I'm sick of the noise

[Dragon Tamer]

I have been working on a radio for my iPod for a couple of months now and a very irritating problem recently showed up. When I have the iPod plugged in to charge and the radio turned on there is a lot of feet-back from the iPod. It only happens when the iPod is charging. I don't think that my filter cap is too low of a value, but I'm going to try adding second cap to see if that helps at all. I can't tell you what the frequency or amplitude of the feed back is because I don't have my oscilloscope. But my multimeter says its somewhere between 15-20Hz, but it sounds like it can sometimes go as high as 100Hz. I've tried just about everything to fix the problem but nothing seems to work. Adding filter capacitors after the 5V regulator actually made the problem worse, adding an inductor made it impossible for the iPod to charge (I don't know why).

If anyone has any ideas about how I can fix this problem please let me know. Better still if you can show me the circuit that they use in the BOSE sound docks or some other high quality iDock that works to. (Or you could put me in touch with someone who works there)

 

[ben7]

Hey, I built an amplifier for my ipod and it makes a high pitch squeal. I know it is the ipod cause I only hear the squeal when the screen is lit up. It might be from the battery charger circuit inside your ipod. Maybe a filter can remove that squeal. What amplifier chip do you use? I used an LM386.

 

[Blueteeth]

I haven't seen the inside of an ipod, but I know that most modern portable (ie: battery powered) audio players have switched mode power supplies in them to generate all the required voltages, as well as charge the battery. The 'charger' you plug in to charge the battery is nothing more than a power supply, the circuitry which controls the charge is in the device itself.

I am unsure what you mean by 'radio for your ipod'... perhaps a short range FM transmitter? It could of course be a ground loop problem, but since you mentioned it only happens when charging the thing, sounds to me like the internal SMPS used to charge the lithium battery inside is kicking out some nasty noise - which isn't always conductive (via the connections/traces/powerlines). Have you tried sheilding?

Ben7: interesting, I'll assume the LCD screen is LED backlit. For efficiency purposes they put the LED's in series, and use a boost converter to step up battery voltage to the LED string voltage - no doubt this is your squeal. However, seeing as how I doubt commercial ipod amps have the same problem, they must be doing something you are not Perhaps ferrite beads, or more likely, the amp is screened. Like the previous poster, it could be that the noise isn't actually being transmitted through the connections, but rather through the air. Stick the amp in a grounded metal box and see what happens

 

[ben7]

I don't think it is transmitted through the air though. I think a filter would be needed. Maybe the power from your computer, DragonTamer, is noisy, and it is leaking into the ipod amplifier somehow.

 

[ben7]

I don't have a lot of experience with filter circuits, except from making power supplies for audio purposes.

 

[Dragon Tamer]

The radio is actually an amp for the iPod, I only said radio because that's what most other people understand. Amp is actually an old radio/amplifier (it does both) from the 80's that has been modified to work with my iPod. I have had a problem with the high pitch squeal that Ben described but that seemed to disappear once I added a series inductor with the power line.
(By the way, my iPod is the 5th gen nano, which has an LED screen instead of an LCD screen)

What I have set up now is an ICL7663 LDR adjusted to 5V, the output of which goes through a 100µH RF choke which seems to be doing a pretty good job at muffling the noise, but the noise is still there. The attached circuit is of what I currently have set up to charge the iPod.

Some of the things that I have tried are:
-standard 7805 regulator
-LDR regulator (currently used)
-series regulator
-shunting regulator
-low pass filter before regulator
-low pass after regulator
-high pass before regulator
-high pass after reg
-band pass before and after reg
-charge pump to isolate the iPod from ground (very desperate attempt)
-filter caps (which made it worse, or it wouldn't charge at all)
-filter inductor (best by far)

The only thing that I haven't tried yet is a switch mode power supply, but I have tried using a 555 timer and a MOSFET to supply the regulator with a second noise source at a much higher frequency than the audible noise that is driving me insane (the 555 was putting out about 220kHz). I don't have much experience with switch mode power supplies, so can you give me a summery of the pros and cons of these power supplies?

I have attached the schematic of my current power supply, the only change that I have made to it since it was made is that the output of all of the 12V regs are connected via a 100Ω 10W resistor, and no that's not the cause of the problem. The charger is connected to where it says "To radio, VU, etc..."

I will also give the shielding a go, though I'm not too optimistic. I also thought about adding a series inductor to the input of the amp since it has an isolation cap but no inductor, do you think that might help?

 

[Blueteeth]

Wow, I must say... I've seen a lot of schems in my time, and hand-drawn one are rarely that clear and readable!

I have only glanced at the schematic, which I may look at more closely later on, so right now I'm just going to blurt out some thoughts.

Firstly: Just checked wiki about the 5th gen nano. Uses a TFT (lcd) display, LED backlit. TFT's tend to be rather quiet when compared to the new OLED displays - which have to multiplex much higher current since they are powering the LED's as pixel, not just liquid crystal segments. So, I'm thinking the display itself is probably fine

Seems you have tried a LOT of standard methods to get a 'clean' supply, but I am still unsure of your basic setup. I mean, you have a nice schematic there, but is your amp powered separately form the iPOD? Does it charge the iPOD? I see a max712 in there, with some batteries, which I'm guessing is for portability (the 712 being a cool on board charger for those batteries) ... is this what you meant by 'putting it on charge'? Or did you mean.... you plug your iPOD into the amp, whilst the iPOD itself is charging?

Apologies for all the questions

About switched mode power supplies. Generally, they are very noisy, switching high peak currents through PCB traces/wires, and inductors and so radiating RF/EM all over the place. There are fixed frequency PWM SMPS, which have advantages, as they don't vary their frequency to maintain efficiency, you can easily filter out the noise. I wouldn't start using one of those until you've got rid of your noise, because it will probably just 'add' to the problems, making it harder to narrow it down. So firstly, I guess trying to spot whats going on.

Noise is either conductive, going though the power lines, or transmissive, magnetic fields. As you've got some heavy filtering there, it seems if it is conductive, it's going to require an larger LC filter, 100uh, depending on the value of the capacitor looks a bit low to me.

It could be transmissive: that is, when your ipod is on charge, even though the noise from the on board SMPS charger (in the ipod) is well filtered from the audio out, the connections are picking up the noise via EM, and transmitting it through your amp, where of course its amplified. That means that the noise is getting into your signal path before your amp! However, this would be noticeable if you were to use headphones as well.

Maybe I'm over complicating things, but it could be noise from a beat frequency from two noise sources. Say, when your ipod charges, it kicks out say 240kHz. Sure your amp will heavily attenuate that, but what if your power supply for your amp produces a heavy line at 245kHz? They mix to produce 5kHz, which you can't filter out, as its slap bang in the audio band. - I@m out of my depth here though.

Ok, so, a coupe of questions which might help myself, and others narrow things down (as in things *I* would try).
1) Charging the iPOD with it playing through headphones. Leaving your amp out of it. Still noise?
2) iPOD going to your amp, both running off batteries. Any nose?
3) iPOD -> mains powered, amp.
4) Using a very cheap, off-the-shelve wall wart instead of your power supply. And running your amp off that. Charging your ipod at the same time and playing tunes. Sure, the wall wart WILL be noisier... but it won't be exactly the same noise (frequency, level etc..) as your power supply. If it sounds clean you know your filter circuits are doing jstu fine, and its the frequency of the noise thats giving your grief.

Again,I could be totally off here, perhaps I@m slow but I'm still unsure exactly when the noise rears its head.

 

[Dragon Tamer]

To answer your questions in order:
1.) No, when the iPod is charging with the home made charger, there is no noise using the headphones
2.) With the iPod and the amp running off of the battery there is noise, right now the iPod does not charge right away, usually needs to be connected to the carger several times before it will charge (sometimes not at all with the batteries). I'm blaming the filter inductor, because once it's been removed, the iPod behaves normally.
3.) I don't quite understand this one but I'll try to explain it, The radio is turned on and off using a CMOS switch (4027 driving a transistor) the iPod charger is worked much the same way, except when the AC power is applied, the transistor is bypassed so the iPod would charge regardless.
4.) If the radio is powered with a wall wart, then there is no noise at all, and the same goes for the iPod, if it's charged with a commercial charger, no noise.

If it would help, I can post the schematic of the "switches" for the circuit I'm using.

 

[ronv]

Maybe just a block diagram of how everything is hooked up would help. If you could include the switches and audio path in that would be useful.

 

[picbits]

The radio is actually an amp for the iPod, I only said radio because that's what most other people understand. Amp is actually an old radio/amplifier (it does both) from the 80's that has been modified to work with my iPod. I have had a problem with the high pitch squeal that Ben described but that seemed to disappear once I added a series inductor with the power line.
(By the way, my iPod is the 5th gen nano, which has an LED screen instead of an LCD screen)

What I have set up now is an ICL7663 LDR adjusted to 5V, the output of which goes through a 100µH RF choke which seems to be doing a pretty good job at muffling the noise, but the noise is still there. The attached circuit is of what I currently have set up to charge the iPod.

Some of the things that I have tried are:
-standard 7805 regulator
-LDR regulator (currently used)
-series regulator
-shunting regulator
-low pass filter before regulator
-low pass after regulator
-high pass before regulator
-high pass after reg
-band pass before and after reg
-charge pump to isolate the iPod from ground (very desperate attempt)
-filter caps (which made it worse, or it wouldn't charge at all)
-filter inductor (best by far)

The only thing that I haven't tried yet is a switch mode power supply, but I have tried using a 555 timer and a MOSFET to supply the regulator with a second noise source at a much higher frequency than the audible noise that is driving me insane (the 555 was putting out about 220kHz). I don't have much experience with switch mode power supplies, so can you give me a summery of the pros and cons of these power supplies?

I have attached the schematic of my current power supply, the only change that I have made to it since it was made is that the output of all of the 12V regs are connected via a 100Ω 10W resistor, and no that's not the cause of the problem. The charger is connected to where it says "To radio, VU, etc..."

I will also give the shielding a go, though I'm not too optimistic. I also thought about adding a series inductor to the input of the amp since it has an isolation cap but no inductor, do you think that might help?

The thing that strikes me about that schematic at first glance is the base resistor for the 2n3055 is 3.3k. Considering the gain of a 3055 this seems rather high. Putting it into a simulator gives me very poor regulation/no regulation at currents over around 240mA.

Changing it to a 1k resistor gives regulation to around 700mA.

Just a thought

 

[Dragon Tamer]

The thing that strikes me about that schematic at first glance is the base resistor for the 2n3055 is 3.3k. Considering the gain of a 3055 this seems rather high. Putting it into a simulator gives me very poor regulation/no regulation at currents over around 240mA.

Changing it to a 1k resistor gives regulation to around 700mA.

Just a thought

Your thought has been noted, and taken into account. I also noticed that adding a 100nF capacitor to the AC input of the bridge rectifier really helped to reduce the noise.

 

[adown]

I had this problem also with an amplified system on my motorcycle. I solved it with a PI filter on the input and output of the regulator. Small ceramic caps and ferrite beads. What Gen and type of IPod.

 

[Menticol]

What about connecting the iPod charger ground to the amplifier's ground?

I had the very same problem when connecting an old stereo and my laptop computer -powered via battery charger.

Just connected a cable between the ground pin of the charger plug and the ground of one of the RCA jacks on the rear of the stereo. The noise was gone.

 

[DerStrom8]

Guys, this post is 2 years old....

 

[Menticol]

Whoops!, you're right Matt! I'm sorry.


A small "Old Thread"warning flag would be great to warn sleepy people like Adown and me.

 

[DerStrom8]

Whoops!, you're right Matt! I'm sorry.


A small "Old Thread"warning flag would be great to warn sleepy people like Adown and me.

They do have warning flags, but only for the first new post on an old thread. Adown should have gotten one.

 

[ChrisP58]

They do have warning flags, but only for the first new post on an old thread. Adown should have gotten one.

Does the warning flag come up before the new post goes up?

 

[DerStrom8]

Does the warning flag come up before the new post goes up?

It comes up when you click "reply" on the post.