Current flow BTL vs. SE

[Dave King]

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

 

[Phil Allison]

"Dave King"

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.

** OK, so far.

The cancellation obviously reaches its highest intensity at the peak of
the waveform.

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

So ... does the same effect occur if I were to use a BTL amp instead?

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

In this case the current flow is bidirectional (push/pull) per single
cycle, not one way.

** 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

 

[Phil Allison]

"George Herold"

What's BTL?

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

I'm assuming that SE is single ended.

** Yep.




.... Phil

 

[Dave King]

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

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.

** Absolute bollocks.

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

 

[Phil Allison]

"Dave King"
"Phil Allison"

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

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.

** Nothing wrong with seeing it like that.

But cancellation is just cancellation.

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."

** 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