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Backdriving an Audio Source

03/06/2019 8:20 AM

Suppose I have a relatively small audio source (Bluetooth earbuds, to be specific), and I have the audio lines hooked up in such a way that under most circumstances, the audio source provides an output voltage without being opposed by anything other than the resistance of a pair of headphones.

However, for convenience purposes, the audio source's output (in both cases where the audio source is playing and not) would occasionally be connected to the output of another (larger) external audio source (like a cell phone or a laptop).

Thus, should I have any concerns about accidentally frying the small audio source with the output from the large one? If so, what would you recommend for an easy way to switch between audio sources (without unplugging anything, preferably)?

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#1

Re: Backdriving An Audio Source

03/06/2019 8:49 AM

This is a very confusing question...Bluetooth earbuds are wireless speakers...you keep saying 'audio source' for everything...Bluetooth by definition transmits with low power radio waves @ 2.45 GHz frequency...

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#3
In reply to #1

Re: Backdriving An Audio Source

03/06/2019 8:59 AM

Unfortunately, I don't have time to do a more detailed diagram, but essentially I plan to cut the actual earbuds/speakers off of the earbuds, then hook up the output wires to a male 3.5mm jack so I can connect the Bluetooth radio to any speaker (i.e. a non-wireless headset). However, I don't want to have to unplug the Bluetooth receiver every time I wish to switch to a wired headset connection, so I'm contemplating just wiring the female AUX connection for the headset wire, the Bluetooth receiver's output, and the speakers all in parallel. That said, I know that due to differences in volumes (and therefore voltage), some current could go from the other audio source to the Bluetooth board, or vice versa (though I'm mostly concerned about the smaller, cheaper Bluetooth board being fried).

I tried to phrase the original question in general terms so as to be more useful to others with a somewhat similar problem (because my specific case might be difficult to draw general lessons from). Apologies if a diagram would help, but I'm at school and won't be free for another six hours or so.

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#4
In reply to #3

Re: Backdriving An Audio Source

03/06/2019 9:02 AM

That description's still as clear as mud!

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#6
In reply to #4

Re: Backdriving An Audio Source

03/06/2019 9:04 AM

Diagram it is, then. I'll see if I can find a free block before school lets out, but if I do I'll likely be preoccupied with something else.

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#8
In reply to #6

Re: Backdriving An Audio Source

03/06/2019 10:31 AM

Here's a diagram of two setups (credit to Del the cat for inspiring the second one). Both diagrams are simplified to only have a single channel, but I figure that adding the other channel shouldn't be too complicated.

The one on the left was what I originally envisioned: a minimal circuit so I wouldn't have to construct a PCB, etc and could just plug in the cable and not worry about flipping any switches.

The one on the right has an SPDT (DPDT for stereo audio) installed on the V+ line so that only one source can provide current at a time. I presume that another pole for the ground channel isn't required because the circuit is already broken.

Thoughts?

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#22
In reply to #8

Re: Backdriving An Audio Source

03/07/2019 5:14 AM

Drawing on right looks good.

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#2

Re: Backdriving An Audio Source

03/06/2019 8:58 AM

"Thus, should I have any concerns about accidentally frying the small audio source with the output from the large one?"

No. The energy in the bluetooth (wireless) signal is tiny. The "size" of the audio input or output to which you are referring depends on the amplifier in the transmitting or receiving device (cell phone, laptop etc.) and its power supply.

Under normal circumstances, you cannot "overload" a bluetooth device by sending too large a signal.

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#5
In reply to #2

Re: Backdriving An Audio Source

03/06/2019 9:03 AM

I just posted a response to SolarEagle with important specifics about my implementation. By audio source, I don't mean the device that broadcasts the Bluetooth signal and provides the original audio stream, but instead the Bluetooth board's output to its speakers.

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#7

Re: Backdriving An Audio Source

03/06/2019 9:26 AM

Don't connect two outputs together without some form of appropriate mixer, switch, buffer etc.
Del

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#9

Re: Backdriving An Audio Source

03/06/2019 12:37 PM

Still not clear what you are trying to do. Why are you changing the speaker? The receiver and amplifier are built into the earbuds, they supply a proper amplified signal to the speakers, for the size and impedance of the speaker. If you change to a different speaker, unless they match, it's not likely to work. If you try to send an amplified signal into the existing earbud amplification circuit to drive a larger speaker, this likely will lead to high distortion and possibly overload the circuit, and burn out some components.

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#10
In reply to #9

Re: Backdriving An Audio Source

03/06/2019 12:55 PM

If you have a larger set of speakers that you want to make bluetooth capable you need an amplifier to feed the signal to, that then feeds the speakers...they sell adapters for $13 online...

The circuitry in the earbuds is going to be micro sized, and too small to alter...

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#12
In reply to #10

Re: Backdriving An Audio Source

03/06/2019 4:00 PM

Here is a versatile adapter that seems to work for what you want....

https://www.amazon.com/Golvery-Bluetooth-4-1-Transmitter-Receiver/dp/B01N4GK874

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#13
In reply to #12

Re: Backdriving An Audio Source

03/06/2019 4:55 PM

Further clarification: the final goal of this project is to (cheaply) modify an existing noise-canceling headset to accept Bluetooth connections. I want as compact of a final electronics assembly as possible so as to avoid adding bulk to the headset.

I did some research, and it appears that headsets of the size that I'm working with (similar to a Razer Electra V2) and earbuds of a size similar to the wireless earbuds I intend to salvage (Razer Hammerhead Pro V2) have similar impedance, so amplification, voltage, and current draw shouldn't be an issue. The reason I'm using the wireless earbuds is because

  1. Left side audio of the earbuds is sketchy and inconsistent due to a wire cracked by fatigue. The board works, but it is not properly connected to speaker output. As it is an extraneous part that I have already, I figured I'd rather repurpose it than discard it.
  2. I'm trying to avoid paying excessively for a product that won't integrate well into the final build I have in mind. If it's a custom build from the start, I'll have more control over the final form factor and features.
  3. This is a learning experience for me. I want to go through the design process myself and figure out how to get this to work, and using an off-the-shelf solution wouldn't be as satisfying. I'm a student, after all.

That said, this isn't completely uncharted territory. I was originally inspired by a YouTube video where a guy used knockoff AirPods and connected the wires for the speakers to 3.5mm male plugs, and then used those to wirelessly deliver audio to 3.5mm AUX devices, albeit separate devices were required to play both the left and right channels. Granted, those speakers doubtless had their own power sources and amps, but as detailed above the impedances will be similar enough that such measures will be unnecessary. Furthermore, my earbuds are wired and have both left and right audio from a single board, so I should be able to wire the board to a single 3.5mm male plug. Additionally, the hardware is highly generic (identical button presses/user interface and voice actress to many cheap wireless earbuds), and the earbuds housing is fairly large. I have also messed with the wiring of these particular earbuds in the past (attempted to resolve the original issue with the left size cutting out), and have relatively easy access to at least the left audio line, left audio ground, and the positive and negative leads of at least one of the batteries of the earbuds (either two batteries total, with one located in each earbud to maximize capacity, or only a battery in the left side because the PCB takes up too much space on the right side, but I digress because either will work).

In essence, I don't anticipate the biggest issues coming from connecting the Bluetooth board to the over-the-ear headset; the speaker drivers are similar enough that it shouldn't be an issue. My original question was summed up by the diagram I posted earlier: assuming that there's nothing else wrong with the diagrams and how each individual board interacts with the speaker, is there any reason why I should not create the configuration on the left? Somebody else gave their opinion, which I believe has merit, but I want to see what you think.

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#14
In reply to #13

Re: Backdriving An Audio Source

03/06/2019 5:18 PM

Something like this then...? Just unplug the portable powered bluetooth module, and plug in the headphones to the audio source of choice...I don't see a problem...

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#15
In reply to #14

Re: Backdriving An Audio Source

03/06/2019 5:32 PM

Definitely something like that. But once again, I don't like the idea of constantly unplugging and replugging the Bluetooth module; that's how audio channels get broken. I want to have a female 3.5mm jack on the headset that functions without physically removing the Bluetooth module. As such, could the left side circuit work just fine without frying any sound cards, or is something like the right side circuit required to ensure that no current flows between the sound cards? Again, it's simplified to be mono audio, but I believe the same principles apply.

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#16
In reply to #15

Re: Backdriving An Audio Source

03/06/2019 6:09 PM

Then you need a switch with 2 in's and one out...

like this...

https://www.amazon.com/dp/B0776JX4JQ/ref=psdc_172546_t2_B073GWCRP3

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#17
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Re: Backdriving An Audio Source

03/06/2019 6:28 PM
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#18
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Re: Backdriving An Audio Source

03/06/2019 7:21 PM
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#19
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Re: Backdriving An Audio Source

03/06/2019 7:58 PM
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#20
In reply to #19

Re: Backdriving An Audio Source

03/06/2019 8:53 PM

Alright, took a look at the links you provided. Interesting to see from the Instructables link that DPDT will work, but would have ground interference.

The switch that you linked to definitely would work, but I'm going to try and find a version that uses a toggle switch instead, as those make troubleshooting a little bit easier. My issue with the button is that one can't tell at a glance whether it's the switch being in the wrong position or an actual circuitry failure, let alone correctly set the input source without guessing. Looking around, there don't seem to be too any easy drop-in solutions that match my spec, so I think I'll get the appropriate parts from Mouser and make my own switchboard.

Thanks so much for your help, SolarEagle! How do you even have time for all of this stuff?

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#21
In reply to #20

Re: Backdriving An Audio Source

03/06/2019 10:19 PM

I'm retired for one, and currently paused between projects....we're experiencing our last gasp of cold weather here in central Florida...50° Brrrr plus I always manage to pick up some knowledge while helping others it seems....Good Luck on your project

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#23
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Re: Backdriving An Audio Source

03/07/2019 9:45 AM

Thanks for all the help (even though as a New Englander, I scoff at the idea of 50 degrees being cold)! When my project is done, I think I might send you a DM with the link to the writeup on Hackaday.io

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#11

Re: Backdriving an Audio Source

03/06/2019 2:14 PM

Separate the two cables from your earbuds and give each of them an audio plug for your sound system inputs. I suppose your phone or laptop don't have two inputs for stereo. In that case you need to wire the stereo adapter I presume keeping the wires separated as Del pointed out.

If there is any risk to your bluetooth receiver from being wired up to plug in as an input to a bigger amplifier, I don't know of it.

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