powering 8 ohm 2W speaker with teensy audio

[orbitronics]

Hi!

As the title says, i salvaged a 2W 8ohm speaker from a radio and i'd like to play audio from the line out of the teensy audio on it, but i'm not exactly sure if i should include an amplifier.

Would anyone be able to point me in the right direction ?

Thanks!

 

[Theremingenieur]

You need an amplifier. A line output never delivers enough voltage and current to power a 2W 8Ohm speaker. Since P = U * I and I = U /R, U = sqrt(P * R) which gives a required effective drive voltage of 4Veff which is about 11.2Vpp. The current is thus 0.5Aeff. There are nice small bridged class D audio amplifier modules which, even when powered with only 5V, can do this job.

 

[orbitronics]

Ahh, i was wondering what voltages a speaker like this would want, seems like it's 12V. no doubt line out is absolutely not adequate.

Do you mind me asking, What is Veff, or just the _eff?

 

[Theremingenieur]

Veff is interesting for AC only. It is defined as the effective Voltage (corresponding to the DC voltage) which dissipates the same amount of heating power into a given resistance. Since with AC, the momentary voltage changes all the time, the relationship between the peak voltage, the peak-to-peak voltage, and the effective voltage depends highly on the waveform and is mathematically obtained by integrating the voltage over a period of the AC frequency.

As a rule of thumb and assuming that the AC voltage is symmetrical and close to a sine wave (which is ok for most audio considerations), you might use :

Veff = 0.7 * Vp (peak voltage) or Veff = 0.35 * Vpp (peak to peak voltage).

 

[PaulStoffregen]

He probably meant "effective" voltage. It's commonly called RMS (root mean square) or just AC voltage.

The basic idea is you need 4 volts AC (and 0.5 amp AC) to get 2 watts power delivered to an 8 ohm load. It's simple math.

The less simple part is what "4 volts AC" really means. For a sine wave signal, that's a sine wave with 5.66 volts peak, or 11.3 volts peak-to-peak. There are many very detailed explanations of AC waveforms and power online, if you really want to dig into a deeper understanding those details.

The peak voltage is an important detail. In these modern times of directly coupled transistor amplifiers without any magnetic elements (like impedance matching transformers), you can generally assume an amplifier can't output a peak voltage any more than whatever power supply voltage it uses. Of course the peak voltage output may be much less, depending on the design of the amplifier.

 

[orbitronics]

That's the one, the equation you used for Veff was identical to the formula i learnt at university for V_rms <--> V_pp, so i got confused!

You may have noticed my weakness is with analogue electronics, so i appreciate help from the two of you as i seem to have forgotten the basics :S

 

[Theremingenieur]

He probably meant "effective" voltage. It's commonly called RMS (root mean square) or just AC voltage.

Yes, that's what I meant and explained above.

BTW: Mathematically (ok, this is a "purist" thing), the RMS method which corresponds to the Newton approximation for integrals is always a little less precise than "true" integration...

 

[orbitronics]

Btw i ended up purchasing a breakout for the PAM8403 which seems to be quite a nice little IC for speakers around this size.

I noticed with these class D amp ICs there's usually a SHDN and MUTE pin; if you wanted to mute, surely you'd just enable SHDN to mute AND keep your system efficient by stopping unnecessary flow to the IC, than enabling MUTE? Would the main difference be startup times?

 

[Theremingenieur]

longer startup time and probably "pops" in the speaker for full shut down/wake up

 

[orbitronics]

Thanks for the advice.

So I've gone off and purchased one of these amps for one of these speakers, and I am planning to hook it up to a teensy 3.2 W/ audio board like so (to the line out's L G R)

In your opinions, would this design be ok? (i know i may have some issues with the pinouts on the PAM8403, it was quicker to insert that IC than to make the pinout for the breakout).

[Edit: I just remembered, i will only consider attaching the usb 5V from the teensy to the class D amp given the current draw will be greater than 500 mA].
As an abstract for the rest of the circuit: I'm aiming to play a wav track from the SD card when certain RFID tags are read by a reader.

Does it look sane?