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Old 29th March 2013, 07:17 AM   #1
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Default RFI problems on diy headphone amp.

Hello. I've been working on a couple headphone amp designs for weeks now. One is a design using a maxim max9722 headphone amp IC and the other is a slightly more complex build using NJM4556AD dual op amps. Both amplifiers share the same power delivery and charging system composed of a texas instruments BQ24072 li-po battery charger and STM6601 smart push button controller to handle soft on/off and low voltage protection. The 4556 based headphone amp also has a LTC3122 based DC/DC boost converter circuit configured to deliver +/- 12v to the 4556 op amp.

Both amplifiers are nearly silent almost no hiss or other odd noises I can attribute to the amplifiers themselves and I'm quite happy with the audio quality but I've been having persistent issues with RFI from my 4g cell phone when it's transmitting or receiving data within approximately 1 foot of either headphone amp.

I have read numerous guides on dealing with RFI and have implemented a number of suggestions. low pass filters on input and feedback circuits, ferrite beads, obsessive ground fill on both board layers. No matter what I try it doesn't seem to affect the signal bleeding in from the cell phone. I've held the amps right next to a number of other RF sources including routers, microwaves, AM/FM transmitters etc. and the only component that seems to introduce any noise at all is cell phones.


I've been a forum lurker for quite a while and I apologize for my first post being one asking for assistance but I'm kind of stumped currently on how to go about this problem and thought perhaps somebody with experience could give some suggestions.
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Old 29th March 2013, 09:47 AM   #2
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Pics of the device, both sides of pcb (stuffed/empty) will help.
I'd guess you've overlooked some sensitive loop...
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Old 29th March 2013, 09:09 PM   #3
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Sorry took so long. I've tried a couple other things including a test board that contains only the audio section to try and eliminate noise coming in from the rest of the circuit. it's currently running from 2 3.7v li-po batteries for testing to produce +/- 3.7v. Nothing has changed with the RF noise. Below is a current schematic for the test board.

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Old 30th March 2013, 01:37 AM   #4
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Oops. Actually I missed 2 input load resistors on the schematic that are in the actual circuit. The fixed schematic below.

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Old 30th March 2013, 09:56 AM   #5
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Schematics appears to be fine. The RFI thing is mostly layout/connections thing.

Please show the pics of actual devices from both sides of PCB, connected to your test equipment (this one matters too).
Have you tried the amp with batteries, headphones and portable source - do you still get that buzz?
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Old 31st March 2013, 03:01 AM   #6
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Here is the board layout top and bottom. The input is connected to a HTC Rezound 4g phone using a 12 inch shielded (braided copper + foil wrap) 3.5mm male to male TRS cable. The output is only driving various types of headphones. It's not used as a preamp or feeding anything above a 40ohm load so far.

Click the image to open in full size.Click the image to open in full size.

Thanks for the assistance by the way . I've been fighting with this for more than 2 weeks now. It's no wonder so many retail products have RFI problems.
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Old 31st March 2013, 05:20 AM   #7
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Just a couple of suggestions :-)

What you have there is basically a cmoy with a very high gain of 11, try reducing the gain to 2 or 3,

your virtual ground would benefit from a resistor divider, just the two caps will not work as well to divide the voltage

The 2.2K resistors should be much larger say 100K

good luck with the build

cheers
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Old 31st March 2013, 05:52 AM   #8
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yes, gain of 11 plus driving only low impedance headphones, a poorly decoupled power supply and highish impedance ground, are all things that can lead to RF interference. I would also suggest a common mode filter/choke part to replace the discrete inductors
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Old 31st March 2013, 07:52 AM   #9
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Sorry for the confusion. I only posted the output section of the schematic but if you look in the OP you can see I'm not using a virtual ground system. The ground is 0v ref common ground all the way through.

For testing purposes to try and isolate the RFI problem I built a board with only the output stage replacing the power end of the design with 2 li-po cells center tapped but in the full amp I'm generating a true + and - 12v using a LTC3122 DC/DC converter with output regulation. Not a fan of virtual grounds so I added the LTC3122 operating at 2mhz to the mix.

Again I've temporarily eliminated all the power supply circuit and went with a test board with only the amplifier section driven by 2 li-po cells in series. Sort of a divide and conquer approach. Since the RFI still exists with only 2 batteries powering it I know that it's not being introduced by the battery charging system or DC/DC converter.
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Old 31st March 2013, 08:13 AM   #10
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Also I started out with much higher standard "cmoy" resistor values but decided to lower them to tighten up op amp dc offset and lower hiss etc.

For the intended purpose of the amplifier a gain of 2 to 3 would end up being too low for the output voltage swing on the source device and quite a number of other cell phones and mp3 players. The amp is driving mostly inefficient larger low ohm headphones like the sony XB series or stuff like the Beats by Dre line . A gain of 3 wouldn't provide enough output with the max clean input level my rezound can provide.

Something like an iPhone has more clean voltage swing at the headphone jack and can get away with lower gain levels and still push the power output of the amp near it's limit.

Gains of 10 seem to be fairly common for the "high" setting on tons of existing amps. So I've been aiming on achieving that.

Maybe I should switch to a newer generation of op amp with better noise rejection?
The output section of my amp idles at around 23mA. A bit higher than I was shooting for anyway. Would like to get it down to under 10mA to extend battery runtime.
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