Basic Qs on LM3886 power supply design

[Mahendra Reddy]

I have some bsic questions in designing power supply for LM3886
amplifier for 4ohm speakers. Please help me in designing this. Not
only for this requirement but in general I need to understand these
things. Sorry for asking so many basic questions.

LM3886 datasheet page 16 says it requires +/-21V at 4.5A for 40W into
4ohm load.

1.) Does it mean it dissipates 42V(+/-21V dual supply) * 4.5A = 189
watts? i.e Pd = V * I

2.) To calculate power dissipation in the above equation is it correct
to consider +/-21V(dual supply) as 42V? If I need to consider 21V in P
= V*I then what should be the voltage if I use 42V single supply
configuration of LM3886? Power disspation and current requirement
should be more or less same in +/-21V dual supply and 42V single
supply. Am I correct?

3.) Is 4.5A at +/-21V equal to 2.25A on +21V supply and 2.25A on -21V
supply? If I use ammeter to measure the current drawn by the amplifier
I see some amps drawn on +V supply and some amps drawn on -V supply.
So is it sufficient to use a 18V-0V-18V transformer supplying 2.5 amps
to get approx +/-21V and 4.5amps(2.25 at +21V and another 2.25 at
-21V) required for 40W into 4 ohms in dual supply configuration?

4.)What is continuous output power and what is Instantaneous peak
output power of an amplifier?

5.)What if I give required voltage but the trnasformer delivering less
current(for example 1A against the required 4.5amps) and turn up the
volume to maximum? Will it distort the signal? or simply doesn't sound
loud?

Thanks in advance
Mahendra Reddy

 

[Mainlander]

I have some bsic questions in designing power supply for LM3886
amplifier for 4ohm speakers. Please help me in designing this. Not
only for this requirement but in general I need to understand these
things. Sorry for asking so many basic questions.

LM3886 datasheet page 16 says it requires +/-21V at 4.5A for 40W into
4ohm load.

1.) Does it mean it dissipates 42V(+/-21V dual supply) * 4.5A = 189
watts? i.e Pd = V * I

There is a certain amount of power lost in the amplifier but nowhere near
as much as what actually is lost across the speaker. If it asks for that
much voltage and that much current you can be sure that the output
voltage is going to be something like +/- 21 volts going to the
loudspeaker at a similar amount of current. there is a small voltage drop
across the transformer junctions which is, among other things, where the
power is lost in an amplifier, but it is nothing like 189 watts for such
a small amp! apart from which I suspect no plastic pack in common use
could dissipate such a large amount of heat.

What you should understand is that at its core an amplifier is
essentially just a few transistors that are driven by a small load (the
input signal) to switch a much larger load (the current going to the
speaker) and there is just a small amount of power lost across the
semiconductor junctions that carry the output load current. A bit like
the way a relay works.

2.) To calculate power dissipation in the above equation is it correct
to consider +/-21V(dual supply) as 42V? If I need to consider 21V in P
= V*I then what should be the voltage if I use 42V single supply
configuration of LM3886? Power disspation and current requirement
should be more or less same in +/-21V dual supply and 42V single
supply. Am I correct?

You should use the +/- 21V configuration in preference, for a single
supply you will need a large capacitor in series with the speaker to
block the DC and then you run into the problem that it poses a frequency
dependent resistance to the current. So it ends up being a relatively
large value to minimise this resistance.

3.) Is 4.5A at +/-21V equal to 2.25A on +21V supply and 2.25A on -21V
supply?

No 4.5A on both supplies.

+/- 21V means that the supply voltages are +21V and -21V instead of +42V
and 0V. The difference between the two is 42 volts in both cases. The
current will be flowing in both supply leads, same amount. It flows in
one lead and out the other, effectively. We use dual supplies so that we
don't have to use a capacitor in series with the speaker.

So is it sufficient to use a 18V-0V-18V transformer supplying 2.5 amps
to get approx +/-21V and 4.5amps(2.25 at +21V and another 2.25 at
-21V) required for 40W into 4 ohms in dual supply configuration?

4.)What is continuous output power and what is Instantaneous peak
output power of an amplifier?

5.)What if I give required voltage but the trnasformer delivering less
current(for example 1A against the required 4.5amps) and turn up the
volume to maximum? Will it distort the signal? or simply doesn't sound
loud?

If the transformer delivers insufficient power then the amplifier will be
incapable of delivering its full load. If the amplifier is then
overloaded with excess signal level the result will be distortion of one
sort or another. At its worst, excessive clipping can result in
significant amounts of DC delivered to the speakers which can damage
them.

 

[Phil Allison]

I have some bsic questions in designing power supply for LM3886
amplifier for 4ohm speakers. Please help me in designing this. Not
only for this requirement but in general I need to understand these
things. Sorry for asking so many basic questions.

LM3886 datasheet page 16 says it requires +/-21V at 4.5A for 40W into
4ohm load.

1.) Does it mean it dissipates 42V(+/-21V dual supply) * 4.5A = 189
watts? i.e Pd = V * I

** Not at all. To deliver 40 watts ( sine wave ) into 4 ohms requires
a voltage of 12.7 volts rms or a current that peaks at 4.5 amps.

2.) To calculate power dissipation in the above equation is it correct
to consider +/-21V(dual supply) as 42V? If I need to consider 21V in P
= V*I then what should be the voltage if I use 42V single supply
configuration of LM3886? Power disspation and current requirement
should be more or less same in +/-21V dual supply and 42V single
supply. Am I correct?

** No. The power dissipation is a complicated function of the output
power level - but you only need to know the worst case with 4 ohms which is
when the output is around 65% or 25 watts. The dissipation in the power IC
is about 45 % of max output 20 watts at this power level.

3.) Is 4.5A at +/-21V equal to 2.25A on +21V supply and 2.25A on -21V
supply?

** No.



If I use ammeter to measure the current drawn by the amplifier

I see some amps drawn on +V supply and some amps drawn on -V supply.
So is it sufficient to use a 18V-0V-18V transformer supplying 2.5 amps
to get approx +/-21V and 4.5amps(2.25 at +21V and another 2.25 at
-21V) required for 40W into 4 ohms in dual supply configuration?

** No.

4.)What is continuous output power and what is Instantaneous peak
output power of an amplifier?

** The first means the power delivered by a constant level sine wave -
the latter is the power figure calculated from a single peak of voltage.

5.)What if I give required voltage but the trnasformer delivering less
current(for example 1A against the required 4.5amps) and turn up the
volume to maximum? Will it distort the signal? or simply doesn't sound
loud?

** Distortion will be heard long before 40 watts is reached - at least
on any sustained sounds.





............... Phil