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Old 5th April 2013, 03:21 PM   #1
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Default LM1875 hum, strange problems.

Hello. This is my first post in diyaudio. Sorry if something not right. I have to say that my english isn't very good too.
I have a problem with my LM1875 project.
When i connect it to my cellphone, everything is normal, no hum no noise, volume potentiometer works fine.
But when i connect to PC, strangeness begins... When i turn potentiometer, one chanel speaker cone moves back and forth. When not turning potentiometer everything is ok, although there is some low level hum. Anyways, biggest problem is moving speaker cone. And why only one cone moves i cant get it
Both chanels are on one PCB.
PCB layout:
http://www.part.lt/img/eefd6753db0bd...cf96dc5263.png
I hope someone will help me
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Old 5th April 2013, 03:36 PM   #2
AndrewT is online now AndrewT  Scotland
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This could be a combination of two different but connected problems.

DC from the source is entering the DC coupled amplifier.

Check the output of the Source.
Check to see if the input of the amplifier has a DC blocking capacitor.
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Old 5th April 2013, 04:41 PM   #3
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Originally Posted by AndrewT View Post
This could be a combination of two different but connected problems.

DC from the source is entering the DC coupled amplifier.

Check the output of the Source.
Check to see if the input of the amplifier has a DC blocking capacitor.
In pcb image you can see that i use input capacitors, and feedback capacitors.
Or you cant see that image? If not i will try put it in different place. Source don't have any DC, another amps that i building / built works fine.
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Old 5th April 2013, 04:49 PM   #4
Mooly is offline Mooly  United Kingdom
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Is the PC a headphone type output or a proper line output ? Some headphone type outputs need to work into a low impedance because the 'phones form part of the Class D output filter network.

Hard to diagnose these things at a distance but if it works with one source and not another you have to look for obvious problems.
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Old 5th April 2013, 08:25 PM   #5
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Originally Posted by Mooly View Post
Is the PC a headphone type output or a proper line output ? Some headphone type outputs need to work into a low impedance because the 'phones form part of the Class D output filter network.

Hard to diagnose these things at a distance but if it works with one source and not another you have to look for obvious problems.
Thats headphone output, but i tried line out to on PC. Results same.
Also i said that other amplifiers work fine from PC. Only this one do that.
Strangest thing is that only one speaker moves
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Old 6th April 2013, 06:23 AM   #6
Mooly is offline Mooly  United Kingdom
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Strange problem that really needs a scope to see what's going on. In the first post you say its OK with a cell phone. Perhaps the amp is on the ragged edge stability wise and some hf hash from the source is pushing it over the edge. As an experiment you could try a filter cap on the input to shunt any hf.
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Old 6th April 2013, 09:17 AM   #7
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Originally Posted by Mooly View Post
Strange problem that really needs a scope to see what's going on. In the first post you say its OK with a cell phone. Perhaps the amp is on the ragged edge stability wise and some hf hash from the source is pushing it over the edge. As an experiment you could try a filter cap on the input to shunt any hf.
What value i should choose?
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Old 6th April 2013, 10:18 AM   #8
AndrewT is online now AndrewT  Scotland
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A simple passive RF filter needs two components.
A series resistor and a shunting capacitor.
The RC of that pair give the attenuation of the filter.
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Old 6th April 2013, 10:33 AM   #9
FoMoCo is offline FoMoCo  United States
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Originally Posted by valtra103 View Post
When i turn potentiometer, one chanel speaker cone moves back and forth. When not turning potentiometer everything is ok, although there is some low level hum. (
A lot of PC outputs have DC on them. Both the L and R are floating at some voltage. The common floats at the same voltage. The net DC is zero across the headphones. Look at some single supply headphone driver datasheets for a more technical explanation.

Chances are your amp has a variable resistor and then a DC blocking cap. As you turn the pot, the level varies and is passed through the cap and amplified. When you stop turning it, the DC level is static and blocked by the capacitor.

To test: Put a capacitor in series with each input. It doesn't have to be bigger than 1uF to test with. You can even use a polarized cap if you place it such that it doesn't have a negative bias. (Most likely positive terminal tied to the PC.) See if the problem goes away.

To fix: Use a bigger non-polarized cap in series, just like the test. Without knowing the input impedance of the amp, I can't give you an exact value.
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Old 6th April 2013, 12:54 PM   #10
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Originally Posted by FoMoCo View Post
A lot of PC outputs have DC on them. Both the L and R are floating at some voltage. The common floats at the same voltage. The net DC is zero across the headphones. Look at some single supply headphone driver datasheets for a more technical explanation.

Chances are your amp has a variable resistor and then a DC blocking cap. As you turn the pot, the level varies and is passed through the cap and amplified. When you stop turning it, the DC level is static and blocked by the capacitor.

To test: Put a capacitor in series with each input. It doesn't have to be bigger than 1uF to test with. You can even use a polarized cap if you place it such that it doesn't have a negative bias. (Most likely positive terminal tied to the PC.) See if the problem goes away.

To fix: Use a bigger non-polarized cap in series, just like the test. Without knowing the input impedance of the amp, I can't give you an exact value.
But my pcb already have 2,2uF capacitor, that you can see in first post on my pcb. Input impedance set with 47K resistor. Sorry that i don't have schematic.
But my input part is copied from LM4780 Reference board schematic.
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