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Old 3rd August 2012, 06:04 AM   #1
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Default lm3875 noise problem--suggestion required

Hello friends,

I am building lm3875 chipamp based guitar amp. currently I am only preparing the power amp section. I had breadboarded it and connected with my computer output. There are some noise issues.

1. There is 'hiss' sound even in the absence of computer input when the amp input pot level above 9 clock... though it is not so loud but still annoying.
2. There is very loud audible click/pop when i press any electrical switches in my home and even worse when changing ceiling fan regulator..

Once i satisfied with the outcome I will make PCB......

Please suggest your views......

thanks
Paku
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Old 3rd August 2012, 07:49 AM   #2
wups is offline wups  Lithuania
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Quote:
Originally Posted by paku5535 View Post
Hello friends,

I am building lm3875 chipamp based guitar amp. currently I am only preparing the power amp section. I had breadboarded it and connected with my computer output. There are some noise issues.

1. There is 'hiss' sound even in the absence of computer input when the amp input pot level above 9 clock... though it is not so loud but still annoying.
2. There is very loud audible click/pop when i press any electrical switches in my home and even worse when changing ceiling fan regulator..

Once i satisfied with the outcome I will make PCB......

Please suggest your views......

thanks
Paku
For the second problem, you need to install an EMI filter in the power suply.
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Old 3rd August 2012, 08:12 AM   #3
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Quote:
Originally Posted by wups View Post
For the second problem, you need to install an EMI filter in the power suply.
Thanks WUPS for your reply. Can you suggest any schematic though I have already finished the power supply section.....
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Old 3rd August 2012, 01:25 PM   #4
macboy is offline macboy  Canada
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You can build up the EMI filtering a little at a time until you get enough filtering to do the job. Start with a snap-on ferrite on the AC power cord, at the amp side (not the wall side). This can be surprisingly effective. You can also add these to devices that you suspect are making a lot of electrical noise. I have many them around my house and have added them to video cables in addition to power cables to reduce RFI that causes visible noise in the video.

Next you can add a X-rated capacitor across the line and neutral of the AC inside the power supply. It must be an X1 or better rated AC safety capacitor, you can't just put any capacitor on there, since failure of this component can be dangerous. You can scavenge such capacitors from old PC power supplies, or an switching power supply from dead consumer electronics. They will be located closer to the AC input, prior to the rectifiers and big filter cap. They are often blue and always have a rectangular plastic shell. They bear the mark "X1", often "X1Y2" (meaning also safe to use as a Y2 capacitor). If you are using a IEC socket, you can solder the cap directly to the legs of the socket. You can also put a AC rated film capacitor (does not need to be safety rated) across each secondary of the transformer.

If that doesn't help, you could try to do more, google EMI RFI AC filtering.

You may also be picking up airborne noise. To help with this, make sure that the amp is in a grounded metal case.
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Old 3rd August 2012, 01:34 PM   #5
djoffe is offline djoffe  United States
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Paku,

It might be EMI filtering, but a very good possibility is that you haven't put on the "civilizing" parts for the feedback loop and snubber networks. It turns out that the data sheet can be quite misleading about what is really required. Please post your present schematic, and we can see if this is the case. Without these parts, the amp always sits on the edge of oscillation.

All the best...

dan

Akitika GT-101
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Old 3rd August 2012, 03:28 PM   #6
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hello friends

thank you all for your reply.. I have only done the power amp circuit on the breadboard certainly does not have star ground and other benefits PCB offer. But the annoying thing is the key bouncing sound..

Circuit design: input portion of the power amp taken from Single Chip 25W Amplifier (Project 72) and rest of the circuit components are from lm3875 data sheet.

power supply is regular bridge rectifier with 4700 bypass caps and 0.1 uf filter caps with +-25v in each rail. and pre amp 9v is taken out from lm 317. for relay switching and micro controller I have separate transformer that is rectified and have +5 +12V output from fixed low dropout regulator.
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Old 3rd August 2012, 07:10 PM   #7
djoffe is offline djoffe  United States
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a picture would still help us know just what bits and pieces you have adapted to your design.

for example...do you have the RL output network? It's really quite important.
do you have the 220 pF across the input leads (e.g. positive and negative inputs of chip)? It's also quite important.

Akitika GT-101

Last edited by djoffe; 3rd August 2012 at 07:11 PM. Reason: clarify input leads
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Old 4th August 2012, 07:32 AM   #8
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I HAVE FEW QUESTIONS TO ASK

1. DO I NEED SEPARATE GROUND WIRE FOR SPEAKER DIRECTLY FROM POWER SUPPLY?

2. I DID NOT ADDED ZOBEL NETWORK AT THE OUTPUT. IS IT NECESSARY?

3. CURRENTLY I AM TESTING WITH MY COMPUTER OUPUT.. WHEN THE COMPUTER IS TURNED OFF OR INPUT IS OPEN, AMP IS PRODUCING GREAT 'HISS'. BUT WHEN COMPUTER IS TURNED ON THEN THE NOISE REDUCED TO 95%..

4. WHAT IMPROVEMENT CAN BE MADE WITH POWER SUPPLY AND AMP SECTION?

5. A STRANGE CASE-- WHILE THERE IS INPUT ON THE CHIP FROM COMPUTER IF I POWER OFF THE AMP SECTION WITHIN 2 SECONDS THE CAPACITORS COMPLETELY FLUSHED OUT. BUT THEN AFTER 5-6 SECONDS THE AMP MOMENTARILY BECOMES ACTIVE AND PLAY FOR VERY SHORT TIME AND THEN FINALLY STOPS. I CAN'T FIGURE OUT WHAT IS HAPPENING...

CHECK OUT THE ZIP FILE CONTAINING CIRCUIT SCHEMATICS...
Attached Files
File Type: zip POWER AMP.zip (3.9 KB, 19 views)
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Old 4th August 2012, 02:17 PM   #9
djoffe is offline djoffe  United States
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these changes may help stability...see enclosed...

Update My Dynaco
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File Type: png CHIP_AMP2.png (52.6 KB, 155 views)
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Old 4th August 2012, 04:04 PM   #10
wups is offline wups  Lithuania
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Some why i have a feeling, that C7 shouldn't be an electrolytic capacitor(according to the datasheet).
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