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Old 1st April 2013, 10:22 AM   #21
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Well a small update. I've tried all of the above suggestions.

10uf, 1uf and 0.1uf decoupling caps bridged right from the power pins on the op amp to ground for both + and - power supply lines (all 0603 SMD multi-layer X7R caps).

Ferrite beads on not only the audio inputs and outputs but also on the ground and +/- voltage source.

Shorted both non inverting inputs to ground right at the op amp to eliminate the entire input path.

Temporarily removed the 150pf caps from the feedback loops.

Lowered the gain to 6.


Nothing at all seems to affect the amount and loudness of the interference except the position of the phone relative to the board and cabling. Changing the gain, changing input ground path load resistor value, adding the T circuit to the headphone outputs.

One interesting thing I noticed is if I force my phone back to 3G data connection all the noise goes away. The amp is dead silent even laying right on the back of the cell phone so it appears the problem is limited to 4G at least in this case.

Is it possible that the noise I'm hearing from the 4g radio isn't actually high freq RF noise being demodulated but perhaps some other signal bleed actually in the audible range?
If that's the case no amount of filtering that doesn't impact the actual sound of the amp will remove it.

Last edited by Amperage; 1st April 2013 at 10:39 AM.
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Old 1st April 2013, 12:07 PM   #22
JMFahey is offline JMFahey  Argentina
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Quote:
Nothing at all seems to affect the amount and loudness of the interference except the position of the phone relative to the board and cabling.
Quote:
Is it possible that the noise I'm hearing from the 4g radio isn't actually high freq RF noise being demodulated but perhaps some other signal bleed actually in the audible range?
If that's the case no amount of filtering that doesn't impact the actual sound of the amp will remove it.
I think it's RF bursts (actually coded data, but for us it's the same) which within a certain small distance fom the antenna are strong enough to be rectified by *any* PN junction (of which there are tens to hundreds available inside any audio device); such complex shape pulses are wideband by definition, so filtering amounts to kill audio.
Probably the only solution is to fully enclose the audio equipment in a Faraday cage ... and even so I suspect some strong RF will "travel back" through the earphone wires into the amplifier and be rectified ... in which case such "audio" will mix with the actual Music and be heard on the phones.
So far only defense against such interference is to move the transmitter away so induced voltage does not get rectified (<0.7V peak?)
I usually have my cellphone less than 2 Ft away from my CPU (which is *supposed* to be RF shielded such as to not leak digital noise outside) and I regularly hear through the PC speakers the "bzzt bzzt" sounds it makes every few minutes, probably the "I am still here" signal to the closest tower.
And it becomes crazy a few seconds *before* it starts ringing , so when it does I already have it in my hand.

I commented in other forums a very annoying "mystery" interference I had ***everywhere*** a few years ago.
Everywhere means Radio/TV/Phone/PC/anything.
My friends joked I would soon have it in the Fridge and washing machine
Sound was maddening : "silence for 40 seconds ... tick ... tick ... TICK ... BZZZZT .... BZZZT ... TICK ... tick ... silence .... "
One day a neighbour came and on hearing it commented: "are you repairing a Marine Radar?" (He's a ship Captain).
That solved the mystery.
I live in the old Buenos Aires port, less than 80 meters from the riverside, and they were repairing a Spanish fishing ship, anchored "at the corner of my house".
Looks like Ship Radars have 2 settings: "sailing" (full power) and "Port" (5W average)", because they need to "see" at most 200 meters away ... and they were (illegally) using it full power.
The ticking sound was the transmitted pulse, and the 40 seconds silence was when the antenna rotated away.

Last edited by JMFahey; 1st April 2013 at 12:10 PM.
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Old 1st April 2013, 03:45 PM   #23
qusp is offline qusp  Australia
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afaik, its a ping, I used to get it in 2 situations, when the phone was just about to receive, or send a call/message, or when the phone was forced to switch to 2G when it pinged the nearest tower. I moved to balanced portables, nothing just pure audio bliss now . there are other ways to tackle it, some amps are more affected than others, but balanced is an excellent cheat, if a bit pricey, as it wont work with just using a phase splitter unless its entering the circuit after the conversion to balanced; so you need a balanced dac too. otherwise I would suggest just swapping to something like an OPA1632 or THS4521, which take single ended input and output balanced.
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Old 1st April 2013, 04:35 PM   #24
agdr is offline agdr  United States
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Quote:
Originally Posted by Amperage View Post
That's a pretty sharp cut in max power output but if nothing else I try helps that's something not too difficult or expensive to try.
Here is a solution that I first leaned about from jcx here a couple of years ago, figure 1:

http://www.intersil.com/content/dam/...n11/an1111.pdf (opens PDF)

puts the second half of the OPA2132 in parallel with the first to nearly double the output current to 60 - 80 mA, right up there with one section of the NJM4556A. 1 ohm should work for the two output balancing resistors (Rbm1 and Rbm2), giving you just 0.5R output impedance looking back into it. You could just use a quad OPA4132, but I wouldn't recommend it for power dissipation reasons. Probably best to use two OPA2132s, one for each channel. The slight time delay through the second chip should be insignificant for audio.

Interesting about the tests! Here is another test to try. Tinfoil hat time. Cut a square of aluminum foil about 1 inch wider than your PC board in both directions, then put strips of 2 or 3 inch wide packing tape or duct tape on one side of the foil to insulate it. Then ground one corner with a test clip and just set the thing right on top of the PCB, insulated side down of course, and move the phone above the foil. If that helps, try cutting the foil square down smaller and smaller until it is just right above the chip and see how it goes. The RF can still get in under the edges, but the foil should cut the field strength down substantially if the problem is directly radiated energy into part(s) and/or PCB traces (as opposed to off-board wiring).

Last edited by agdr; 1st April 2013 at 04:52 PM.
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Old 1st April 2013, 05:22 PM   #25
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Cell phones are a PITA. The only practical solution is to physically move the phone away from the amp. Trust me,I've pulled my hair out trying to kill cellphone RFI - it ain't happenin. Certain phones/providers seem to be worse than others.
(I've never had this issue with my own cellphones,but I use an older non-smartphone,and the basic calling plan,no data or other BS.)

The problem is that it's pulsed data,big nasty square wave pulses,with harmonics as far as the eye can see.
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Old 1st April 2013, 05:39 PM   #26
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Noise Suppressing Sheets / Magnetic Sheets | Products - TDK

I have seen some of this TDK stuff attenuate a GSM signal more than a few dB with just a single sheet. The problem is it is very expensive, I think $75-100 per sq ft and it's not a sure thing depending on the mode of coupling.

You can try Laird HF ferrites, normal ferrites are useless at these frequencies. You can also try an X2Y C0G cap as an input filter.
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Old 1st April 2013, 06:02 PM   #27
hpeter is offline hpeter  Europe
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Originally Posted by Amperage View Post
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.
if phone has low signal, it can transmit up to 2W to antenna. that is quite a lot of energy radiated
learn to live with it
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Old 1st April 2013, 06:45 PM   #28
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Well if it's some type of low frequency pulses being sent as a result of the radio equipment in the phone changing power levels etc then I guess there isn't much I'm going to do about it other than move the phone further away or move to a better signal area

I'll play around with the grounded aluminum foil idea a bit. maybe ground to some water lines to test.

The amp that has the cell phone noise issues is inside a full aluminum shell with only openings for plugs and controls and is grounded to the PC chassis while plugged in charging but computer grounds are far from ideal. It's not like I can keep a portable amp earth grounded but just out of curiosity I'll give it a shot.

I'm also tempted to order a FiiO E6 and see if it's really immune to the issue or if perhaps the reviewer forgot to take his phone off of airplane mode .

It's in a plastic housing and from what I can tell by the pics I can see of the PCB it doesn't use any type of crazy chokes etc but if I had one to look at here and it's not affected by my noise problem it wouldn't be hard to study the board and see how they solved the puzzle.

I would have ordered one just to play with already but I've read some negative comments about build durability on it and I wanted to build an amp with enough voltage swing to drive 600ohm cans if need be. Also my amp lets me still use the in-line mic/remote controls on the headphones that have them. AFAIK most retail amps only have TRS (3 conductor) jacks and/or won't pass through the sleeve connection untouched.

Anyone reading this that has a FiiO E6 if you could sandwich it against your cell phone and see if any noise creeps through while the phone is transmitting that would be awesome (4g data connection preferably).

Last edited by Amperage; 1st April 2013 at 06:55 PM.
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Old 1st April 2013, 06:50 PM   #29
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Hey, don't forget the cellphone already has DAC and tiny headphone amp INSIDE, which isn't prone at all to the EMI agressor located SO CLOSE.

It's just the layot bug somewhere.

Get ferrite clamps/rings and put 'em on all the wires...
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Old 1st April 2013, 06:58 PM   #30
dmills is offline dmills  United Kingdom
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Thing us at cell phone frequencies it only takes a inch of wire to present significant impedance, earthing as such is not helpful.

Ensuring all connectors are bolted directly to the box, have 100pf caps across them right at the box (SMT ceramic MLCC is better then leaded disk) and have some inductance in the lines to the board will usually make it go away. Common impedance coupling is the kiss of death for immunity, and there is much to be said for using a 4 layer board to allow a continious ground plane.

Dont forget that pickup in the output wiring can be every bit as troublesome as pickup in the inputs,but with appropriate design and layout cell phones can be beaten.

Regards, Dan.
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