Current flow BTL vs. SE

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Dave King 16 years ago

I am trying to understand the practical implications of the differnece in current flow between a SE vs. BTL amp.

First I take a length single wire and fold it over in half flat upon itself so the two fly leads are at one end.

Then I connect the leads across the output of a earth referenced SE amp and apply a sinewave to its input. The current now opposes in the adjacent conductors and magnetic flux tends to cancel. The cancellation obviously reaches its highest intensity at the peak of the waveform.

So ... does the same effect occur if I were to use a BTL amp instead? In this case the current flow is bidirectional (push/pull) per single cycle, not one way. Does this mean two flux cancellations per cycle will occur, given the above wire configuration?

Keep in mind that everytime the flow reverses at full amplitude represents a cancellation event.

I can't get my head around it for some reason.

Dave King

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Phil Allison 16 years ago

"Dave King"

** OK, so far.

** The magnetic field cancellation effect is not related to the actual current level.

** Got nothing to do with the source of current either.

** Absolute bollocks.

There is nothing so different about a BTL amp.

A good way to imagine how a BTL output stage operates is to imagine the load is made up of two identical loads in series - each with exactly half the resistance of the actual load.

Then note that there are equal and opposite polarity voltages at the ends of that split load at ALL points in time.

Hence, the voltage at the middle point of the split load is ALWAYS ZERO !!

So, it will make NO difference if that middle point is connected to common or ground.

So a BTL stage is essentially the same as TWO amplifiers driving TWO loads grounded at one end - but the load resistance seen by each amplifier is * half * the resistance of the bridge connected load.

..... Phil

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George Herold 16 years ago

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What's BTL? I'm assuming that SE is single ended.

Thanks, George H.

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Phil Allison 16 years ago

"George Herold"

What's BTL?

** Bridge Tied Load or more simply " bridge mode ".

I'm assuming that SE is single ended.

** Yep.

... Phil

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Dave King 16 years ago

Can you please explain how this works. If there is no current or voltage, there would be nothing to cancel. Logically then, at maximum voltage or current cancellation should be more intense.

Here is a quote from the TDA 2005 data sheet. "Voltage and current swings are twice for a bridge amplifier in comparison with single ended amplifier."

This implies to me there would be two flux cancellation events per single cycle of sinewave when using a BTL amp.

If not, can you please further clarify your points?

Dave King

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Phil Allison 16 years ago

"Dave King" "Phil Allison"

** I'll give it just one more try - folks.....

** Nothing wrong with seeing it like that.

But cancellation is just cancellation.

** Silly misinterpretation by you.

In the above, " twice " = twice the size.

If you read the rest of the paragraph that begins page 6, this is perfectly clear.

..... Phil

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miso 16 years ago

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Read the next line in the data sheet. Basically they are indicating you get twice the voltage delivered to the load for a given supply voltage.

I think you are trying to read too much into the "balanced line". The differential signal is merely to get more power for the available supply. Balanced lines are more critical for low level signals where you are trying to reduce interference.

Most differential designs are not really differential. They just put in an inverting stage and a second single ended amp.

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Current flow BTL vs. SE

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