Noise problem

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slrkt
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Noise problem

Post by slrkt » Thu Mar 22, 2007 10:55 am

Need help with a noise problem. I am powering an audio adapter board via the 5&12vdc output from a small touch screen PC. The problem I have is when I play audio thru my adapter board there is an audible noise coming thru from the PC. I have confirmed its coming from the PC's power output by way of using an external power supply (noise goes away). My final design requires I use the PC's pwr supply for my adapter board so I was hoping for some suggestions on what components I should use to mitigate the noise issue. More info: The noise changes tones depending on what the PC is doing (ie: programs running, video playing) Any help or suggestions would be greatly appreciated. My plan would be to build the filter into the power cable between the PC's power output and my adapter board.

Robert Reed
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Post by Robert Reed » Thu Mar 22, 2007 11:51 am

If you swapped supplys and noise dissapeared, your plan is definately right. However, it may take a hefty filter in order not to drop too much voltage. Experiment yada yada.

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haklesup
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Post by haklesup » Thu Mar 22, 2007 12:08 pm

Use an audio isolation transformer commonly used in car audio to eliminate ground loops between amplifiers and the head unit. ANyplace that sells car audio equiptment should sell these. You probably don't need a big one for high wattage. Smallest they have is fine.

This kind of ground loop occurs when there is a difference of the voltage on the ground return of the circuit and the ground return (shield) if the audio connection when the same supply is used to power both. What you are hearing is the ground current flowing back to the supply causing tiny voltage fluctuations in the ground conductor and interconnects.

The isolation xfmr breaks the DC connection path along the audio cable. Using a different supply breaks the common ground connection at the supply side. Either solution will eliminate the noise.

Filtering the noise directly or improving the ground conductors are usually dead ends.

I had a similar problem using a portable DVD player in the car. I had it connected to the line in plug of the head unit. When powered by its internal battery it works great but when powered by a DC-DC car supply adapter, it buzzed horribly when the engine waas running. Solution, audio isolation xfmr.

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Externet
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Post by Externet » Thu Mar 22, 2007 1:34 pm

Hi.
Your plan is the right one, building the filter on the supply cable.
Or at its end, entering the circuit; install a choke coil in series at the power input terminals of your circuit, plus an extra capacitor in parallel.
Use the amount of inductance needed to choke the ripple, and of wire gauge enough to carry the current.
A defunct PC power supply may be a donor for chokes.
Make sure the interference is not entering via air, electromagnetic waves, from a fluorescent backlight or whatever.
Miguel

slrkt
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Post by slrkt » Thu Mar 22, 2007 7:46 pm

Thanks for all the input. I did try an off the shelf car audio noise filter with no improvement. I'll give it another go (funny how things work when you try them the second time). But my gut feeling is the noise in in a different frequency range if that makes any sense. I had a similar plan regarding harvesting parts from a power supply I plan to pull the filter circuit from a pc mother board and see what happens. Beyond that any further input is always welcome. I mostly do repair so ciruit design is not my strong suit.

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Post by Robert Reed » Thu Mar 22, 2007 8:49 pm

Your car filter is mainly for "alternator whine" and covers but a limited band of interference. Why dont you play around with making your own filter. You probably have the neccessary parts on hand and it wouldn't cost you. For the series element you will probably want a choke of suitable value and current capacity. If your add-on circuit doesn't pull too much current, then go with lo-value resistors. For the shunt element you will want to digitize the capacitance by parrelelling an 0.001, 0.01,0.1 and as much 'lytic as it takes to complete the job. Keep the leads short and the filter close or better yet right on the add-on board. But its almost mandatory that you examine this line first with your 'scope to see whats present and plan how to deal with it. If all this fails then you may want to try hacklesup suggestion. One thing I have done in the past with these type of problems is to temporarily disconect the grd./sheild of the signal cable at its termination point (not the source).This may be carrying more garbage than the power ground/common line. Sometimes it cures-sometimes no, but its a quick and easy test.

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haklesup
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Post by haklesup » Fri Mar 23, 2007 10:32 am

A filter is not an isolation transformer unless you're real loose with your terminology. A filter regardless of design would have a DC connection straight through and would attempt to attenuate noise either through series inductance and/or shunt it to ground via parallel capacitance.

An isolation transformer does not filter the signal, for the most part, what you put in you get out. Like I said before, it breaks the DC connection between the two amplifiers while allowing equal currents to flow through the signal and return lines on both sides.

Opening the shield would also break this DC connection but that allows the signal to float and pick up other modes of noise. Now the return current has to take the long way back through the supply conductor encountering unequal resistance to the signal and opening the door to differential mode noise. It may also simply stop the signal if the return is isolated from ground. It does make a good diagnostic test though.

There are however other modes of noise. I occasionally see cheap PC speakers hum along with the digital noise on the motherboard. These were mostly unpowered speakers I think. Most of the Mhz speed noise in a PC you couldn't hear anyway even if a speaker could reproduce it.

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Post by Robert Reed » Fri Mar 23, 2007 12:52 pm

The assumption here is that the signal cable is coaxial and unbalanced with common ground to power ground. My reply as to breaking this connection at and only at the term point still keeps the sheild effect intact. I have experienced in the past horrendous ground currents flowing in this sheild path that should have been flowing in the supplys ground return only. this gave cause for inductively coupled disturbance to the inner signal line.By opening this path it eliminated this from happening and the sheild was still intact. Like I said some times it worked and sometimes no.
The high frequency noise component on power supply lines would not directly translate to audible speaker sound, but may affect processing circuits ahead of it that would. And of course a filter is not an isolation transformer and each deffinately has it place. Noise intrusion can be difficult to track down and sometimes many different schemes have to be tried until eliminated. I am embarassed to say this, but there have been instances where I have hit on a cure and had absolutely no logical explanation of why it worked. But it did work, and after many hours of frustrating debugging, I decicided it was good enough.

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Post by slrkt » Fri Mar 23, 2007 1:42 pm

One thing I failed to mention is the audio signal originates from the PC and connectes to the audio adapter board (basically a pre-amp) via a 3.5mm male to male patch cord. As I wrote before if I switch to an external power supply the noise goes away.

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Bob Scott
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Post by Bob Scott » Fri Mar 23, 2007 7:58 pm

slrkt wrote:As I wrote before if I switch to an external power supply the noise goes away.
Then the problem could be solved by filtering the supply, but that would just compensate for the poor design of the adapter board.

If that adapter board were carefully designed using op-amps properly, the circuit would have a PS noise rejection ratio of over 100Db. I think the adapter board designer may have stuck in a cap, bias resistor, or PWM stage from your signal path to B+. Just one can ruin the s/n. Or maybe it's a simple discrete transistor design with inherently low noise rejection.

Bob :cool:

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Chris Smith
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Post by Chris Smith » Fri Mar 23, 2007 8:02 pm

"IF" your noise is coming up the line, OPTO isolate it.

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Post by slrkt » Fri Mar 30, 2007 12:40 pm

Its coming thru the power circuit so opto isolation is not an option.

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Chris Smith
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Post by Chris Smith » Fri Mar 30, 2007 5:45 pm

Are you sure DE coupling the two wont separate the noise? [feed back]

IF the power is doing the noise, several caps and chokes should kill the noise factor.

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