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Old 13th September 2006, 02:16 AM   #1
henkel is offline henkel  Malaysia
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Default Sheilding from Numerical LED display.

Hi!

Most newer design pre-amps have some form of numerical or LED style display on their front panel. I am building one such kit now, and seems to be getting a high pitch hum as a result.

How do I know it's from the display? Because when I mute the pre-amp volume, the display goes blinking, and the high pitch hum blinks along with it!

Is there any mechanical or electrical sheilding that you can advise?

Thanks for reading.
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Old 13th September 2006, 02:27 AM   #2
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LED displays use a "scan-multiplex" system -- with a clock that pulses the display frequently enough not so as to appear to blink, but not so quick as to draw a lot of current.

if you can, increase the clock frequency

there may also be a poorly designed switch mode power supply as the culprit spewing forth.
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Old 13th September 2006, 02:38 AM   #3
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Quote:
Because when I mute the pre-amp volume, the display goes blinking, and the high pitch hum blinks along with it!
hahahahaha nice!
1. can you dress the wires away from the high gain stuff?
2. can you use a seperate transformer winding to power the display?
3. have you star grounded your returns so that display currents don't "pull-up" on analog stage grounds?
4. have you decoupled display drivers etc?

So this interferrence can get in several way's:
Capacitive coupling. when components are too close and form a capacitor (coupling the signal)
Inductive coupling: when conductors are carrying current and electro-magnetically coupling.
poor grounding: when the current feeding one part of the circuit is modulating the ground voltage feeding another part of the circuit.
poor decoupling: like poor grounding, but the modulation is occuring on the rail rather than ground.

Good ways to prevent these things:

1. keep heavy current traces away from "quiet" or high gain circuits.
2. Try to have all of the ground returns meet smack dab in between the filter caps.
3. use local caps close to sensitive amplifier stages
3. and definitely not last and definitely not least have some copper foil on hand (with a alligator clip to ground) and try different shielding experiments.

You said this is pre-amp. Is this a Phono stage thats picking up the noises??
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Old 13th September 2006, 04:54 AM   #4
henkel is offline henkel  Malaysia
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Guys, thanks for your ideas.

Here's more infomation which I hope you can use to further this topic. It's a line stage pre-amp, not phono.

All signal wires are routed away from transformers and caps.
But due to space constraint of the casing that I used, the power supply regulator's rectifiers and caps are only about 20cm away from display panel(any possible problems there?).

I've built a sheilding panel for it, and will be trying that tonite. I'll need to look for copper foils. I'll be trying on a/c chokes as well to the power supply of the display as suggested by a friend.

And lastly, I'll check if there's any adjustment that I could make for the clock timing, I think there's a rotary pot on the display panel.

With your help, I think I am in good hands.

Will feedback my findings soon.
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Old 13th September 2006, 10:45 AM   #5
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Quote:
Originally posted by henkel

I've built a sheilding panel for it, and will be trying that tonite. I'll need to look for copper foils. I'll be trying on a/c chokes as well to the power supply of the display as suggested by a friend.
Copper foil is nice, but a bit expensive you can try galvanized steel (as from an electrical box -- but I don't know whether you folks use steel or PVC in your locale) or aluminum foil. The folks from Mu-Metal here in the States suggest that for audio, before going the expensive route try the cheaper ones first.

The potentiometer probably sets the current not the clock frequency.

you might try to put a 10K resistor on the display board from each of the "Chip Enable", "Clock" and "Data" lines to ground, and a 1K resistor in series.
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Old 13th September 2006, 02:38 PM   #6
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A common cause for digital noise in the audio of this sort of device is improper grounding. If the digital circuits share a ground-plane with the audio, there will be many small loops formed with many paths for the digital noise to get into the audio. The digital circuit's ground should connect to the analog ground (if at all) at only one point- the star groud of the entire system. Separate power supplies are a good idea, too.

Unfortunately LEDs take quite a bit of current to operate so the display is multiplexed to reduce the load on the LED drivers (only 10 LEDs have to be on at any moment, instead of 100) and simplify (lower cost) the display and power supply circuits.

I_F
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Old 14th September 2006, 03:13 AM   #7
henkel is offline henkel  Malaysia
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Guys,

Thank you very much. I think I have solved most of the noise by having a separate power supply and sheilding panel for the display.

Now there's still a tiny, tweeny bit of noise, which you can only hear if you put your ear right next to the tweeter.

Any further ideas on eliminating that last bit of noise?
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Old 25th September 2006, 04:07 AM   #8
henkel is offline henkel  Malaysia
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Hi! Guys,


The last bit of noise starts to get to me after 2 weeks of listening.

It goes "te............" just a whisper. I noticed one of the transofmer runs very hot, much hotter then the rest. Does hot transformer generate noise as well?
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Old 25th September 2006, 02:03 PM   #9
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Hi Henkel, Good work on removing most of the noise.

So, there's a last little bit of noise? This last bit might be the toughest to get rid of. If the noise is a result of Electromagnetic pick up, you can try loosening/removing the display transformer and change it's position. Most non-torroidal transformers have Electromagnetic lobes around the transformer. If a lobe happens to cut through a sensitive part of the audio circuit, it could couple into the signal path, the supply traces, or the ground path. Youy said you had to put your ear up to the tweeter to hear. That would indicate a small amount of interference. That transformer getting hot bugs me. Is the transformer the right one for the job? If the transformer saturates, it will get warm and I think the EM field will get larger in a hurry. The noise could be coupling from transformer to transformer if the transformer's EM lobes cross. ......it could be piced up on the AC line too. spat out onto the AC line by one transformer and picked up by the other one. You might want to bypass your "switchy" stuff too. ...Add 0.01uF caps across your rectifier diodes.

There's a lot that could cause this. Just be aware of how these noises can couple. Capacitively, Inductively, insufficient shielding, insufficient filtering, or modulating B+ or ground.
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Old 25th September 2006, 02:32 PM   #10
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Quote:
Originally posted by mrshow4u
Hi Henkel, Good work on removing most of the noise.

So, there's a last little bit of noise? This last bit might be the toughest to get rid of. If the noise is a result of Electromagnetic pick up, you can try loosening/removing the display transformer and change it's position. Most non-torroidal transformers have Electromagnetic lobes around the transformer. If a lobe happens to cut through a sensitive part of the audio circuit, it could couple into the signal path, the supply traces, or the ground path. Youy said you had to put your ear up to the tweeter to hear. That would indicate a small amount of interference. That transformer getting hot bugs me. Is the transformer the right one for the job? If the transformer saturates, it will get warm and I think the EM field will get larger in a hurry. The noise could be coupling from transformer to transformer if the transformer's EM lobes cross. ......it could be piced up on the AC line too. spat out onto the AC line by one transformer and picked up by the other one. You might want to bypass your "switchy" stuff too. ...Add 0.01uF caps across your rectifier diodes.

There's a lot that could cause this. Just be aware of how these noises can couple. Capacitively, Inductively, insufficient shielding, insufficient filtering, or modulating B+ or ground.
All transformers have lobes -- the best contained are potted core. Again, consider shielding the transformer with mu-Metal or even the steel from a soup-can. It too bothers me that the transformer is getting hot.

If its just the "te....." sound you probably have a problem elsewhere -- thermally dependent electrical noise from high resistances, suboptimal opamp selection, lack of decoupling of opamp power supply, noisy power supply -- a whole panoply of problems to contend with!!!

FWIW a 10nF cap across the diode just reduces the resonant frequency of the network formed by the diode junction capacitance, transformer leakage inductance etc -- possibly pulling the noise into the audio band -- you need to lower the Q of this resonant circuit with a resistor and capacitor across the diodes. These values can be determined empirically.

I would still go back to one of my original recommendations -- decouple the microprocessor from the display with some resistors, make sure that the Clock, Chip-Enable and Data lines are terminated at both junctions.
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