Audio signal between digital devices shuts down one of the two devices

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
Hi everybody. I'm not sure if this is the correct forum section so i apologize in advance if I'm mistaking.

i'm trying to send audio from a digital device/bluetooth receiver to a Raspberry Pi's audio input through cable (Rpi's bluetooth device is unusable).


As soon as i plug the 2 devices togheter with the cable, the bluetooth device completely switches off.
IF i connect them BUT i keep the Ground of the audio signal unconnected, then all is working. Why and how could i solve this?

Both of this 2 devices are powered from the very same power supply that provide enough current but also when they were powered separately, i had the same issue

I will upload a drawing to be more clear.
 

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MisterBill2

Joined Jan 23, 2018
8,736
This is an educated guess, which is that the "ground" on at least one of those devices is not what I would call "ground". It seems that you are powering both the bluetooth device and the R-berry device from the same power supply and that the audio commons are at different voltages. Some demented designers us the positive as a common side.
But if the problem was the same when you used different isolated supplies then the cause is a bit different, and the solution will be to use a capacitor in series with the audio signal line.
 

BobTPH

Joined Jun 5, 2013
3,308
Most likely the output if the Bluetooth module is bridged and intended to drive a speaker. It is okay to connect either of the outputs to your input as long as the ground is the same. Just don’t connect the other one to ground, which shorts it and could damage the board.

Bob
 

MisterBill2

Joined Jan 23, 2018
8,736
Most likely the output if the Bluetooth module is bridged and intended to drive a speaker. It is okay to connect either of the outputs to your input as long as the ground is the same. Just don’t connect the other one to ground, which shorts it and could damage the board.

Bob
I had not considered that it might be a speaker driver output. That is possible, and if that is the case, then certainly the advice is correct. And a series capacitor is a good idea.
 

BobTPH

Joined Jun 5, 2013
3,308
Agree. Probably not needed, because there is one on the input already, but if there is not, it is needed.

I would wire up the plug to bring the ground of the Bluetooth board and one if the outputs through a capacitor, say 10uF, with the + going to the output if polarized.

It might also work better with a small load, say a 33 Ohm resistor across the two outputs.

Bob
 

Jim Brooks

Joined May 19, 2014
4
if you are are using the 3.5mm Jack on the Raspberry pi it is a four way connect wired as show below the sleeve is used for the video not ground and ground is on ring 2.
Also the audio is output not input



1621434936473.png
 

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Thread Starter

jacopo1919

Joined Apr 12, 2020
41
Thanks for your help, really.

But if the problem was the same when you used different isolated supplies then the cause is a bit different, and the solution will be to use a capacitor in series with the audio signal line.
Well, i was unexact in the description. The problem was the same when i was powering the bluetooth device with a battery and the Raspberry with its power supply BUT their Ground was connected.
This would agree with your first sentence of the fake ground (even if I don't deeply understand it).
i will try with a capacitor in series.. i believe i understand how it can be useful.

Most likely the output if the Bluetooth module is bridged and intended to drive a speaker.
The bluetooth device just receive a signal and sends it out to the Raspberry via cable. No speaker has to be driven at this stage

if you are are using the 3.5mm
I'm using the input of an Hat sound card named ''Pisound''
excuse me if I didn't state it out in the message
 

BobTPH

Joined Jun 5, 2013
3,308
The bluetooth device just receive a signal and sends it out to the Raspberry via cable. No speaker has to be driven at this stage
I didn’t say there was. I said “was intended to drive a speaker”. The two wires of a bridged amplifier will drive a speaker, but cannot be connected to a ground referenced input, as you discovered.

Bob
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
I didn’t say there was. I said “was intended to drive a speaker”. The two wires of a bridged amplifier will drive a speaker, but cannot be connected to a ground referenced input, as you discovered.

Bob
ok i understand now.
now the issue seems disappeared by placing a 10uf in series between the audio grounds but i don't believe is the best practice. i'm afraid noise is rising up.
I'm trying to understand if there is something else that could be done in such a situation.
I'm indeed thankful for the suggestions though :)
@MisterBill2
 

BobTPH

Joined Jun 5, 2013
3,308
That is not what we were suggesting.

The ground of the Pi input should be connected directly to ground on the the Bluetooth board. The audio input on the Pi should be connected to one of the speaker outputs of the bluetooth board through the 10uF capacitor.

I also suggested that it might work better with a 33R resistor across the speaker outputs. Try it with and without.

Bob
 

crutschow

Joined Mar 14, 2008
27,184
I'm trying to understand if there is something else that could be done in such a situation.
You could use a hum-loop isolator audio transformer, such as this, from the two Bluetooth audio outputs, to provide isolation.
You can still connect the Bluetooth and PI's grounds together.
 

MisterBill2

Joined Jan 23, 2018
8,736
The other alternative would be to have a "line level" output from the bluetooth device, so that no connection to the speaker would be required.
Actually, I don't think that we ever got a decription of what the BT device was. That probably matters a bit.
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
The other alternative would be to have a "line level" output from the bluetooth device, so that no connection to the speaker would be required.
Actually, I don't think that we ever got a decription of what the BT device was. That probably matters a bit.
is wireless actually but that shouldn t change the matter
the ''receiver'' is a sort of thing similar to this one
https://www.alibaba.com/product-detail/IN-C043-1-Ucuz-Portable-Wireless_60823653187.html.
I'm feeding the board as described above
 
If it's because of the bridged output, you can just use one side of the output and ground. if you pick the "wrong side", the output will be inverted. Doing, the one side thing, the gain will be down by a factor of 2.
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
That is not what we were suggesting.

The ground of the Pi input should be connected directly to ground on the the Bluetooth board. The audio input on the Pi should be connected to one of the speaker outputs of the bluetooth board through the 10uF capacitor.

I also suggested that it might work better with a 33R resistor across the speaker outputs. Try it with and without.

Bob
the 2 devices already share the ground, they are powered from the same source. the issue occurs when i connect the cable (so the grounds) over the audio path out of the first device to the raspberry audio input.
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
If it's because of the bridged output, you can just use one side of the output and ground. if you pick the "wrong side", the output will be inverted. Doing, the one side thing, the gain will be down by a factor of 2.
i would be happy to learn more if you can give me a key for a google research :)
 
See https://en.wikipedia.org/wiki/Bridged_and_paralleled_amplifiers

Say you had two amplifiers that shared a ground for the speaker. When you "bridge" those two amplifiers, you apply the innput sidnal to one amp and an inverted input to the other amp and you take the speaker output from the two (+) terminals. When one amp is zigging the other is zagging (i.e. 180 degrees out of phase).

A car amp non-bridged and a 13.6V supply might put out +-6V to the speaker. Bridging, makes that closer to +-12V. Power goes up by the difference in V^2. That's also why you see 4 ohm and 2 ohm car speakers. No expensive switching power supply to get the power up.
 
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MisterBill2

Joined Jan 23, 2018
8,736
After reading through all the posts again I see that we have no actual description regarding the output connections from the BT device, except "Audio cable" . That tells us very little. Is it a connector intended to have a speaker connected? Or a headphone connected? Or possibly it is a Bluetooth transceiver, so that it can send as well as receive. We have none of that information, and so all of the responses have been guesses based on what some guess the connection to be. The fact that the device gets a wireless signal is not part of the issue.
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
After reading through all the posts again I see that we have no actual description regarding the output connections from the BT device, except "Audio cable" . That tells us very little. Is it a connector intended to have a speaker connected? Or a headphone connected? Or possibly it is a Bluetooth transceiver, so that it can send as well as receive. We have none of that information, and so all of the responses have been guesses based on what some guess the connection to be. The fact that the device gets a wireless signal is not part of the issue.
the output of the wireless receiver is a headphones output. the audio cable I was talking about is 3.5mm to 6.35mm. I could send you detailed photos as well but all is very messy around here and I doubt that it could be helpful.
i thought that was an easy answer question, i neglected some details at first since i thought to make it easier--> it turned out to be the opposite
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
See https://en.wikipedia.org/wiki/Bridged_and_paralleled_amplifiers

Say you had two amplifiers that shared a ground for the speaker. When you "bridge" those two amplifiers, you apply the innput sidnal to one amp and an inverted input to the other amp and you take the speaker output from the two (+) terminals. When one amp is zigging the other is zagging (i.e. 180 degrees out of phase).

A car amp non-bridged and a 13.6V supply might put out +-6V to the speaker. Bridging, makes that closer to +-12V. Power goes up by the difference in V^2. That's also why you see 4 ohm and 2 ohm car speakers. No expensive switching power supply to get the power up.
thanks a lot. i Will read more into this :)
 
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