Stereo 3.7W Class D Audio Amplifier - MAX98306 (ADA987)

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“Stereo 3.7W Class D Audio Amplifier - MAX98306”

This incredibly small stereo amplifier is surprisingly powerful - able to deliver 2 x 3.7W channels into 3 ohm impedance speakers. Inside the miniature chip is a … read more

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Hi, I would like to add audio to a 3d printed Star Wars Battle Droid (Roger Roger), using an Adafruit FX sound Card 16mb and triggered by a distance adjustable motion sensor. Can you please recommend what amp, speakers and sensor I should use? I was considering it the Stereo 3.7W Class D Audio Amplifier - MAX98306 but it says it works with 3ohm speakers and I don’t see them on your web store. Will 4 ohm be ok? Thanks kindly.


Hi Rosalyn
use any 3ohm or greater impedance speakers.
4Ω will be OK. Possibly a smidgen less power but you won’t hear the difference.
Cheers Bob


Thank you. I suspected that but didn’t want to chance it without confirmation. I’m a bit of a newb to all this.


class d is a sort of magic device it either fully saturates or turns off the output so fast thus making them the most efficient but with any amplifier …if you put in crap you will get out crap its worth much more buying speakers that are efficient with a lower powered amp…the sound of some class-d`s combined with digital audio source is a much better combination…i prefer vynal…however…


Hi Rosalyn,

Welcome to the forum!!

Bob and Brian have some good advice as to what speakers to use, there are a couple of audio triggers with built in amps that you could use to keep the device cost down a little (note they are 2W amps):

I couldnt track down an adjustable distance motion sensor but you are able to change the sensitivity of this PIR: PIR (motion) sensor | Adafruit ADA189 | Core Electronics Australia

Let us know if you have any other questions!


Hi Liam,
I already have the Adafruit fx sound card with the built in 2w amp and it’s not very loud. I am looking at the 3.7w option so the sounds can be heard in a room full of people. I currently use the 2w in a dinosaur puppet and you have to be really close to hear it. I have 2x small 4 ohm speakers connected in stereo to it and volume buttons to keep it at max and still it is very low. Maybe I am doing something wrong?

Also, I am starting to think the motion sensor is not a great idea because in a crowded room, the droid would never shut up! :grin: So I am starting to think a wireless way of triggering the sound card like with an RF 433mhz receiver and a 4 button fob or something similar, would be better. Have you got any suggestions?

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Hi Rosalyn,

While I couldn’t find anything out of the box for triggering your bot remotely, it’s worth mentioning that normalising or compressing (that’s amplitude compression, not data compression) your your audio files may eke out a bit more output from your amp. Also worth noting is that the Watts/dB relationship is not linear, see this:


Hi James.
Off on a tangent a bit.
A worthwhile read. Explains everything well, especially this statement
“The decibel is not an absolute measurement, but indicates the relationship or ratio between two signal levels.”
The only criticism I have here is insert “Power” in place of “signal”.
In other words “db” is a power ratio and the term when used as an absolute level on it’s own is meaningless without a reference. For instance “dbm” is referenced to 1mW while “dbW” references 1W, “dbV” 1V and “dbµV” 1µV and so on.
A bit of care has to be taken if calculating say the gain of an amplifier using the voltage gain formula (20log instead of 10log) as this only holds true if the INPUT and OUTPUT IMPEDANCES are the same. If not, which would mostly be the case, a conversion to power needs to be done to express the proper gain in “db”. This would be one case where a reference would not need to be quoted. ie; “The gain of this amplifier is 20db” would be a valid statement. The reference is needed when quoting absolute levels like +30dbm (1 Watt).

All this can be very confusing to the newcomer but it can make life so much simpler. One scenario would be the expected signal at a receiver. Start with TX level expressed in dbm, subtract cable loss (to antenna), add antenna gain, subtract free space loss, add RX antenna gain, subtract RX cable loss, and with simple addition and subtraction you now have the expected RX signal level expressed as dbm and converting this can get to microvolts (or millivolts). This would be something of a nightmare if working in volts or even watts especially as all those gains and losses are expressed in db anyway.

If newcomers find “db” difficult to understand at first don’t feel too bad about it. When I was at the Marconi School many years ago the confusion was so bad the class size almost halved literally overnight when the subject came up. Fortunately I was working at AWA at the time and had gone out of my way to find out what this “decibel” business was about. Either that or walk around in a daze as the expression was used many times on a daily basis as a routine thing.
Cheers Bob

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