Feed a speaker only with positive voltages

Question

I am using the timer of a microcontroller to create square wave signals from 10KHz to 40KHz, but my microcontroller generates the signal only with positive values (0 - 3.3V). I have generated the same signals from my PC with the sound card, the signals in the oscilloscope have a range from -2V to 2V (at maximum volume) this is 4V peak to peak amplitude.

So my question is:
If the speaker is able to support 4V of amplitude, will it be ok to use a 0 to 3.3V signal?
Is there any way that this positive going signal can damage my speaker?

Answer 1

The 0-3.3V signal has a 1.65V DC component, and this will effectively be shorted by the speaker. You should not connect the speaker directly to the microcontroller as the speaker presents a very low impedance at DC (assuming a regular moving magnet speaker with a voice coil), and it will essentially short the microcontroller.

To ensure that DC voltages do not reach the speaker, put the speaker in series with a capacitor. With a capacitor, you need not worry about the microcontroller driving 3.3V, the capacitor will block DC voltages. You should also ensure that your microcontroller's output pin has sufficient current handling capabilities to drive the speaker directly; buffering the output pin with a MOSFET would be a good idea, or you could get more sophisticated and add an amplification stage with high-pass filter to remove the DC component.

If you have a higher impedance speaker (more of a buzzer), then driving it directly may be perfectly fine. You simply need to find the DC impedance of the speaker, and verify that the current draw is within the limits of the microcontroller GPIO driver.

Answer 2

Driving a speaker with DC-biased signals will cause it to convert more power into heat than driving it with bipolar signals. In addition, speakers are usually designed for balanced mechanical stresses, and a DC bias will cause unbalanced stresses. Speakers will often be more tolerant of this at higher frequencies than lower frequencies (as frequencies approach DC, the fraction of power converted into heat approaches 100%), and at 10kHz the effects might not be too bad. On the other hand, adding a coupling cap will avoid the issue and at 10KHz the required coupling cap may be fairly small and cheap (the impedance of the speaker will likely increase with frequency, so even a 10uF cap might be more adequate).

As an alternative to using a coupling cap, or in combination with it, you could drive both ends of the speaker with separate port pins. This would let you drive twice the voltage and could eliminate the need for a coupling cap (though a coupling cap could provide protection in case of software malfunction).

Answer 3

It's not ideal, but actually this is frequently done when you just want a noise from the electromagnetic transducer, and the 'speaker' is relatively high resistance.

I've designed several products with such small electromagnetic transducers controlled by an MCU and a single transistor.

It is not a good idea to put significant DC voltage across something like an 4-8 ohm speaker- it will displace the cone, cause unnecessary power dissipation and could overheat the voice coil. Use a blocking capacitor in that case.