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[ftlum]

Hi All.

I am confused about the comments that have been made re: analog output protection for the MP3 trigger.

Is a small portable battery / USB powered speaker safe to use with this MP3 trigger? Technically, it amplifies sound, but I wouldn’t call it a traditional amplifier per se.

http://www.amazon.com/gp/product/B004OA ... 2JP1F32SRQ

thanks in advance,

Frank

[Ross Robotics]

I hook my Trigger to a powered speaker and never had any problems. If I were to build my own amp, then yes, I would some kind of protection.

[Mee_n_Mac]

Here's the potential problem. The IC that decodes and then outputs the analog audio signals runs off 3.3V. The right and left audio signals that are output are the AC music voltage plus a DC voltage offset (= ~ 1.65V). Now if you plugged the R&L audio outputs into a set of headphones, there's a question of how the signal is returned to the IC. That is you need a complete circuit for the current from the outputs to flow back into. Normally that return path is just a simple ground.

If that path path is a simple ground, the speakers in the headphones would "see" the AC music signal and the DC offset. That DC offset doesn't do the IC any good and doesn't do the speakers in the headphones any good. If the speakers impedance is high enough there may be no damage to either the IC in the Trigger nor to the headphones. The loudness you can achieve with the headphones will be limited though. So the Trigger also provides a low impedance path to a "buffered" ground ... basically a path back to the DC 1.65V offset. The mini-jack on the Trigger has the L and R audio signals (AC music + DC offset) and the buffered ground (= 1.65V DC offset) on it's connections. When the headphones (or any pure speaker, no amp) is connected to the mini-jack, the L/R audio signals are connected to the speakers + side(s) and the speaker - side uses the GBUF signal as the "ground" (aka return) path. This way the speakers only "see" the music AC signal, just as they should. The AC + DC signal on one speaker connection minus the DC buffered ground on the other speaker connection = the AC signal only across the speaker.

Now go and plug the Trigger into a powered speaker. It has an amplifier in between the input connections and the speaker(s). Most powered speakers take the precaution of putting a DC blocking capacitor in the R & L input audio lines. So any AC music + DC offset turns into just the AC music signal before it gets the amplifier. That's good. But now you have the problem of how to have a return path. Given the DC blocking caps, that path shouldn't tie the amp ground to the Trigger's "buffered ground" (which is what's output as "ground" on the mini-jack). How the powered speakers handle this I can't say. Perhaps they also DC block the "ground" input. That would be good and everything should work. Perhaps the amp circuitry floats, that is the amp has no tie to any other ground, like the AC outlet ground. It only "knows" the input "ground" and all it's circuitry uses this as it's ground point. This is also good and this is almost certainly the case with battery powered speakers. USB powered speakers represent another case. They get 5V power and ground from the PC (or USB hub). If there's no tie between the Trigger's ground and the USB ground, then that's OK. You'll likely have a floating situation just like battery powered speakers. In all of these cases the "buffered" ground that is provided by the Trigger will allow the AC music current to complete it's circuit back to the Trigger but the 1.65V DC offset will not be present at the amp.

In summary if you plug it in and it works (and you don't get distortion very early in the volume range) then everything is OK for some reason or another. If I had the option of making the connections ... I'd run the L&R signals into the amp'ed speakers (and trust there's DC blocking caps inline) and tie the Trigger's true ground (not the buffered version) to the amps true ground. In practice the latter's ground might not be easily accessible.

[ftlum]

Thanks for the explanation!
I'll probably get a ground loop isolator just to be safe.

Frank

[ljbeng]

Any advice for connecting the MP3 Trigger to the Audio Amp Kit by SFE?

https://www.sparkfun.com/products/9612

Confession... I already fried one MP3 Trigger board and can't afford to fry this one.

I am thinking 2 caps in series with audio, .1uF and using board ground to board ground.

Thanks.

[alberto2000]

I am having the same question but with the MP3 Shield https://www.sparkfun.com/products/10628

Any help would be GREATLY appreciated!

I am using this amplifier: T.Amp PM40C http://www.thomann.de/gb/the_tamp_pm40c ... nmodul.htm

The amplifier has the information on the image attached to this post.

I am using the mini jack on the mp3 shield and then into the XLR (+/-/ground) of the amplifier. Everything seems to work EXCEPT the output is very very low. I am using a 8ohm RMS 30W/program 40W speaker http://www.lb-lautsprecher.de/Richtlautsprecher/RL-140

Anyone has an idea why that could be?

[Mee_n_Mac]

While the XLR input is designed for a balanced (aka differential) signal and that's what you need to connect the output of the shield to, there's no guarantee the levels are compatible. The shield will output perhaps a 2V pk-pk signal. If you've got the level knob on the amp turned up to it's max and commanded the shield to it's max volume and the speaker volume is still too low ... I don't know what else to say.

[Mee_n_Mac]

Any advice for connecting the MP3 Trigger to the Audio Amp Kit by SFE?
https://www.sparkfun.com/products/9612
Confession... I already fried one MP3 Trigger board and can't afford to fry this one.
I am thinking 2 caps in series with audio, .1uF and using board ground to board ground.
Thanks.

So long as Trigger board ground is not the GBUF signal, yup I think that will work though with 2 caveats.

First I don't think it's necessary for you to put additional DC blocking caps in line. Looking at the schematic, the amp already has 0.47 uF caps to block the 1.25VDC that the AC music signal is riding on coming out of the MP3 Trigger. There is a 10k trimpot (for volume control) prior to the blocking cap (in each channel) but I don't think that would upset the MP3 at all. We're talking about 125 uA ... hard to see that being an issue.

Second, if you decide to be uber safe then I think the 0.1 uF value is a bit smaller than I'd like, it might cut off some of the bass. The aforementioned 10k pot with a 0.1uF cap will form a high pass filter with a low frequency cutoff of about 160 Hz. I think you'd rather have that down in at least the 30 - 40 Hz region if you're playing music. That's a cap that's 4x-5x larger ... like the 0.47uF caps used in the amp.

The one thing (again) NOT to do is tie the GBUF signal from the MP3 Trigger to actual ground. That will short 1.25VDC to ground and certainly damage the board.
https://www.sparkfun.com/datasheets/Wid ... output.pdf

[CaptInfinity]

I'm trying to build my own simple amp to use with the MP3 trigger.
I'm using this simple TI 386N (currently breadboarded)
http://www.ti.com/lit/ds/symlink/lm386.pdf
http://www.instructables.com/id/LM386-Audio-Amplifier/

The amp works fine plugging in an iPhone or other device, though if the volume is too high there is distortion.

When using the amp with the mp3 trigger, I connect a 3.5mm cable from the mp3 trigger board to a 3.5mm jack on the breadboard. I'm also powering the mp3 board and breadboard with the same 9V battery. However, when I connect everything up, I get a recurring ~1 sec pulse that clicks the speakers and flashes the green LED on the trigger board. How should I wire the two boards together to prevent this from happening? I'm a newbie and don't want to blow the mp3 board. I have a feeling it may be because the battery ground is the same as the GBUF signal ground. How do you separate the signal and power grounds in this scenario?

[Ross Robotics]

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