Lightsaber: Powering Speaker from 3.7V

Hi there

My lightsaber has a 3.7V LiPo powering a 12W LED and I am currently working on a sound solution. While the code is done by now, I am struggeling with the hardware part. I basically produce a 7-bit sine wave at PORTD, which runs into a R2R-ladder (I build a 220 Ohm version myself and recently received a 100k package I ordered). The signal then needs to go though some kind of amplification to power an 8 Ohm speaker while taking the load from the Arduino. All components except the LED are fed by a 5V step-up/step-down VCC (max. 500mA).

I initially took a PN2222A with a 220k resistor before the speaker to power the speaker. The transistor was fed by the 220k R2R-ladder. The sound was ok, but I observed three problems:

1) The lower part of the sine wave is cut off.

2) As soon as the Transistor is under load, the amplitude of the signal from the R2R-ladder is significantly reduced.

3) When I remove the 220k resistor before the speaker, I get the desired volume, but the transistor gets hot very quickly.

I then ordered the 100k R2R-ladder (fore size reasons - it's only 1/10th of my self-built) and an OP2134PA op-amp. However, I wasn't able to successfully implement these components due to the following reasons:

A) new 100k R2R-ladder: While the oscilloscope shows the same nice sine wave as with the self-made R2R-ladder, the signal gets crippled to 1V as soon as the output-pin of the R2R-ladder is under load resulting in a very faint sound.

B) OP2134PA: I currently only have +5V and GND to supply the op-amp, but no -5V or even more.

Would an op-amp even make sense given that the R2R-ladder should produce a signal amplitude near +5V?

Do you have an idea, how to get the signal to about 2 watts at the speaker without clipping though a transistor threshold or heating issues?

Your help would by much appreciated!

Cheers Burney

The higher value resistor means that the output impedance of your audio is high. This means the input impedance of your amplifier needs to be high. The clipping of your signals means your amplifier is not capable of going rail to rail. You need a rail to rail op amp followed by a linear power driver stage.

At that low a voltage a bridged configuration for the amp would be best. That means you can get 4 times the power out of your speaker for a given supply voltage. A 4 ohm speaker might be a better choice too.

These days when wanting to drive a speaker your first thought should be "class D" since this is the 21st century ;) However 3.7V supply may be a challenge...

I suspect that all cell phones use class D amplifiers. Lots of choices that run from low voltages, here's one that can use 2.5 to 5.5V: