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Is using PWM tones from Arduino amplified by a HiFi amplifier safe? Can I damage the amplifier or speakers because the PWM signal is not a "real" audio signal? Taken in consideration that I do not clip the signal and using a RC Low Pass Filter to filter out inaudible frequencies and using a LM386 amplifier chip to amplify and control the volume?

I am developing a synth/sampler project based on the Arduino Uno r3. This is my first electronics project ever and because of that reason I have some questions which I can't find a definite answer for on internet.

The synth has two function. It has a tone generator using the ToneAC library. With code I have mimicked LFO and VCO functions, using potmeters I can adjust the frequency and LFO speed realtime. The other function is that I can trigger samples stored on a SD card using the TMRpcm library. The lowest frequency which can be played is 100hz and the highest 3100hz.

The device will be used in nightclubs on high end soundsystems and PA. I am afraid I will damage the equipment with my device because I do not send a pure audio (AC?) signal.

Currently the flow is like this:

PWM output (pin 9, 10) -> voltage divider to 2.5v -> RC Low Pass Filter 15Khz -> LM386 amp with volume control -> mono jack output -> mixing desk -> amplifier -> speakers

(the TMRpcm and ToneAC library share the same pin (9), I have fixed this by disabling the TMRpcm when the ToneAC is playing and vice versa. The mono jack is wired as follow. pin 9 goes to plus side of mono jack output, pin 10 to negative side. The TMRpcm pin 9 also needs to go to plus side of mono jack output, and mono jack negative needs to go to ground. Because I disable the ToneAC at that time I consider port 10 grounded because it not generating any sound so there is no signal/voltage running out of it, aka the IO pin is set to LOW)

I am hoping that using the voltage divider I am within range of the Audio Line levels of a default mixers Line-IN (like Pioneer, Behringer, etc). The RC Low Pass Filter is used to filter of inaudible frequencies and clean up the sound a bit, the LM386 audio amp is used to amplify the signal and (hopefully) make it a real audio signal.

Because I lack a lot of knowledge in this field and because I do not own a oscilloscoop I can't say for sure if my output signal is harmful for the equipment and was hoping to get some guidance on my project on this forum.

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    Questions on stack exchange forums get better answers, the more specific you can make your question. Asking for guidance doesn't help someone know how to help you and the responses are likely not to be well focused. Also, this question sounds more about audio electronics than Arduinos, even though you're using an Arduino host. If so, it may be a better fit for the Electrical Engineering stack exchange site. – JRobert Jan 20 '15 at 0:10
  • Point taken, I've redefined the question – erik404 Jan 20 '15 at 19:48
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In theory, the peak signal accepted from your amplifier should be 0.894V p2p if you want to rescpect the common -10dB signal strength/level. I don't know why you would want to have a 5V to 2.5V "voltage divider" in the first place, but in any case if you use that you already limit the current probably enough, it all depends on the resistor values used.

Using the LM386 amplifier is also simply not required at all, since the Arduino is already providing more than enough power. If you want to tune the volume at that stage, simply combine all functions (volume, low pass and current limit) by connecting the 5V PWM output thru a resistor (eg. 4k7) to a POT (eg. 1k) in parallel with the capacitor (eg.2n2) would give you approximately what you want. Then to remove de DC component, add a series capacitor (eg. 1uF) and you're all set. You may still hear a "click" when you connect it to the amplifier though, so keep the faders low when connecting.

Of course you can adapt the values quite liberally, but these values are both safe and should be close to what is needed (cutoff 15kHz, Vpp 0.88V max).

  • Thanks for the detailed answer. I already found out that I need to pass the signal through a 10uF capacitor so it looks like the +/-2.5V AC signal. Send that through a resistor divider and bring it down to +/- 1V (line level) for feeding into audio amplifier. LM386 is indeed not necessary. This should do the trick :) – erik404 Jan 21 '15 at 15:01

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