What are they caused by and how to eliminate crackles in bluetooth headphones

Good morning. I have the following problem: I connected the XX.Y BTT009 Bluetooth transmitter to the Denon PMA-720 ae amplifier via the headphone output. This set was designed to transmit sound to the JBL Synchros E50BT bluetooth headphones. When listening to music every few minutes at irregular intervals, the signal is disturbed (something cracks, hisses, etc.), after a while everything returns to normal, and after a few minutes the signal breaks down again. The distance between the headphones and the transmitter is usually about 2m. When the headphones are connected to a phone or laptop, the problem does not occur. What could be the cause of this problem and how to eliminate it. Thank you very much in advance for your help

If there is a Wifi network at home, change its broadcasting channel, this usually helps.
It's best to switch to channel 1 or 11.

From what I can see, I have channel 11 set, I'm about to change to 1 and see if it helps

Added after 1 [minutes]:

Unfortunately, it didn't help, I checked on several different channels

Are these crackling sounds also heard when silence or a single monotonous signal is transmitted?

NO. When there is silence, there are no crackles.

Could a colleague connect these headphones to the "Pre-out" of the amplifier - for testing purposes?
It is true that it is a 2xCINCH output, but it should not be a problem.

Of course I can, I'll try it right now

Added after 5 [minutes]:

On the "Pre-out" output, the headphones work but very quietly and the sound is not clear

Hmm, I don't think the crackling problem was present when connected to this "Pre-Out" output.

stoppaleniu wrote:

On the "Pre-out" output, the headphones work but very quietly and the sound is not clear

Hmm, the signal level on this output depends on the position of the amplifier's volume potentiometer.
Have you tried changing the volume?
The speakers can always be turned off.

Theoretically, you could also use the "REC-OUT" output.
The difference between this output and the "PRE-OUT" is that the signal level on the "REC-OUT" is independent of the volume setting on the amplifier.
Also, it can be used provided that the headphones have their own volume control.

I tried increasing the volume on the pre-out, but at almost maximum volume, the sound is barely audible. Headphones have their own volume control so I will try to connect to the rec-out output

I tried the operation of the headphones on the rec-out output, it works fine, but after a few minutes the interference also appeared

Hmm...

stoppaleniu wrote:

When the headphones are connected to a phone or laptop, the problem does not occur.

This makes me wonder...

The only difference is that the Denon uses the mains power directly, and the above-mentioned devices do not.
Who knows, maybe it's mains-borne interference.
Maybe the BT transmitter wakes up for some reason?

Maybe a colleague will check the suggestions below and answer them;

Does the socket where the Denon is connected have a ground pin?
Is the amplifier powered by a power strip with some sort of noise filter?
Can a friend do a test with the amplifier connected to a different socket?
Could a colleague make a test by connecting these headphones to another mains-powered device, e.g. a TV?
The amplifier has a separate ground terminal on the rear housing. Is it connected to ground? Please try it.

Can there be any device in the installation in the apartment / house that periodically turns on / off?
Does your friend have any LED or fluorescent lights? If so, please turn them off for a test and check the operation of the headphones.

stoppaleniu wrote:

I tried the operation of the headphones on the rec-out output, it works fine, but after a few minutes the interference also appeared

Was the amp gain turned to "0" with this test?
If not, please check it out

It is probably a dysfunction resulting from the incomplete compatibility of different Bluetooth versions. I have a similar situation with a phone and a speakerphone in the car. The range decreases to a dozen or so cm and then it's OK. As soon as they move away, there are crackles, noises, etc.
I propose to do a test by reducing the distance between the transmitter and the headphones and whether the situation improves or does not have any effect.

Does the socket where the Denon is connected have a ground pin? Yes, but generally in the entire home network I do not have grounding, only phase and zero
Is the amplifier powered by a power strip with some sort of noise filter? Yes
Can a friend do a test with the amplifier connected to a different socket? Tried, no improvement
Could a colleague make a test by connecting these headphones to another mains-powered device, e.g. a TV? I'll check tomorrow as I don't have a working TV right now
The amplifier has a separate ground terminal on the rear housing. Is it connected to ground? Please try it. How to try? What's going up there?

Can there be any device in the installation in the apartment / house that periodically turns on / off? NO
Does your friend have any LED or fluorescent lights? If so, please turn them off for a test and check the operation of the headphones. I have and after disconnecting from the power supply, I have not observed any interference yet, but I will write after longer tests

Added after 5 [minutes]:

Polon_us wrote:

The only difference is that the Denon uses the mains power directly, and the above-mentioned devices do not.

I also tried connecting to an external sound card in a computer powered by usb.

Added after 1 [minutes]:

stoppaleniu wrote:

Does your friend have any LED or fluorescent lights? If so, please turn them off for a test and check the operation of the headphones. I have and after disconnecting from the power supply, I have not observed any interference yet, but I will write after longer tests

However, after prolonged listening, the interference also occurs with the LED lighting disconnected

Added after 1 [minutes]:

Interference is in the form of crackling sounds with very short but frequent pauses in sound reproduction (approximately 0.5 seconds - 1 second)

Added after 2 [minutes]:

zmyslonyy wrote:

It is probably a dysfunction resulting from the incomplete compatibility of different Bluetooth versions. I have a similar situation with a phone and a speakerphone in the car. The range decreases to a dozen or so cm and then it's OK. As soon as they move away, there are crackles, noises, etc.
I propose to do a test by reducing the distance between the transmitter and the headphones and whether the situation improves or does not have any effect.

This is a possible reason because the transmitter has bluetooth version 2.1 and the headphones have 4, but after getting closer (about 10 cm from the transmitter) the interference also occurred after a few minutes.

Hmm, one thing came over me...

stoppaleniu wrote:

When the headphones are connected to a phone or laptop, the problem does not occur.

Was this BTT009 transmitter used in these trials?

No, because both the laptop and the phone have their own bluetooth

Added after 16 [minutes]:

I connected the transmitter to the mp3 and the phone and the interference also occurred. Could it be that the transmitter is faulty?

stoppaleniu wrote:

No, because both the laptop and the phone have their own bluetooth

That's what I just realized.

stoppaleniu wrote:

I connected the transmitter to the mp3 and the phone and the interference also occurred.

I was going to suggest it.

stoppaleniu wrote:

Could it be that the transmitter is faulty?

That's what it looks like.
If a friend has the opportunity to exchange for another, I would suggest doing so.
Please check what BT version your headphones have, as well as what protocol they support.
And from this angle find the right transmitter.

I will try to return the transmitter under warranty and buy another one. I consider the matter closed for now. thank you very much everyone for your help

Added after 6 [minutes]:

And one more question. This headphones support BT: 3.0, A2DP v1.3,AVRCP v1.5, HFP v1.6, HSP v1.2 The old transmitter has Bluetooth 2.1+EDR, A2DP, AVRCP. What parameters should the new one have? BT 3.0 or later?

stoppaleniu wrote:

What parameters should the new one have? BT 3.0 or later?

I would approach this issue in a different way.

A colleague is sure that the headphones work with the phone and laptop. You can check which versions of BT and AVDP they have installed.
And from this angle I would look for a suitable transmitter.
This, in my opinion, is the best solution.

Who knows, maybe the current transmitter meets the requirements and is simply damaged.

I don't know at what stage my friend is with this transmitter, but one more thing came to me ...
I've had a "late start" lately :(

Question - were the headphones + transmitter - phone and / or laptop turned on during the tests with the amplifier?
If so, did they have BT enabled?

If so, could a colleague do another test?
It would consist in temporarily disabling the BT function on both the phone and the laptop.
After that, the colleague would test the operation of the transmitter connected to the amplifier again.

Here I want to make sure that the headphones were not interfered with by these devices.