.
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Old 19th May 2012, 03:39 PM   #32
AndrewT is offline AndrewT  Scotland
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No, it is not true.

If you to take extreme car amplifiers as an example, then look at the real specifications.
I take a hypothetical example that can drive 1r0 test load.
max Vac . . load
. 20V . . . 1r0
. 22V . . . 2r0
. 23.5V . . 4r0
. 23.9V . . 8r0
This amplifier is rated to drive a 2ohm speaker load.

Into 2ohm it will deliver a maximum of 22^2 / 2 = 242W into 2r0
into 4r0, 138W, into 8r0, 71W.
Now let's bridge these amplifiers.

into 2r0, 800W, but this bridged amp is not capable of driving a 2ohm speaker.
into 4r0, 484W, into 8r0, 276W, into 16r0, 142W.
There is no 4times for any of the test loads.
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Old 20th May 2012, 11:30 PM   #33
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
No, it is not true.

If you to take extreme car amplifiers as an example, then look at the real specifications.
I take a hypothetical example that can drive 1r0 test load.
max Vac . . load
. 20V . . . 1r0
. 22V . . . 2r0
. 23.5V . . 4r0
. 23.9V . . 8r0
This amplifier is rated to drive a 2ohm speaker load.

Into 2ohm it will deliver a maximum of 22^2 / 2 = 242W into 2r0
into 4r0, 138W, into 8r0, 71W.
Now let's bridge these amplifiers.

into 2r0, 800W, but this bridged amp is not capable of driving a 2ohm speaker.
into 4r0, 484W, into 8r0, 276W, into 16r0, 142W.
There is no 4times for any of the test loads.
hi,

Your just being tedious about the simple fact an amplifier never usually
delivers double the power into a half impedance load, and we all know
that is true, irrespective of bridging, 4 ohm power is not 2 times 8 ohm.

Say something useful, rather than negating the impossible.

rgds, sreten.

It is stupid to say an amplifier capable of driving a 1R load cannot
drive 2R loads bridged. its also stupid to simply say a 8R rated
amplifier can't drive 4R loads, of course it can up to half power.

Last edited by sreten; 20th May 2012 at 11:47 PM.
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Old 21st May 2012, 12:28 AM   #34
CBS240 is offline CBS240  United States
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If the test load Andrew is referencing is a 1R0 resistor, then the amp will blow up driving a 1R0 reactive speaker load. Speakers are not resistors, and the output transistors will feel the difference.

Overloading an amp but only turning it up half way does stress the amp more, it's not a 50-50 ratio because load Z is not closely matched to the Zout. Besides, this is bad practice because at some time some wank-er will crank the volume knob and 'puff' will go the output stage.
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Old 21st May 2012, 11:38 AM   #35
AndrewT is offline AndrewT  Scotland
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Sreten,
test loads and speaker loads are quite different.
If you don't want to believe what I say about that then maybe you just might listen to Cbs.

Sreten,
go and read up about SOAR into reactive loads.
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