Why does a Bluetooth pairing fix ground loop hum in my circuit?

Audio Components Amplifiers
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I’m powering some small amplifier modules with a 12V AC wall adaptor which is causing the speakers to buzz. This is 100% a ground loop hum problem because when I powered the amps with batteries, there was no buzz.

This isn’t a problem though, as I only use the system with Bluetooth, and as soon as I pair the Bluetooth receiver, almost all of the hum is eliminated. With a Bluetooth pairing and without streaming anything, if I turn the volume up very high and place my ear directly in front of a speaker, I can hear a little bit of hum. Can somebody please enlighten me and explain how pairing the Bluetooth receiver with a Bluetooth transmitter fixes the circuit’s problem with ground loop hum?

Edit - I’m assuming this is a ground loop buzz and not an induced radiation hum. I got the terms and explanations from here - https://www.psaudio.com/pauls-posts/hummin-along/.

Whatever the below noise is, it is mostly eliminated with a Bluetooth pairing.

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Lets start from the beginning.

What amp modules, what power supplies, what source(s)?

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If I have to, I can link a pic of my convoluted setup, but I think my description of the circuit should suffice.

Edit - as a new user, I am only allowed to post a maximum of two links, so please go to the Parts Express website and query 120-040 for the AC adapter, and then query 320-612 for the amplifier modules. I’m using the power supply to power both amp modules.

I cut the tip off the power adapter and split the output with a 2 to 4 lever splicer.

I just noticed that the speaker buzz goes away as soon I power on the Bluetooth receiver, even if it isn’t paired. It does, however, have to be in receiver mode. If it is in transmitter mode, the buzz remains.

The source is either a Taotronics TT-BA07 Bluetooth receiver/transmitter in receive mode, paired to the same device in transmitter mode, or the same Bluetooth receiver paired to my iPhone, or the same Bluetooth receiver powered on and in pairing mode, or the same Bluetooth receiver powered on, just before it enters pairing mode.

https://fccid.io/2AFDGBAXX/User-Manual/15-TT-BA07-UserMan-pdf-2956728

The Bluetooth receiver then connects to a 3.5 mm audio cable, then to a coupler, then to a splitter. Each splitter goes to a 3.5 mm cable, which then connects to a terminal block panel mount connector. I wired one connector only to left and ground, the other to right and ground, and each connector goes into one of the two amp modules. I wired the two speakers accordingly.

Not high-fidelity system, but it was a fun proof of concept project to create a stereo system from two mono amps. It’s also an emergency power loss system, as I can easily and quickly swap out the power supply with two 9-volt batteries.

My audio cables are all shielded, so I’m assuming the multiple paths to ground are what is causing the speaker buzz.

Thanks

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Most likely a ground issue. This should tell you a lot:

The bluetooth is your problem based on this:

Do you have another source you can use to see if you’re still having problems? Cable you can plug between your PC headphone jack and the amps?

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Edit - At everybody looking at the topic. Some forum spam bot deleted my last reply and the reply is awaiting moderator approval, which is why we can only see portions of my previous post in the sections where @Jedi quoted them.

Appreciate the last reply, but we aren’t on the same page right now. I know this is a ground issue. The Bluetooth receiver isn’t the problem, it’s a solution. The receiver has a switch and can also be used as a transmitter, so I mentioned that when it was in transmitter mode, it didn’t fix the hum.

I did go ahead and bypass the Bluetooth receiver by connecting a radio directly into the system, but the buzz continued, even when I powered the radio on, so something about the Bluetooth receiver being connected to the circuit is magically eliminating most of the buzz.

I wasn’t posting to ask for a solution to the buzz. I started the topic to get an answer as to why the Bluetooth receiver eliminated the buzz.

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Don’t you think you ought to know what’s causing the buzz in the first place to understand why the blue tooth can be a remedy? It’s all about circuit schematics.

1 Like
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Are the potentiometer bodies (on the amp boards) grounded? If not, they need to be.

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I have addressed this. Your entire initial reply is now a few posts up here.

Sorry!

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Thanks

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I don’t fully understand the science but seems to me like the whole setup was a floating ground input without the Bluetooth receiver being powered on which picked up interference from the power supply, and when the Bluetooth receiver was powered on, the existence of the source negated the hum.

@Jedi - looks to me like the potentiometer bodies on the amp boards were not grounded. I tried to ground them but was having difficulties.

I tried fixing the buzz by replacing the 2 prong AC adapter with a 3 prong AC adapter, but that didn’t help

For clarification, part of the original post was in error when I stated that all of my audio cables were shielded. The wires going into the amplifier channel and ground inputs were insulated 22 AWG wire, however, when I replaced them with shielded audio cable, the buzz didn’t go away. When I removed the input cables completely, the buzz persisted, albeit at a lower volume.

I solved the buzz problem with this setup, which doesn’t require the Bluetooth receiver to be powered on:

I split the main output of power negative with a conductor terminal block and wired up the ground prong of a 3 wire grounding plug with one end. With that plug in an outlet, neither speaker buzzes when powered on without the Bluetooth receiver being powered on.

I tend to come with unusual solutions to problems. Is my solution a known hack that’s been used to eliminate hum or buzz in audio circuits?

So, I solved the problem, and now if I ever decide to use the system without Bluetooth, the speakers will not buzz, but I still don’t understand why the powered Bluetooth receiver eliminated the buzz.

@Job_Jalink - knowing the fix to my buzz problem which doesn’t require powering on the Bluetooth receiver, as well as the additional information I provided, are you able to explain what the powered Bluetooth receiver did to eliminate the hum.

Also, I’m thrilled that I solved the root problem, but can you please explain exactly why this works, as well as why switching to the 3 prong adapter did nothing to eliminate or reduce the buzz?

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Hi dragon49,
First of, congrats on finding a working solution!
Before responding to your questions: I’m not an electrical engineer and my answer’s based on my high school and general understanding.
From what I understand the original amp design doesn’t have ‘true’ ground. Therefore, the plus and minus are relative to eachother, thus giving opportunity to pick up hum. Apparently, switching on the blue tooth is ‘equalising’ the plus and minus to a level equal to when having true ground.
Probably, you have been researching yourself. Anyway, when trying to understand the common cause of hum, I did some investigating and stumbled upon this link: https://www.psaudio.com/ps-how/how-to-find-and-fix-hum/ from which I comprehend you have reached a similar solution as with the AC Chester plug.
This link I also found helpful understanding: https://www.techwalla.com/articles/how-to-eliminate-amplifier-hum-from-speakers. I don’t quite understand what you did when applying the three pong connector. What did you connect the ground prong with? It seems that you created true ground by using the ‘main output of power negative’ and connecting it to ground with a separate wire, although I don’t really understand what this does electrically. It also seems you might combine this solution with your first three wire grounding plug connection.
In the above some relevant information might be missing for full understanding.
I hope this helps providing further understanding, but please do fill in where our common understanding of the schematic can be further augmented and ask anything to further deep dive into the matter.

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I used a Conductor Terminal Block to split power negative between the amplifier connections and the plug. The three-prong plug has different screws for ground, positive, and negative. I just connected the extra negative wire to the ground screw.

Not sure what you mean by suggesting I combine this solution. I had already tried switching the two prong power adapter with a three prong one and that didn’t eliminate the buzz.

Appreciate the reply and congrats. I’m proud of having found the solution but would like to know why it works. Hoping somebody can explain it.

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When I understand correctly, at first the ground of the three prong plug wasn’t connected to anything (and didn’t work for you), now it’s connected to the power negative (and it’s working for you). Great.
From my understanding, I unfortunately can’t give a better explanation. So, if anyone else would like to give it a try, please have the floor.

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Not sure if you have the complete picture.

I’m now using two different plugs. The 12V AC adapter with two prongs goes into one outlet and this is connected to both amplifiers. The 3 prong plug that is only connected to power negative (to its ground prong) from the AC adapter is plugged into a different outlet.

I had earlier attempted to eliminate the buzz by switching to a different power supply that had a third grounding prong, but that setup didn’t eliminate the buzz.

Appreciate you trying to figure this out. I’m sure there are electronic gurus reading this that are trying to figure out how to phrase an explanation that I would understand. :slight_smile:

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Thanks for the update. Just wondering whether the adapter with a third prong is really grounded. It might not be, leaving you with wrong impressions. And when grounded, your call remains.

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Good catch. I wish I still had the adapter to look at it again, but the problem was that I cut the wire (to get the tip off) too far back and removed an RF choke that prevented the switching adapter from causing interference with nearby radios. I didn’t want to be an RF polluter, so I trashed the unit.

But, when I split and stripped the wires, I don’t remember seeing anything other than positive and negative, and when I look at images of the product, I only see two wires going into the plug, so maybe the third prong didn’t serve any purpose and was indeed not connected to ground.

This also brings up a follow-up inquiry. If the adapter would have had a ground wire, could I just have used that one plug and joined negative and ground?

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Presumably and IMO, everything would have been alright in the first place. Now you’ve worked up to a similar solution.