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Old 2nd May 2013, 12:26 PM   #131
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It's been a little while since I updated the thread so I figured I'd give anybody who is still wondering a heads up on the issue I've been chasing a while. After working on things a bit I thought of a very simple way to find out if the noise I've been having trouble with is higher frequencies being demodulated or lower frequencies being magnetically induced.

The solution was to just hook up a tiny speaker to my oscilloscope and hold it near the phone when it's transmitting... and what do you know.

The same low frequency pulses I saw at the output of the headphone amp.. Looks like it's just magnetic fields after all.

Having said that I did find ways to significantly reduce the noise in my newer layouts. It may fly in the face of conventional wisdom but I broke up the ground foil into sections with no overlapping pieces. any connections from components to ground are done with vias as close to the component pads as I could manage to keep stray capacitance between the layers to a minimum.

The result in the latest amp is pretty good. I can only hear the noise if I wave the phone along the headphone cable. Sandwiching it against the amp enclosure produces nothing.

The latest portable amp is using that power pig of a TPA6120 paired with a ST MC33078DT input buffer stage. Still manage ~15h run time and pretty decent volumes but the amp is half battery.
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Old 2nd May 2013, 12:32 PM   #132
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This is the new amp. Sounds great and tons of grunt. The DC boost converter is outputting +/- 12v @ up to 0.8A

The size in the picture is hard to make out but it's about the same size as a iphone 4 but thicker 0.6"

Click the image to open in full size.
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Old 2nd May 2013, 03:14 PM   #133
agdr is offline agdr  United States
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Outstanding work on your troubleshooting! So inductive pickup by the ground plane. That explains why the (shielded) metal case wasn't solving the problem. B field lines would cut right through. It would take mu metal or some such. On conventional wisdom, I think back to that old Monty Python skit with a crowd saying "we are individuals" and then one guy pipes up "I'm not!".

Last edited by agdr; 2nd May 2013 at 03:19 PM.
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Old 2nd May 2013, 07:48 PM   #134
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Outstanding work on your troubleshooting! So inductive pickup by the ground plane. That explains why the (shielded) metal case wasn't solving the problem. B field lines would cut right through. It would take mu metal or some such. On conventional wisdom, I think back to that old Monty Python skit with a crowd saying "we are individuals" and then one guy pipes up "I'm not!".
Thanks.

It was kind of a facepalm moment when I realized that I could use a simple coil to detect magnetic fields around the phone so I first tried a little 10uH unshielded inductor and was seeing something but wasn't a big enough waveform on the scope to tell for sure then I found the small cell phone ringer speaker and that proved to be sensitive enough to get the scope to lock onto it and get a good look at the signal.

Because none of the filtering I was playing with had any effect I was suspecting inductive coupling a while ago but had no way to prove it.

I enjoyed the puzzle while it lasted and I learned quite a bit along the way so time well spent.

My new task with the current amp is to get idle power draw under control. With a 2200mah 3.7v li-po cell I'd prefer to have a couple days of "on" time and only significantly reduce that when really hammering the output load. I've been considering having the DC-DC boost circuit operate at +/- 5v while no music is playing then ramp up the output voltage to the full +/- 12v when it sees source input.

Complete on/off transients into the headphones from turning the converter on and off are fairly minor. a slight "thud" vs a harsh pop and the settled DC offset on the outputs are 0.8mv and -0.3mv which I understand isn't too shabby for the TPA6120 so I think ramping the output from the converter from 5v to 12v shouldn't be too bad in operation and will reduce the quiescent power consumption by more than half while idle.

It would be simpler to just power down the converter while no music is playing but I don't want to hear the thing come out of "power saving" at all. My phone does this already and it's pretty annoying. I can hear the slight pop generated on the phone's output when the phone sits for a few seconds with no music playing and again right as output starts again. Just feels low quality

Last edited by Amperage; 2nd May 2013 at 07:55 PM.
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Old 3rd May 2013, 01:29 AM   #135
jcx is offline jcx  United States
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http://www.ti.com/lit/ds/symlink/ths6132.pdf
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Old 3rd May 2013, 02:21 AM   #136
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Interesting looking chip. Looks like even at full bias it's below the TPA6120 in quiescent current draw but I'm somewhat confused as to the 2 tier switching function. From what I understand looking at the datasheet it has 2 input voltage levels and the current draw values are valid for Vcc(L) when the output voltage is below ~3.5v and only uses the Vcc(H) input when the output voltage level exceeds roughly 3.5v?
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Old 3rd May 2013, 02:42 AM   #137
jcx is offline jcx  United States
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some explanation http://www.ti.com/lit/an/snaa128/snaa128.pdf

Class G power amp but OK explanation, example power calcs Class-G Amplifiers
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Old 3rd May 2013, 03:06 AM   #138
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some explanation http://www.ti.com/lit/an/snaa128/snaa128.pdf

Class G power amp but OK explanation, example power calcs Class-G Amplifiers
Ahh nice. that explains it nicely. Pretty much what I was thinking they were doing with the chip. Maybe I'll get a couple to play with on a later revision.
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