Iphone-charger-TV ground-loop.

[David_]

Hello.

I have a Iphone and when talking to people I like to plug it in to my TV with the 3,5mm audio jack so I can here them as if there where in the room and I can talk straight out to the as well.

It works great until I need to charge the phone because when I plug in the charger I start here a hum, the hum goes up and down in volym in sync with me turning the TV sound up or down, I directly thought that its a ground loop.

Not that the following changed my mind but even when I hold the charger in one hand and the phone in the other hand I get that annoying hum.

Does anyone know anything about why this hum appears and is there any easy way to enable me to charge my phone and use it with the TV?

Or is it just a ground-loop situation that I have to live with or build some isolation circuit?

Regards

- - - Updated - - -

Hmmm, the charger has no PE earth connection.

- - - Updated - - -

Might it be noise from the switcher in the charger perhaps?

 

[Audioguru]

I think you have acoustical feedback howling. A sound goes into your phone's microphone, is amplified in the phone and some of it comes out of the TV speaker where it goes through the air into the mic on your phone and the sound goes around and around.

Speakerphones use a "voice switching circuit" that prevents feedback by allowing only transmitted or received sounds to occur but not both at the same time. A Polycom speakerphone uses an expensive echo canceller circuit that cancels speaker sound from being re-transmitted.

Maybe the charger produces hum that is not passed by the telephone call but is passed by the TV. I doubt there is a ground loop because the TV and the charger are not grounded.

 

[Vbase]

I always thought that iphones are perfect. If you had a TV with narrow band speaker like the one in your phone then there wouldn't be a problem.
David, you have to make it your new project: 'the hum-less charger'. Because there is a risk in getting it wrong and damaging your iphone the safe way is to make 12V supply with no ripple and plug it to the iphone charger you use in the car.
Another way is to make your own RC high-pass filter and to add it to the cable that plugs to the back of your TV.

 

[David_]

Okey so ground-loop is out, but if it was acoustic feedback then it should stop if I take a pillow and try to smoother my phone.
I can surround the phone with any amount of insulation and it does not affect the hum.
I wonder how many would by a 'the hum-less charger'

But since the audio cable I use now have a ... whats the english word for when you cut a cable and then connect a new(or the same) cable?
I might as well try adding a RC high-pass filter, I suppose the cut-off frequency should be about 20kHz.

 

[kam1787]

.....when I plug in the charger I start here[sic] a hum, the hum goes up and down in volym[sic] in sync with me turning the TV sound up or down, I directly thought that its a ground loop.

Not that the following changed my mind but even when I hold the charger in one hand and the phone in the other hand I get that annoying hum.

Might it be noise from the switcher in the charger perhaps?

So it is only the charger? Does the hum increase/decrease as the proximity of the charger to the TV changes? Does the hum increase the more tightly you grasp the charger?

Sounds [no pun intended] like the charger's PWM is causing interference in TV's audio circuity [in the AM or IF band]. In another words the charger is acting like a RF transmitter.

Answers? Metal shielding. Don't use charger at the same time. Get a newer/better charger.

 

[Vbase]

Hi David.
It is low probability that the hum is transmitted but it is east to check, unplug the cable from the phone and if the hum stays the same then it is transmitted.
If there is no hum when the charger is unplugged from the mains then it can't be acoustic feedback.
Hi-pass filter is for cutting the low frequencies; 50Hz. Connect 1K resistor from the TV input to ground and series cap of 0.22uF between the output of the phone to the input of the TV. If it is a stereo connection then you need 2 RC filters. The value of the components isn't critical, you can use half or twice the value.
If it helps then audioguru can advise you how to make the perfect filter.

 

[Audioguru]

If it helps then audioguru can advise you how to make the perfect filter.

Hee, hee. There is no such thing as a "perfect" filter. Simply unplug the charger when you are on the phone.

If you make a second-order Butterworth Sallen-Key highpass filter circuit with its -3dB at 300Hz so adult males sound like chipmunks, then 50Hz would be reduced -32dB which is substantial. But I betcha the hum is at 100Hz which the filter would not attenuate much. Maybe the hum has a sawtooth waveform with many harmonics that the filter would not touch.

 

[Vbase]

Hee, hee. There is no such thing as a "perfect" filter. Simply unplug the charger when you are on the phone.

Thanks audioguru, we needed the approval of the top authority in audio.

If you make a second-order Butterworth Sallen-Key highpass filter circuit with its -3dB at 300Hz so adult males sound like chipmunks,

How come that even without the filters we sound like chipmunks.

 

[Audioguru]

How come that even without the filters we sound like chipmunks.

I get a phone call every day from an adult guy in Pakistan (they showed the huge call center on TV) who has a soprano voice that sounds like a chipmunk. He is selling "Duck Cleaning". I tell him to stop bothering me because my Ducks (QUACK QUACK) are clean enough. I bet he has never seen a air duct. 10 local duct cleaning companies have been fined for soliciting business on the phone. The companies thought they would not be fined because they did not make the phone calls that were from the other side of the world.

 

[Vbase]

10 local duct cleaning companies have been fined for soliciting business on the phone. The companies thought they would not be fined because they did not make the phone calls that were from the other side of the world.

I've made for myself a bell for the phone that ignores the first 7 rings and then rings. I learnt that those people that want to sell something ring 5 or 6 rings.

 

[David_]

The sound is stable and the only thing that alters it is if I turn up or down the volym on my TV, if I charge My phone and am holding it in my left hand and then put one finger on the loose 3,5mm plug with my right hand it hums just as much as if I hade them both plugged in the phone at once.
Oh yes of course I meant a low-pass filter, my brain is not always in the loop

I started this thread out of curiosity to find out what it is but the fact is that this really bothers me and needs a solution, my battery is so bad that every night I find my self with low battery and the need to talk to people(I don't have to but its like hanging out with friend even if there at there place and I'm at mine).

By the way, I have not been able to alter the hum in any way other then the TV volym.
But I will take a power nap and then solder in a couple of resistors and caps, when using caps in series with a AC signal to filter DC, how does the value impact performans?
does lower capacitance allow higher or lower frequency's to pass and vice versa?

Next time a seller calls you, interrupt him/her as much as you can while trying to sell them your own product (of imagination). They usually hang up quite quickly when they realize what your saying and after a couple of calls they stop bother with that nut job or whatever they might be thinking about you, why not sell them the same product only "better"
But seriously, its a good way of making them stop calling you.

 

[Audioguru]

To cut low frequency hum, you use a high pass filter that passes the highs. The filter must feed an impedance that is much higher than the resistor value. A series resistor and a capacitor to ground pass high frequencies and reduce low frequencies. The cutoff frequency is calculated simply with the formula "1 over 2 pi RC".

But it is a very gradual filter. The cutoff frequency (300Hz?) will be reduced slightly (-3dB) and 50Hz will be reduced only a little (-15dB). I think the hum is 100Hz which will not be reduced much with such a simple filter but low frequency audio will be cut (chipmunk voices).

You said there is the same hum when you hold the plug which is normal because your body picks up hum from all the electrical wiring all around you. Then you said you plug both into the phone, why two? Don't you use one 3.5mm plug connected with a shielded audio cable? The shield should be connected to the circuit ground so that it blocks hum from being picked up by the signal wire.

The problem with the sellers that phone me from the other side of the world is that there are thousands of them in each call center and there are many call centers. I never get the same person again.

 

[Vbase]

T
Next time a seller calls you, interrupt him/her as much as you can while trying to sell them your own product (of imagination). They usually hang up quite quickly when they realize what your saying and after a couple of calls they stop bother with that nut job or whatever they might be thinking about you, why not sell them the same product only "better"
But seriously, its a good way of making them stop calling you.

The other way is to ask the salesman for his home phone and when he answer that he doesn't like people calling him at home, you tell him: "now you understand how I feel!"

A series resistor and a capacitor to ground pass high frequencies and reduce low frequencies.

I guess you want to correct this one.

The problem with the sellers that phone me from the other side of the world is that there are thousands of them in each call center and there are many call centers. I never get the same person again.

Maybe you get the same person using different high-pass filter.

 

[Audioguru]

I guess you want to correct this one.
My statement: "A series resistor and a capacitor to ground pass high frequencies and reduce low frequencies."
OOps, I got it backwards. A highpass filter uses a series capacitor feeding a resistor to ground.

 

[David_]

Hello.

I've had forgotten about this thread, typical ADD behaviur couses me to not have tryed anything to get rid of the hum.
But I find this to be kind of on topic, I have instead of my phone connected to my televison my laptop connected in order to listen to music.
Then I connect a arduino Due which outputs a sinewave that I then measure with a oscilloscope, however when I touch the Oscopes probes ground sleve on a GND point on the due there is lots of noise induced into the speakers. It sounds as when you unplugg something or plugg in something and the speakers..... Its sounds kind of Chrshhhchrishchrish. I have no charger connected to the laptop so there is no GND connection through the power rails of the laptop.

I have very little understanding of these things, what would you say is to cause this sound?

 

[D.A.(Tony)Stewart]

The hum is due to charger SMPS modulation of input AC and feedthru leakage capacitance to floating output of DC in either or both TV and phone.

The common mode noise on unbalanced audio lines induces a differential noise.

The solution is either use large CM ferrite choke wrapped with audio cable or clamshell type or easier, ground secondary DC or audio or disconnect phone charger and above.

But then any ground faults in home may induce leakage currents to phone if you touch an unsafe ground.