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/eefd6753db0bd57dd00fd17a2cf96dc5263.png
I hope someone will help me
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/eefd6753db0bd57dd00fd17a2cf96dc5263.png
I hope someone will help me
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.
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.
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
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.
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.
Your first post only shows us a PCB Layout. It is a little difficult to make out any input Caps but if you say they're there, they're there.
You also quote the LM4780 Ref Board Schematic...........but don't provide a link or a schematic.
Is it this?
http://www.ti.com/lit/ds/symlink/lm4780.pdf
I can see no Input DC Blocking Caps on the first 'Typical' application but there are some on other circuits further down the pdf.
The LM1875 datasheet
http://www.ti.com/lit/ds/symlink/lm1875.pdf
does show input DC Blocking Caps.
When connected to the PC, can you take some measurements with a Multimeter with reference to the Amp's supply 0Volts point?
If you can measure at the input RCA's, both the 'inner' and 'outer' terminals. The 'outer' connection should be at 0Volts. The inner may have some DC potential but this should be blocked by those Caps you assure us are there.
Please try and supply a schematic, even if it's hand drawn. For us to help you, you need to supply us with as much info as possible with as many Voltage measurements written on the circuit. That will help us immensly.
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Schematic (sorry for drawing skills)
http://imageshack.us/photo/my-images/825/1875schema.jpg/
Sorry it is rotated. Please use rotate tool in upright corner on image.
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Hum removed succsesfully
Problems was... the AC filter. Now installing normal AC socket without filter.
I touched input from pc with finger when both PC and amp were ON. And my finger measured pretty high voltage between GND and input from PC at the RCA connectors.
Multimeter show that my finger was right There was 115Volts between GND and RCA middle pin. Socket voltage is 230V.
When changed to normal input socket, this voltage disapeared.
I don't know if filter is faulty?
Or they cannot operate without safety earth (YES i do NOT have safety earth)
Our house is very old...
So i think what happened:
Filter have capacitors connected to GND. Both PC and Amp had filters, none of them had safety earth, so i think that AC voltage was coupled from PC filter to PC GND, then across input cable to my amp GND and then to amp filter and caused some nasty voltage difference. Now no HUM when connected to pc.
Now only problem is moving speaker.
And this problem only exists when i turn potentiometer.
Later i will measure PC output. Maybe there is some dc in one output.
Problems was... the AC filter. Now installing normal AC socket without filter.
I touched input from pc with finger when both PC and amp were ON. And my finger measured pretty high voltage between GND and input from PC at the RCA connectors.
Multimeter show that my finger was right
There was 115Volts between GND and RCA middle pin. Socket voltage is 230V.When changed to normal input socket, this voltage disapeared.
I don't know if filter is faulty?
Or they cannot operate without safety earth (YES i do NOT have safety earth
)Our house is very old...
So i think what happened:
Filter have capacitors connected to GND. Both PC and Amp had filters, none of them had safety earth, so i think that AC voltage was coupled from PC filter to PC GND, then across input cable to my amp GND and then to amp filter and caused some nasty voltage difference. Now no HUM when connected to pc.
Now only problem is moving speaker.
And this problem only exists when i turn potentiometer.
Later i will measure PC output. Maybe there is some dc in one output.
Hum removed succsesfully
Problems was... the AC filter. Now installing normal AC socket without filter.
I touched input from pc with finger when both PC and amp were ON. And my finger measured pretty high voltage between GND and input from PC at the RCA connectors.
Multimeter show that my finger was right
When changed to normal input socket, this voltage disapeared.
I don't know if filter is faulty?
Or they cannot operate without safety earth (YES i do NOT have safety earth
Our house is very old...
So i think what happened:
Filter have capacitors connected to GND. Both PC and Amp had filters, none of them had safety earth, so i think that AC voltage was coupled from PC filter to PC GND, then across input cable to my amp GND and then to amp filter and caused some nasty voltage difference. Now no HUM when connected to pc.
Now only problem is moving speaker.
And this problem only exists when i turn potentiometer.
Later i will measure PC output. Maybe there is some dc in one output.
Yes, one pc output have 0.3Volts of dc
Another 0.01Volt
When hum is gone, looks like my amp is running fine now. PC is causing problems
It's sad knowing that pc is one year old with 200usd motherboard.Maybe next project some DAC or just buy sound card?
Thanks for everyone who helped.
i will post a picture of my amp soon
Yes, one pc output have 0.3Volts of dc
Another 0.01Volt
When hum is gone, looks like my amp is running fine now. PC is causing problems
Maybe next project some DAC or just buy sound card?
Thanks for everyone who helped.
i will post a picture of my amp soon
Picture:
http://img198.imageshack.us/img198/3161/18755.jpg
Still lots of work needs to be done to finish it
What FoMoCo said is right. DC blocking capacitors allow current to flow (creating temporary DC offset) when the DC across them changes. As you change the volume, the DC usually changes, and until the capacitor charges to the new voltage, there's more DC offset from the amp.
Putting an additional DC blocking capacitor between the volume pot and the source is the test.
Putting an additional DC blocking capacitor between the volume pot and the source is the test.
Thank you both.
This makes sense. I can put capacitors before potentiometer. Thats not a problem.
But then capacitor after potentiometer is needed too? So in other words 2 blocking capacitors needed?
And if source have capacitors or no DC on output, then everything should be fine with only capacitors after potentiometer?
EDIT: I remembered that i have few 3.3uf metalised polyester film capcitors, i think with 10K pot Low freq response will be good with them. I probably try them tomorow.
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0.3V is not zero. It's actually really high for a source. Add that your 100uF capacitor to ground on your feedback is quite a high value (it affects low frequency less, which also makes it slower to respond to DC), it could be exactly as I said. If a capacitor before the pot fixes the problem, then try removing the capacitor after the pot.
What value is your pot?
What value is your pot?
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