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Capacitor to eliminate speaker hum

audioguru

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Most Helpful Member
The LM386 amplifier IC produces 0.45W at low distortion into an 8 ohms speaker with a 9.0V supply. Then at that output power it heats with 0.5W and the battery provides 0.95W.
With a 12V supply, the output power at low distortion is 0.53W and the heating is 0.8W. The battery provides 1.3W.
0.45W sounds the same as 0.53W.
The wasted heating is why the ebay amplifier comes with a 9V battery connector.
 

audioguru

Well-Known Member
Most Helpful Member
Your photo has a pins connector on the amplifier that is used when the input is from a low level microphone.
With a guitar, the connector should be removed because it increases the gain then hiss and hum are increased.
 

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Hacdrag

Member
To everybody, helping out. I really appreciate all the advice. When the system is using Bluetooth, the hum isn't bad and I'm fine with the current state of my system, but would of course like to eliminate it, if possible, but this isn't a big deal. I'm also learning about circuits and electronics from all the replies. With all of that in mind, things are getting weird though:

audioguru "Simply disconnect the non-shielded input wires and listen to the hum disappear."
Unfortunately, disconnecting the non-shielded input wires makes the situation worse. Not only does the hum remain before I power on the Bluetooth receiver, it also doesn't go away once I power it on, as it does when it's connected. Does this shed any light on the situation? For best practices, I may replace it with shielded audio cable, but I'm not confident this will eliminate the hum.


Nigel Goodwin - ty 4 the terminology correction regarding my incorrect use of the term "splitter."

rjenkinsgb "OK, that is an ungrounded power unit." Can you please elaborate. My initial description was incorrect. The AC adaptor is only a 2-prong adapter, but if it were a 3-prong adapter, I don't understand why it would be considered ungrounded.

audioguru "
Your photo has a pins connector on the amplifier that is used when the input is from a low level microphone.
With a guitar, the connector should be removed because it increases the gain then hiss and hum are increased."

I have some familiarity with the board, and if I remove the jumper, this decreases the voltage gain from 46 dB to 26dB. I just removed both jumpers and I much prefer the sound with the extra gain. With the jumpers disconnected and the system powered on without the Bluetooth unit powered on, the hum is still present. It may be a tiny drop less, but irrelevant to me.

rjenkinsgb "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."

Ty -I may try this. Right now, the wires are literally on top of each other.

audioguru - regarding your suggestion to lower the voltage. I can try a 9V power supply instead. Maybe I'm wrong, but I do feel as if I prefer the system with 12V. It seems more powerful to me than when I ran it with 9V batteries, even the few minutes when I got the actual 9.72V from fresh ones. Maybe I like noise and imperfection. :)

I think I've covered everything. :)
 
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audioguru

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Most Helpful Member
The Bluetooth output probably becomes a low impedance when it is powered which shorts the hum interference picked up by the unshielded input cable.

Maybe the cheap Chinese amplifier's volume control picks up hum because its metal case is not connected to 0V?
 

rjenkinsgb

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Most Helpful Member
"OK, that is an ungrounded power unit." Can you please elaborate. My initial description was incorrect. The AC adaptor is only a 2-prong adapter, but if it were a 3-prong adapter, I don't understand why it would be considered ungrounded.

The DC output is completely floating, with no connection for mains ground or connection between mains ground and negative.

They do have some inherent capacitance between the "live" side and output via the transformer, and usually a Class Y capacitor to reduce high frequency interference emissions.

In effect, the PSU is adding a small AC voltage to the amp power side. The amp only sees a difference between 0V and its input, so that has a similar effect to injecting "hum" at an input.

The fact that you mentioned the hum goes away when you touch a volume control (grounding it via your body capacitance) implies that the floating PSU is the main problem.


I've just tried some power adapters to see what voltage is superimposed on the negative output:
The two switch-mode ones I picked up have high leakage voltage and are indistinguishable:

That's over 300V peak to peak! [Almost full UK 240V mains]
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.

PSU_Hum_1_small.jpg


And older one with a 50Hz transformer is not so bad; around 30V peak to peak. No suppression cap in that, at a guess, so only transformer capacitance.

PSU_Hum_2_small.jpg
 

Hacdrag

Member
The fact that you mentioned the hum goes away when you touch a volume control (grounding it via your body capacitance) implies that the floating PSU is the main problem.
The hum doesn't go away when I touch a volume control knob, it gets worse. Does this indicate something else is the main problem?
 

audioguru

Well-Known Member
Most Helpful Member
Connect the metal case of the volume control to your circuit's 0V.
 

Hacdrag

Member
Connect the metal case of the volume control to your circuit's 0V.
I'm still a relative "newbie" at all of this and don't know where my circuit's 0V is. I've attached the amplifier board schematic, as well as a photo of my circuit, and a separate photo of one of the amp boards. The only parts of the circuit that aren't pictured are the power supply, and Bluetooth receiver + 3.5 mm audio cable that sit outside the project box.

Please let me know where my circuits 0V is. Also, where specifically on the potentiometer would I make one end of the connection? I do have some basic soldering skills but would prefer not to have to solder anything. Could I just wrap some wire behind the washer and secure it by tightening the washer?
320-612-amp-board-schematic.PNG
project.jpg
IMG_9717.JPG
 
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rjenkinsgb

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The hum doesn't go away when I touch a volume control knob, it gets worse. Does this indicate something else is the main problem?
OK, that likely just means the metalwork is not connected to 0V, so it's coupling to the input.

A proper mains ground is still the answer.

For info, the power and input negative terminal is the PCB "ground"; they have an "Earth" symbol on the schematic. (It should really be "chassis" symbol, but a lot of people don't seem to know the difference).

That negative / PCB ground is what signals within it are referred to. You can add a wire from power negative to behind the pot washer on each amp.
 

audioguru

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Usually an amplifier is in a metal box as a shield which is connected to the circuit 0V. Then the metal case of the volume control is bolted to the box and 0V.
You probably cannot solder to the metal case of the volume control but try soldering a 0V wire to one of its washers.
 

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Hacdrag

Member
OK, that likely just means the metalwork is not connected to 0V, so it's coupling to the input.

A proper mains ground is still the answer.

For info, the power and input negative terminal is the PCB "ground"; they have an "Earth" symbol on the schematic. (It should really be "chassis" symbol, but a lot of people don't seem to know the difference).

That negative / PCB ground is what signals within it are referred to. You can add a wire from power negative to behind the pot washer on each amp.
Ty, and Ty for the diagram as well - I have to pick up some more 22 AWG wire as I'm out. I'll try the solution later in the week and reply back.
 

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.
I replaced the sets of 22 AWG input wire with shielded audio cable, yet the hum persisted. I ended up putting the original wiring back in place, because the shielded audio cable is too thick to comfortably fit in both sets of screw in connectors.

I also tried grounding the potentiometer base, but nothing I did eliminated the buzz. One of the attempts was a connection from 0V to the potentiometer washer nut. I also wrapped wire behind
the nut and tightened the nut around it.

For now, I no longer care about solving the root cause of the problem, as the Bluetooth receiver has the unintended benefit of eliminating the unwanted noise.

I do very much appreciate all the help though, as I've learned a lot.

Before I put the topic to bed, I do have an additional inquiry.

rjenkinsgb - had mentioned that my circuit had a floating input. I've been researching floating inputs and how they generate noise without a source. So, it appears that my powered Bluetooth receiver is a source and eliminates the noise. Can somebody spell this out for me? When powered on, does the Bluetooth receiver act as a virtual ground which then effectively cancels the ground loop condition, or does the device induce some magic phase cancellation into the circuit which eliminates the frequency of the unwanted noise. The receiver does have a rechargeable Lithium ion battery. Can this be injecting a frequency into the circuit that cancels the hum?

audioguru had mentioned that perhaps the Bluetooth receiver's low impedance was shorting the hum. Not discounting the theory, I just don't understand it.
 

audioguru

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Most Helpful Member
I think that the Bluetooth receiver is a high impedance when turned off or is not paired then the audio wire from it to the amplifier is an antenna that picks up electricity hum and other interference. Then when the Bluetooth receiver is turned on its output becomes a very low impedance that shorts the hum.

Are the audio and shield connections wrongly reversed?
 

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Hacdrag

Member
I think that the Bluetooth receiver is a high impedance when turned off or is not paired then the audio wire from it to the amplifier is an antenna that picks up electricity hum and other interference. Then when the Bluetooth receiver is turned on its output becomes a very low impedance that shorts the hum.

Are the audio and shield connections wrongly reversed?
I know these amp boards and the other components well, and my wiring is correct. The middle screw of the 3.5mm Stereo Female terminal block panel is ground. I haven't tried, but not sure if I would get sound from the speaker if I reversed ground and one of the channels. Please see my attachment.

Regarding my previous inquiry, can you please describe how a very low impedance would short the hum?

61T1aPXw+rL._SX522_.jpg


I don't feel like touching this unit anymore, but I have the components to replicate it, and will most likely replicate the circuit to troubleshoot and figure out how to create it without it producing any hum, even with the Bluetooth receiver being powered off.
 
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rjenkinsgb

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Most Helpful Member
The only thing the amplifier "sees" is a difference in signal voltage between the two input terminals.

With nothing to to hold the voltage down to near zero (like when the bluetooth module is enabled), a signal can be applied to either terminal and cause an output.

In other words it can act almost like a differential input rather than a single ended (signal plus ground) as it should be.

(That would not be the case if the amp negative / input ground was connected to true earth, which is why I keep bringing that up).

As it is, you have capacitive leakage from the power adapter superimposing an AC voltage on the amp 0V, which is also signal common. With the amp floating, that acts as an input.

That plus the stray capacitance of whatever is connected to the actual input terminal causes a small voltage drop across (between) the input terminals, resulting in a hum or buzz.
 

Hacdrag

Member
(That would not be the case if the amp negative / input ground was connected to true earth, which is why I keep bringing that up).
Appreciate the detailed reply. Regarding the quoted text, I've given up trying this for a while, as I broke two of the amp boards while trying to ground them. I physically broke one potentiometer by moving it out of place when placing a wire inside, trying to find a grounding spot. With the next board, I caused some sparks to fly when I touched the wrong part with a wire, then the volume was severely limited.
 
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audioguru

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Most Helpful Member
TRS (RST) connectors are frequently used with low impedance headphones that do not have shielded cables.

To quickly test to hear if the Bluetooth power supply or if unshielded cables are causing the hum:
1) Unplug the Bluetooth's power supply from the AC electricity but with the audio cables connected.
2) Unplug the audio cables from the Bluetooth but still connected to the amplifiers at the amplifier ends.

Most audio products and all power amplifier outputs have a very low impedance to 0V which is an short at low frequencies. The Bluetooth output is a low impedance to 0V when it is paired and is turned on. Then any hum interference picked up by an unshielded output cable is shorted to 0V.
 

Hacdrag

Member
TRS (RST) connectors are frequently used with low impedance headphones that do not have shielded cables.

To quickly test to hear if the Bluetooth power supply or if unshielded cables are causing the hum:
1) Unplug the Bluetooth's power supply from the AC electricity but with the audio cables connected.
2) Unplug the audio cables from the Bluetooth but still connected to the amplifiers at the amplifier ends.

Most audio products and all power amplifier outputs have a very low impedance to 0V which is an short at low frequencies. The Bluetooth output is a low impedance to 0V when it is paired and is turned on. Then any hum interference picked up by an unshielded output cable is shorted to 0V.
The Bluetooth power supply isn't causing the hum. The Bluetooth receiver is powered by a rechargeable LION battery and when the receiver is powered off, there is hum.

I've already tried step 2 and the hum remains.

I'll eventually get rid of the hum. Last night, I replaced the only two sets of wires (the ones that were directly connected to the amp inputs) that weren't shielded with shielded audio cable, yet the hum remained.
 

audioguru

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Most Helpful Member
Do the amplifiers produce hum when the input cables are disconnected from them?
 

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