NXT Speaker Failure

[tcwan]

Hi,

Does anyone have experience with the NXT speaker failing?
This morning I found my NXT speaker does not generate sound anymore.

Last Friday the NXT firmware startup tone sequence sounded more clipped than usual (i.e., harsh metallic tone), and today, I managed to get some weak sound by pressing down on the speaker diaphram for a while (the cover was off for JTAG debugging), but that too eventually won't work anymore.

Is there any programmable gain for the speaker output driver which may cause it to overload and fail? I thought it was just a PWM driven circuit.
What kind of replacement should I look for (impedance)?

[timpattinson]

Try looking in the schematics in the Hardware Dev Kit on mindstorms.lego.com

[mattallen37]

I haven't tried replacing any NXT speakers. I don't know for sure, but I think I remember hearing that they are 32 ohm, but I don't really remember for sure.

As far as how they are powered, I don't believe you could drive one with PWM. For driving a speaker from a digital device, you need to use a DAC.

If I were to have an NXT with a non-working speaker, I wouldn't replace it. I would either not worry about it, or I would replace it with an audio jack for line out.

[nxtreme]

It's hard to accurately measure a speaker, just because of the way they work. However, you can usually just measure the impedance of the speaker and round up to the nearest type of speaker. In my case, I measured ~14.5 ohms, so it's probably a 16 ohm speaker and given it's size, probably not any larger than 1/2 W, more likely 1/4. However, lots of guessing was involved in this post, so take the information with a huge grain of salt .

-EDIT- I browsed through the hardware development kit that the Lego Company offers, but the only mention I could find of a speaker was the text "speaker" and a jumper pad, no specs. So, it appears the Lego Company wouldn't exactly know either...

[pbenco]

Hello dear tcwan



You can use the mattalen37 suggestion to plug external speaker.
Try to ask Lego for a replacement part suggestion, or find a to with little speaker, and test...
Hope this help
Ben

[gloomyandy]

The speaker is driven via a small amplifier using pulse density modulation (PDM), from a single I/O pin. There is no hardware DAC involved (unless you count resistors/capacitors)... PDM is an interesting technique see here for a few more details:
http://en.wikipedia.org/wiki/Pulse-dens ... conversion

Sorry about the off topic post...

Andy

[tcwan]

Thanks for all the suggestions.

While it is not critical to the operation of the NXT, I do miss the audio cues since I'm doing debugging with frequent reboots.
It is good to know the state of the brick without having to look at the LCD, and whether a button press registered correctly or not.

I'll have to try and hunt down a replacement one of these days, 2 cm diameter speakers are not so easily sourced where I am.

[mattallen37]

~Andy, PDM with a cap is a DAC. According to that wiki page, the only thing needed to convert PDM into a DAC is a low pass filter (capacitor in parallel, with maybe a couple extra parts to help smooth it out).

[gloomyandy]

Yes Matt I know that is why I said "unless you count resistors/capacitors". I was just trying to point out that the NXT does not use a convential hardware DAC

[mattallen37]

Oh okay, right.

@tcwan, what did you decide on doing.