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Capacitor to eliminate speaker hum

Hacdrag

Member
Do the amplifiers produce hum when the input cables are disconnected from them?
Just when I thought I was done unscrewing the box and disconnecting things. :)

Good question:

Yes, the amplifiers produce hum when the input cables are disconnected from them, but at a noticeable lower volume. With Bluetooth powered off and one set of input cables disconnected and the volume at 100%, 15 seconds of my iPhone's decibel meter reading with the phone flush against the speaker = AVG 54.5 decibels. Same test with the wires connected = AVG 69.5 decibels.

Edit - the room isn't a controlled environment, so I'm not sure how accurate or relevant the decibel readings are, but it is 100% clear that the buzz remains with the cables disconnected, and it is also 100% clear that the volume of the buzz is most-likely at least twice as loud with the cables connected, then with the cables disconnected.

Second Edit - I'm guessing this means that even though the rest of the wires are shielded, the shielding isn't powerful enough to block all the power adapter leakage. Last night, when I replaced the 22 AWG insulated amplifier input wires with shielded audio cable, the buzz persisted. Too much of a pain to put those thicker wires back in for another volume test, but if those cables reduced the volume of the hum, it wasn't apparent, so if it did, it wasn't by a lot.
 
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audioguru

Well-Known Member
Most Helpful Member
Good, you finally found the problems, if the metal case of the volume control is connected to 0V:
1) The traces on the pcb are picking up hum. Then the amplifiers must be inside a metal box connected to 0V.
2) The shielding of the input cables is poor quality and/or electrical wires are very close to the amplifier box.

I have built many amplifiers and many have way more gain than yours and none produce any hum.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
OK; to determine once and for all what the cause is, I found a place selling that exact amp module, with the circuit included - and replicated it.

The IC in use is an LM386N-3, probably somewhere early 1990s vintage.

ps. To earth your amp, just put an extra wire in to any of the power negative screw terminals and connect that to an appropriate external earth.

Video proof:
(It is HD, but it takes youtube quite a while to process anything above the initial low res release).

 
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JimB

Super Moderator
Most Helpful Member
What a star!
A nicely done practical demonstration.

JimB
 

Hacdrag

Member
OK; to determine once and for all what the cause is, I found a place selling that exact amp module, with the circuit included - and replicated it.

The IC in use is an LM386N-3, probably somewhere early 1990s vintage.

ps. To earth your amp, just put an extra wire in to any of the power negative screw terminals and connect that to an appropriate external earth.

Video proof:
(It is HD, but it takes youtube quite a while to process anything above the initial low res release).

WOW - You went above and beyond the call here! Your YouTube channel has one more subscriber. :)

Regarding external earth, can I connect the wire to any of the third prongs on my Power Strip/Surge protector?
 
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audioguru

Well-Known Member
Most Helpful Member
The LM386 was designed by National Semi of USA. It is still made but by Texas Instruments who purchased National Semi.
The JRC386 is an old version made by Japan Radio Co.
The NJM386 is a newer one also made by Japan Radio Co. but the N is for the word New. I do not know what is new.
Chinese assemblers use old and new Japanese ones or Chinese copies of them.

Nothing in the video is shielded (he said "not screened"). The buzz increases when the guy touches the volume control but the buzz stops when he touches the 0V wire or when the 0V is Earthed.

None of my many audio products are Earthed (the video guy called it Grounded) and none produce any hum or buzz. All my audio cables are high quality shielded.

Maybe when audio amplifiers have unshielded input cables are not shielded and volume control metal that is not connected to 0V then even a battery powered amplifier circuit produces buzz and hum unless its 0V is Earthed?
 
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rjenkinsgb

Well-Known Member
Most Helpful Member
None of my many audio products are Earthed (the video guy called it Grounded) and none produce any hum or buzz. All my audio cables are high quality shielded.
It's down to the small external switched-mode PSUs having filter capacitors from the DC input side to the output - so a trace of the AC voltage is directly coupled to the DC side, making it "live" but at quite high impedance.

eg. See CY1, the 100pF cap, at the top of the example schematic (page 4) for this PSU IC:

No normal audio amplifier supply works like that, so it's not a problem in purpose-made amps - but is exists in a large percentage of "wall wart" type PSUs.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Regarding external earth, can I connect the wire to any of the third prongs on my Power Strip/Surge protector?
Yes, absolutely - but be extremely careful that there is never any possibility of a misconnection!

I'd suggest a new plug with a wire permanently connected to the ground pin and sealed up; never a loose wire than is pushed in to a socket outlet, or anything else that could be confused in the future.

Also make sure the wire is heavy enough to not break in the plug, and has a proper strain relief where it leaves the plug body.
 

JimB

Super Moderator
Most Helpful Member
Chinese assemblers use old and new Japanese ones or Chinese copies of them.
Nothing in the video is shielded (he said "not screened"). The buzz increases when the guy touches the volume control but the buzz stops when he touches the 0V wire or when the 0V is Earthed.
None of my many audio products are Earthed (the video guy called it Grounded) and none produce any hum or buzz. All my audio cables are high quality shielded.
:rolleyes:


One philosophy of life is
"If you cannot say something nice, it is better not to say anything at all"

But, today in this thread, AG you are really excelling at being a right old sourpuss.

Someone (in this case RJenkins) has gone to the trouble of making a video of a good demonstration of the possible reasons for the OPs problems, and all you can do is is go off on another of your anti-sino rants and burbling on about earthing, grounding, shielded, screening, hum, buzz.
Sheesh!
Give us a break!

JimB
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It's down to the small external switched-mode PSUs having filter capacitors from the DC input side to the output - so a trace of the AC voltage is directly coupled to the DC side, making it "live" but at quite high impedance.

eg. See CY1, the 100pF cap, at the top of the example schematic (page 4) for this PSU IC:

No normal audio amplifier supply works like that, so it's not a problem in purpose-made amps - but is exists in a large percentage of "wall wart" type PSUs.
There's usually a high value resistor across the capacitor - and there are there to provide a static discharge path, to prevent the output from accumulating a high static voltage.
It's a very high impedance coupling though, so you cannot feel it if you touch it, and a proper electrical ground kills it completely - but it's present if nothing grounds it.

You certainly can feel it (at least most people can) - it's a VERY, VERY common complaint from customers - you just have to reassure them that it's supposed to be like that, and the 'tingle' is so small as to be completely safe.

Generally most TV (and associated) equipment is class 2 (no earth), so every item it floating in that way - even many audio amplifiers are only class 2 as well. Often though, it's earthed 'somewhere' by one item of equipment, or by the aerial system (if that's earthed - but there's no UK requirement for that).
 

Hacdrag

Member
Yes, absolutely - but be extremely careful that there is never any possibility of a misconnection!

I'd suggest a new plug with a wire permanently connected to the ground pin and sealed up; never a loose wire than is pushed in to a socket outlet, or anything else that could be confused in the future.

Also make sure the wire is heavy enough to not break in the plug, and has a proper strain relief where it leaves the plug body.
Safety is of course of the utmost importance and I want to do this as neatly as I can. If I just power this up with a 3-prong wall adapter, will this just solve the problem? No need to save $ and make my own plug. :) I'm just worried about getting something that will cause my amps to draw too much current. The one I'm using has these specs:

Output voltage: 12 VDC @ 300 mA

If I get something like this that has 3 prongs:

https://www.amazon.com/Facmogu-5-5x2-1mm-Switching-Transformer-Converter/dp/B09BQBWKKT?th=1 - Choose 12V 3A with 3 prong plug:

I'm assuming that the amps will only draw the power that they need. My internet research claims the setup shouldn't be a problem. I don't care about the polarity of the tip, as I'm going to cut it off anyway and just wire the positive and negative leads accordingly after I strip the ends.

Pls let me know if this is a workable solution.
 
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rjenkinsgb

Well-Known Member
Most Helpful Member
If I just power this up with a 3-prong wall adapter, will this just solve the problem?
There is no guarantee that the ground pin connects to the output side, so it may well still be floating.
It's less likely to pass so much "hum" as a two wire one, as the suppression caps etc. can [hopefully] connect to true earth - but no way of knowing without trying it..

The amp will only draw the current it needs, as you thought.
 

Hacdrag

Member
There is no guarantee that the ground pin connects to the output side, so it may well still be floating.
It's less likely to pass so much "hum" as a two wire one, as the suppression caps etc. can [hopefully] connect to true earth - but no way of knowing without trying it..

The amp will only draw the current it needs, as you thought.
Ty - I threw $14.43 USD at the problem to try and solve it. Should get delivered by late Tuesday and I'll reply to let you know how it works out.
 

Hacdrag

Member
rjenkinsgb - Getting closer to perfection!

Unfortunately, the three prong adapter didn't eliminate the hum without a powered Bluetooth module, however, your suggestion does. For one of the amplifiers, I split the power negative between the power negative input and a conductor terminal block. I shoved the other end of the split power negative wire into the third grounding prong of my power strip/surge protector (also works directly into a wall outlet.) and when I powered that amp without the Bluetooth receiver being powered on, the associated speaker did not hum!

So, now, I need a safe and clean permanent solution, since I'm not going to leave 22 AWG bare wire forced into the outlet hole. Would this work?

Could I purchase some kind of 3 prong outlet and only wire power negative to ground, then plug the entire thing into my power strip? For my test, I just split the power negative from one amp. Should be completely irrelevant to the result whether I do this with fewer steps by splitting the master power negative one more time, and then sending one end to outlet ground.

If my idea is valid, I'm assuming I could just use this. Pease confirm:


Guys with crystal radios use outlet ground all of the time and I'm sure they have safe, and proper fitting solutions, so if the above isn't viable, there must be a recommended, safe and secure solution.

I ended up putting my 300 mA 12V power supply back in and will save the new one for future projects that need to draw more current. I do have a question though. Please see the attached. I ended up cutting the wire before this thing, which I believe is a strain relief component. But since I'm not going to use the tip which I cut off, did I do anything wrong or make the power supply less safe?

IMG_9790.JPG
 
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audioguru

Well-Known Member
Most Helpful Member
The "thing" in the power cord is probably a filter to prevent the high frequency switch Mode Power Supply from causing interference to nearby radios.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Could I purchase some kind of 3 prong outlet and only wire power negative to ground, then plug the entire thing into my power strip? For my test, I just split the power negative from one amp. Should be completely irrelevant to the result whether I do this with fewer steps by splitting the master power negative one more time, and then sending one end to outlet ground.

If my idea is valid, I'm assuming I could just use this. Pease confirm:

Yes, the Leviton plug is perfect for the job.

Note that you should use reasonably thick stranded wire, similar to what's in power cords, and build up the diameter with sleeving or tape where the plug shell clamps to the cable outer when closed up, so the wire end cannot move or flex inside the plug as the free end of the wire is moved around.


The lump on the old cable is a ferrite sleeve, with a moulded strain relief over it. It's to reduce any possible radio interference, to help with compliance with EMC regulations.

Something like this type of thing:
 

Hacdrag

Member
The system with the earth grounding now works without any buzz! I'll wire up everything neatly tomorrow. I have to wait for the Flex Shot to dry. I just used some to affix another push in conductor terminal block to the project box. I love those things! With a temp proof of concept wiring job, I split power negative and wired up one end to the ground screw of the Leviton 3 prong grounding plug. With that in an outlet and the amplifiers powered on without the Bluetooth receiver powered on, there is no speaker buzz!

With the temp wiring, I used 16 AWG wire to connect to the grounding plug. If I can fit 14 AWG in it and around the screw, and in the terminal block, and in the side hole of my project box, I'll use it. Your point about building up the diameter is well taken. I did feel a slip as I was closing the unit. I'm going to pick up a barrel cord grip.

WOW, three weeks and three topic pages later, I have a solution, as well as a decent understanding of the root problem. I went through a heck of a lot of time and effort to fix a problem with these cheap amp boards, but in the process of converting a flawed floating input system to a proper earth grounded system, I learned a lot and am glad I went through it all. I use this as my bathroom sound system. It's powerful enough to get the sound through the shower and light fan noise.

Thanks for all of your help!
 
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Hacdrag

Member
There is no guarantee that the ground pin connects to the output side, so it may well still be floating.
The problem is solved, but I'm still learning here. Too bad I threw out the three prong adapter. Since I cut off the RF choke, I decided to be a good citizen and not be a future RF polluter.

The thing is though that when I split and stripped the wires, there were only two sections, and I don't remember seeing anything other than positive and negative. Pictures of the device from my purchase link also show only two wires going into the plug, so does this mean that the ground pin on the plug served no purpose? I can't imagine that cutting off the tip would have defeated the ground.

Also, if the adapter would have had a third grounding wire, could I have eliminated the buzz by joining power negative and power ground at the beginning of the chain, before sending those wires to the amplifiers? If not, could this have been handled differently just using one plug?
 

Hacdrag

Member
The problem is solved, but I'm still learning here. Too bad I threw out the three prong adapter. Since I cut off the RF choke, I decided to be a good citizen and not be a future RF polluter.

The thing is though that when I split and stripped the wires, there were only two sections, and I don't remember seeing anything other than positive and negative. Pictures of the device from my purchase link also show only two wires going into the plug, so does this mean that the ground pin on the plug served no purpose? I can't imagine that cutting off the tip would have defeated the ground.

Also, if the adapter would have had a third grounding wire, could I have eliminated the buzz by joining power negative and power ground at the beginning of the chain, before sending those wires to the amplifiers power negative inputs? If not, could this have been handled differently just using one plug?
 

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