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Capacitor to eliminate speaker hum

Hacdrag

Member
My stereo Bluetooth project is almost perfect, but when I switched from 9-volt batteries to an AC wall adaptor, this introduced a little bit of hum into the speakers. It's not that bad, but I'm wondering if I could eliminate it altogether by placing a small value capacitor between the power supply and the amplifiers.

What's the recommended way to handle this?

Ty
 

Dick Cappels

Active Member
That all depends upon the source of the hum.

Can you measure the amount of ripple on the output of the power supply?

What is the power supply voltage and what is the minimum voltage needed for your Bluetooth project?
 

Hacdrag

Member
That all depends upon the source of the hum.

Can you measure the amount of ripple on the output of the power supply?

What is the power supply voltage and what is the minimum voltage needed for your Bluetooth project?
How do I measure the amount of ripple on the output of the power supply? The power voltage supply is 12V which is the amps max rating. You had answered another question about my setup here - https://www.electro-tech-online.com/threads/am-i-driving-this-chip-with-too-much-voltage.163436/
Your opinion differed from another responder. Not writing to stir anything up, but so far, you were right. I ran the modules for nearly six hours last night at high-volume and no "magic smoke" or any other problems at all.

If the hum were worse and a 9V power supply would eliminate it, I would consider switching, but it isn't awful. I like the sound and feel of the extra power and intend on keeping the 12V power supply as part of the setup.

Getting to the core of the issue now. Listen to the first audio recording. This is the hum I hear if I turn the units on without having paired the Bluetooth receiver:

Warning - on some platforms Imgur auto-mutes the volume on videos and you have to click on the volume icon to hear the sound.

This is a normal listening volume. The higher the volume, the louder the hum. I raised the volume in the middle of the recording to prove this. As soon as I pair the receiver with a Bluetooth transmitter, (without playing anything) almost all of the hum is gone. I have to put my ear directly next to a speaker in order to hear a little bit of hum. Most of the time, I don't notice the hum when playing music, but it was clearly present in a few seconds of a YouTube stream last night. I am concerned though, that is degrading the quality a drop (especially if I listen at high-volumes) and if possible, would like to eliminate it.

This recording is the sound of the hum after I pair the Bluetooth receiver with a Bluetooth transmitter:


Again, this isn't a big deal, but if there is a simple way of eliminating the hum without lowering the voltage or otherwise reducing the power or volume, I'd like to consider the solution.

Ty
 

Dick Cappels

Active Member
I've tried two browsers and neither can pull in the external links you posted.

I only answered your question about power supply voltage. audioguru went an extra step and took into account what might result (excess power dissipation) if driving an 8 ohm speaker or lower. Not a conflict at all, only that audioguru, who is more experienced than me spotted an additional potential problem that should be considered.

Measuring ripple...I think that if you have an oscilloscope you can AC couple a vertical input and tell us what he peak-to-peak ripple looks like. If you don't have a scope, plan on buying on - they are cheap these days and may change your (electronic) life.

There are two common approaches to reducing ripple, which might be the cause of the hum are to place a HUGE capacitor across the output of your power supply (which might give your power supply grief but most are well protected against this sort of thing) and the other is to interpose a lower voltage regulator. In this case, something like and LM7809 which is a 9 volt regulator.

The LM7809 can reduce the ripple voltage to somewhere between 1/200th and 1/1,000th of what the ripple voltage coming in is. (too late at night here to look it up).
 

crutschow

Well-Known Member
Most Helpful Member
You could try a large capacitor (e.g. 1000µF,15v) across the power supply output terminals to see if that helps.
 

Hacdrag

Member
I've tried two browsers and neither can pull in the external links you posted.

I only answered your question about power supply voltage. audioguru went an extra step and took into account what might result (excess power dissipation) if driving an 8 ohm speaker or lower. Not a conflict at all, only that audioguru, who is more experienced than me spotted an additional potential problem that should be considered.

Measuring ripple...I think that if you have an oscilloscope you can AC couple a vertical input and tell us what he peak-to-peak ripple looks like. If you don't have a scope, plan on buying on - they are cheap these days and may change your (electronic) life.

There are two common approaches to reducing ripple, which might be the cause of the hum are to place a HUGE capacitor across the output of your power supply (which might give your power supply grief but most are well protected against this sort of thing) and the other is to interpose a lower voltage regulator. In this case, something like and LM7809 which is a 9 volt regulator.

The LM7809 can reduce the ripple voltage to somewhere between 1/200th and 1/1,000th of what the ripple voltage coming in is. (too late at night here to look it up).
Imgur links are always problematic with forums. I have no idea why.

The first video is the hum without the Bluetooth pairing. The second video is reduced hum with the pairing. My multimeter has been very helpful in my beginning journey into electronics, but I don't have an oscilloscope. I'll be sure to pick one up. I'll also try adding a HUGE capacitor across the output of my power supply.



crutschow "You could try a large capacitor (e.g. 1000µF,15v) across the power supply output terminals to see if that helps." Thanks - I'll try it.
 

audioguru

Well-Known Member
Most Helpful Member
It sounds like the buzz produced by the "antenna" interference picked up by an unshielded audio cable. Maybe the low output impedance of a paired Bluetooth reduces the buzzing.
Audio devices are always connected together with a shielded audio cable.
 

Hacdrag

Member
It sounds like the buzz produced by the "antenna" interference picked up by an unshielded audio cable. Maybe the low output impedance of a paired Bluetooth reduces the buzzing.
Audio devices are always connected together with a shielded audio cable.
I checked, and all my audio cables were shielded except for two that I wasn't certain of. I replaced them with shielded cables and the exact level of hum persisted. If it helps solve the problem, I noticed something else: Touching the potentiometer knobs increases hum. Without a Bluetooth pairing, and nothing streaming to the device, if I touch the metal potentiometer volume control knobs, the humming increases significantly, something like two to three times the volume. With a Bluetooth pairing, and nothing streaming to the device, if I touch the knobs, the hum goes from almost not present to noticeable, but still lower than without the pairing. With music streaming to the unit from a Bluetooth pairing, I can only hear the hum if I'm touching a knob, the volume is set very low, and an ear is directly in front of a speaker. When I listen at a normal or loud volume, and am touching a knob, I can't hear any hum at all, even if an ear is directly in front of a speaker. I'm assuming it is present though, and slightly degrading the quality of the sound.

I'm still going to try the huge capacitor across the power supply output to try and eliminate hum but was wondering if there are alternate solutions you can provide, based on the additional information I provided.

While we are here, is it 100% certain that the attached is not shielded because it isn't braided? All the other audio cables in my chain are braided, and I found my purchase links and the descriptions state that the cables are shielded. For best practices, I would have normally left the pair of shielded cables in the circuit that I replaced the suspect ones with, but I really didn't feel like squeezing 3-foot cables into the project box. I haven't measured the suspect cables, but they are less than six inches, probably three or four. I have decent Googling and Amazon skills, but I can't find any short, shielded 3.5 mm male to male cables. I'd appreciate any links you can provide, or any info on retailers that would stock these.

Anyway, I've been doing a lot of troubleshooting and a lot of typing :) for a very minor problem, but I'd love to solve it, am learning a lot in the process, and very much appreciate the help.

IMG_9625.JPG
 

Dick Cappels

Active Member
There are shielded cables that do not have braid shields, though they may have multiple strands of wire wrapped around an insulated center conductor, or which I've seen in a lot of TV cable, foil with a bare wire under it (for termination) wrapped around the center conductor.

When you bring your hand closed to the gain control you are probably demonstrating the "antenna effect" that audioguru mentioned.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Touching the potentiometer knobs increases hum. Without a Bluetooth pairing, and nothing streaming to the device, if I touch the metal potentiometer volume control knobs, the humming increases significantly, something like two to three times the volume.
Solder a wire between the metal body of the pot and it's ground terminal, it's VERY common to see pot's treated like that if the pot body isn't grounded by been bolted to a metal chassis.
 

audioguru

Well-Known Member
Most Helpful Member
Some shielded audio cables are VERY poor quality.
I bought and installed an expensive one I bought at Radio Shack that worked well. Then I saw exactly the same Chinese cable at The Dollar Store for 1.5 bucks so I bought it but returned it to Radio Shack and got my money back.:)
 

Hacdrag

Member
Keeping the same thread with a follow-up question. I've been researching ground loop hum and I think I figured out the root cause of my circuit's problem. I have a single AC wall adapter powering two amplifiers, each amp with its own ground. The two terminal block panel mount connectors have their own grounds as well, so I'm not sure of the exact cause of the ground loop hum, but I'm confident it's apparent here. Is my circuit a perfect example of how to create ground loop hum? If so, could I wire this differently to eliminate the hum without a Bluetooth pairing? I'm not going to throw money at this with a ground loop eliminator plug adaptor, or use a "cheater plug," just asking about the wiring inside the box.

Forgive my overuse of Flex Shot :), (this "As seen on TV" stuff really does work though!) and any disorganized or sloppy wiring. I'm just getting started with all of this.
project.jpg
 
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rjenkinsgb

Well-Known Member
Most Helpful Member
What power supply are you using? (And what country - what type of power plugs?)

Your amp does not appear to have a "ground" in the sense of a mains earth - which could be the cause of the problem, as it will be "live" with a trace of leakage though a typical ungrounded power supply & that is seen as an input; the difference between PCB 0V and other wiring.

It would explain the hum going away when you touch a pot, as you become a capacitive ground.

Try connecting the audio ground from one of the amp inputs to an earth on some appliance (exposed metalwork) & see if that eliminates the problem?

For info, you should where possible twist cables where you cannot screen them, to minimise pickup and noise coupling - you could do it with the input links, and/or shorten them as much as possible.

And also the power pairs to & from the connector.
 

audioguru

Well-Known Member
Most Helpful Member
Your ebay "cigar box amplifier" is missing important shielded (screened) audio input cables.
Then the ordinary audio input wire is an antenna that picks up interference hum from electrical wires all around you.
 

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Hacdrag

Member
Your ebay "cigar box amplifier" is missing important shielded (screened) audio input cables.
Then the ordinary audio input wire is an antenna that picks up interference hum from electrical wires all around you.
Regarding your attached picture:

"Is this an electricity AC transformer? Is its output AC or DC? It's just a splitter, so I can use one AC power adapter to power both modules. The positive and negative wires from my AC adapter come in from the right side of the box. Positive goes into red, and negative into blue. On the left side of the splitter, one set of positive and negative wires go into one of the amplifier modules, and the other set into the other amp. If I makes any difference, the power adapter wire is 22 AWG, as are the wires I connected to the other side of the splitter. The amp board power supply inputs are to the left of the potentiometer, as it is facing you.

"If these are audio input wires, then it appears they are not shielded audio cables." Correct, they are audio input wires, the ones that go into the right side of the amp boards. I'm just using 22 AWG insulated copper wire for the connections. These go from the terminal block mount connectors to the boards. So, you are suggesting that if I replace these sets of wires with shielded audio cables, it may eliminate the hum. I hadn't thought of that as it is much easier for me to strip the ends off of normal wire, but I'll try that as a solution to the hum.
 
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Hacdrag

Member
What power supply are you using? (And what country - what type of power plugs?)

Your amp does not appear to have a "ground" in the sense of a mains earth - which could be the cause of the problem, as it will be "live" with a trace of leakage though a typical ungrounded power supply & that is seen as an input; the difference between PCB 0V and other wiring.

It would explain the hum going away when you touch a pot, as you become a capacitive ground.

Try connecting the audio ground from one of the amp inputs to an earth on some appliance (exposed metalwork) & see if that eliminates the problem?

For info, you should where possible twist cables where you cannot screen them, to minimise pickup and noise coupling - you could do it with the input links, and/or shorten them as much as possible.

And also the power pairs to & from the connector.
I'm using this power adapter:


USA - normal 3 prong grounded power plug. It's in the same power strip as the rest of my appliances in that room are connected to. I may try some of your suggestions.
 

audioguru

Well-Known Member
Most Helpful Member
Simply disconnect the non-shielded input wires and listen to the hum disappear.
How many DC volts is the power supply?
What is a "splitter"? Why not just a terminal block instead?
 

Hacdrag

Member
Simply disconnect the non-shielded input wires and listen to the hum disappear.
How many DC volts is the power supply?
What is a "splitter"? Why not just a terminal block instead?
Good idea. I'll try that tonight or tomorrow. If the hum doesn't disappear, then there is no point in swapping out the non-shielded input wires with shielded audio cable. I'll reply with the result.

The power supply is 12V. My multimeter measures it as 12.38V.
It is a terminal block - https://www.amazon.com/dp/B087P2XFN1?psc=1&ref=ppx_yo2ov_dt_b_product_details
I just referred to it as a "splitter," as that's what it essentially is.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Good idea. I'll try that tonight or tomorrow. If the hum doesn't disappear, then there is no point in swapping out the non-shielded input wires with shielded audio cable. I'll reply with the result.

The power supply is 12V. My multimeter measures it as 12.38V.
It is a terminal block - https://www.amazon.com/dp/B087P2XFN1?psc=1&ref=ppx_yo2ov_dt_b_product_details
I just referred to it as a "splitter," as that's what it essentially is.
Not in electronic terms it's not :D
 

rjenkinsgb

Well-Known Member
Most Helpful Member
USA - normal 3 prong grounded power plug. It's in the same power strip as the rest of my appliances in that room are connected to. I may try some of your suggestions.
OK, that is an ungrounded power unit.

Try an earth from the amp [audio] input common "ground" to a true electrical ground, it may well cure the problem.

AG is technically correct about screened cable, it is good practice - but the small amount that is unscreened in your setup is not likely to be a cause of a significant problem. Shortening & twisting the wires is a good compromise.

I would suggest keeping them away from the power connections though - where they must cross, try to keep some separation and the two different sets of wires at right angle rather than near parallel.
 

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