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Continuous power without batteries?

  • PabloMDiez Friday, July 22, 2011 3:25 PM Reply
    Hi there! I'd like to buy and mod this product, and embed it into my old stereo system. The idea is to wire audio output of this product to aux input at my stereo.
    Given it wouldn't need to be portable, and the stereo has AC in, i was thinking about using a DC transformer instead of batteries. Would it work? Is this safe?
    Cheers!
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  • jobarjo78 Thursday, August 18, 2011 6:35 PM Reply
    It unfortunately auto power off.
    You need to switch it on everytime you come back.
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  • PabloMDiez Thursday, August 18, 2011 7:33 PM Reply
    Thanks for your answer!
    It auto powers off even if it's playing something?
    Anyway, that's not a big deal, since I can wire out a button to the outside of the stereo to manually turn it on..
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  • PabloMDiez Tuesday, August 30, 2011 7:01 AM Reply
    So, I bought this product. I'm testing it before send a review.
    So far, it's excellent. I haven't had any problems setting it up with my ubuntu laptop. everything worked seamlessly, and I really haven't expected this audio quality. A2DP is just plain amazing.
    I've been using it for 5 hours, and the battery still up.
    Then, I see no need to disassemble it and do what I was about to do..
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  • gossg Tuesday, May 29, 2012 1:56 AM Reply
    I had the same question as Pablo. I want to use this to connect my phone music player to my car stereo's aux in. The car stereo has a USB that can provide the power. But I have to unplug the headphones to use the charger. :(
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  • PabloMDiez Tuesday, May 29, 2012 10:30 AM Reply
    Hey! I forgot to update this thread.
    I did manage to power it up with continuous power.
    I just replaced the included battery with an old Sony Ericsson W300i charger, and that was pretty much everything.
    Everything works perfectly. Sometimes I forget to unplug it when not in use, but still not making big problems of that.
    What I have to notice, is that the continuous power source, should be stabilized. Whenever my fridge starts or stops, it plays this huge crack on my speakers. Also, when not in use (but powered on and connected), after 10 secs (more or less), it starts playing a white-kinda-noise. I think it's because the same problem.. but also not too much of a big deal :)

    Cheers!
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  • eliasalberto Thursday, July 5, 2012 9:16 AM Reply
    I wouldn't recommend doing what PabloMDiez did. The battery is lithium, so it should work only between 3 and 4.2v. If you REPLACE the battery with a 5v supply, you'll be providing 5v to the circuits that were designed to receive 4.2v. Best case scenario: there's a voltage regulator converting these 4.2v to the voltage required by the bluetooth chip, so you'll be overheating the regulator and eventually burning it. Worst case scenario: instead of replacing the battery with a charger, you wire the battery in parallel with your charger, effectively throwing 5v directly into the sensitive lithium battery; there will be no current limitation and the battery will literally blow up. But let's say you used a bad DC source with high internal resistance so it won't be able to provide current enough to promptly blow up the battery; then the battery will blow up when fully charged because the charger will keep pushing it to 5v but it only should go up to 4.2v.

    I've managed to simultaneously get audio output and charge the battery without modifying the A2DP receiver. Rough instructions on next paragraph, advanced details on the subsequent ones.

    Get a usb extension cable (male a to female a) or a usb-to-4-pin-p2 cable, peel the insulation in the center. Cut the white and green wires, they are the left and right audio channels (no idea which is which); red wire is +5v to charge battery, black is 0v. If you plan to power the device using a USB port you don't have to cut the red and black wires, but you'll have to peel the black wire because it will be both the ground for charging AND the ground for audio. Solder the black, white and green wires to a regular P2 plug and you're ready to go. When you apply power you'll turn off the device, so remember to apply power before pairing it.
    If you plan to let it plugged constantly and don't want to kill the battery, maybe it's a good idea to somehow limit the charging voltage to 3.8v or 4v since lithium batteries age faster with greater voltages (haven't tested how much current is drained while charging, but maybe you could use a shunt circuit with a 3.9v zener like 1N748/1N5228/1N4622/1N4730/etc). You'll still need at least the main button of the a2dp receiver, so if you're gonna hide it somewhere you better route that button out.
    When I REALLY crank up the amplifier's volume I hear chirping digital noises while pairing and playing, maybe because I'm using a switched power supply, maybe because there's internal switching or digital noise on the a2dp unit; I believe both are occurring. When I turn off the power supply, chirping is gone, regardless if the a2dp device is off or on. When only the PSU is on, there's little chirping. When both the PSU and the device are on, there's more chirping than before. Maybe you can solve this with a low-pass filter on your PSU (there's even room for a small capacitor inside the a2dp unit) or on your L/R channels, or maybe the zener scheme I've mentioned already solves this.
    When the a2dp device is not charging, there's no DC on L/R channels, regardless if the device is ON or inactive.
    When it's charging and active (briefly after pressing a button, streaming audio, etc), there's 1.7~1.8v DC on L/R channels.
    When the device is charging but inactive, the DC component on the left/right channels drops from 1.7v to 0.1v or lower, the chirping noise disappears and then faintly appears a typical 60hz line hum noise (less than 0.1vac, apparently). This 60hz tone appears whether the charger is connected or not, so it's most likely a problem on my setup.
    Chances are your amplifier already has AC coupling capacitors on its input so the DC won't be a problem, but you'll definitely have DC if you use earphones (I've tested it; screws up audio quality, DC voltage dropped to ~1.4v, headphones were not damaged) and you'll also have problems on amplifiers without AC coupling on its input. To make your own AC coupling you can grab 6 electrolytic capacitors (probably you're good to go with 10uF, but for 16 ohms earphones you might want to use 10000uF, google 'coupling capacitor calculator'), "unpolarize" them (solder the negative leads of each two, so you'll end up with 3 capacitor arrays), then put each of them in series with each output (L/R/audio ground) of your a2dp device. If you're picky about audio quality, use ceramic capacitors (or even better, metalized plastic film/metal film). Maaaaaybe it's a good practice to put a big (in value, not in size) resistor in paralell with each array of capacitors, just to avoid big voltages building up.
    Can I use this space to annoy people?
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  • eliasalberto Thursday, July 5, 2012 12:05 PM Reply
    Now the curious part. If the device is charging and playing music, but then you disconnect the charger, it maintains the DC voltage on the L/R channels until the next time it goes inactive and then active again (or if you discharge it using a resistor - btw, you'll only have to discharge one channel, the other one will be discharged automatically). Even more curious: after discharging this voltage, if you short the +5V and 0V terminals (while not charging, obviously) the 1.7v on L/R channels will reappear. You can repeat this as many times as you want, but if you discharge the voltage keeping the 5v and 0v terminals shorted, the voltage wont come back UNLESS you un-short the 5v and 0v terminals and then short them again. What puzzles me most is: the voltage won't come back if you let the device go inactive (or shut it down) and then wake it up again. Let's find an explanation to this.

    I've mapped the pins on the usb plug, on the 4-pin p2 connector, on the connector inside the a2dp adapter and on a regular 3-pin p2 connector.
    Let's number the contacts on the USB plug. http://forums.techguy.org/attachments/192235d1305262850/patillaje_conector_usb.jpg
    (in case image goes offline: put the usb cable on a table, point the usb plug away from you, put the usb logo facing up and number the contacts on the usb plug from 1 to 4, left to right, and you'll have 1=+5v, 2=D-, 3=D+ and 4=GND)
    Now get the 4-pin P2 plug. The contacts, from TIP to BASE, are 2, 3, 4, 1.
    If you open the a2dp receiver, you'll find 4 metallic contacts over the p2 socket. Put the receiver on a table with the buttons facing down, put the P2 hole entrance pointing away from you and count clockwise: 1, 3, 2, 4. (top right contact is 1, mid right contact is 3, bottom contact is 2, top left contact is 4)
    On a regular audio p2 plug with 3 pins, from TIP to BASE, you'll have 2, 3, 1+4.

    In case you haven't noticed, a regular P2 audio plug will create a short-circuit between pins 1 and 4, which are +5v and 0v. That's why you CAN'T just pull a couple wires from the bottom of the BCK-08 and use them to recharge the device while you use a regular audio plug; you'll short the charging wires as soon as you plug the regular audio p2.
    This sort of explains why there's a DC voltage on the L/R pins while charging. I guess the +5v pin is loaded with 1.7v when not charging (maybe to feed the microphone/"action" button usually present in 4-pin headphones; i'm not sure because I don't have one to test here). If you plug a regular audio plug, this 1.7v pin will be shorted with ground and ground will "become" 1.7v. For the output to be unaffected, this DC component is also added to the left and right channels.
    However, during charging, the ground must remain at 0v and obviously can't be shorted with the other pin, BUT the 1.7v compensation mechanism is active (probably it throws the dc offset at the L/R audio pins whenever there's current on the 1.7v/5v pin), so if you try to use anything without AC coupling (such as a headphone) while charging, this DC offset will go to the device which is not prepared to deal with it, screwing sound quality and/or causing damage.
    However, I'm still unable to explain why this 1.7v isn't applied when the device comes back from shutdown/inactive state. I guess it should be applied in order to enable the device if what's plugged is a regular headphone, a headset with microphone, or a high-impedance output.
    Oh, btw: everytime you press the main button and hear a "click" on your headphones, that's because 1.7v has just been applied to them (and compensated by adding this DC component to the L/R channels).

    On a side note, my unit starts the periodic "low battery 3-beep warning" when the battery is at 3.7v. I haven't noticed at which voltage it effectively stops working.
    Another side note: Inside the charger that came with my unit, the two leftmost pins (ground and data+) are purposefully shorted using a via in the PCB, and they're also shorted with the metallic "cage" of the usb plug. I opened the a2dp receiver, applied +5v to the appropriate pad and then applied ground to any of those two pins, and the device lit up the charging red light.
    Can I use this space to annoy people?
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