| LM3886 problem, HELP! - Click HERE for Original Thread |
| Matthewong |
| I'm building an amplifier using 2 LM3886 I.Cs. for stereo. Right now I'm facing humming problem. First the humming is very loud when I didn't connect the RCA cable from my pre-amplifier, after 3-4 seconds, my left channel will generate weird sound, something like shzzzzzzz, quite loud, only left channel, but it dissapear when I connect the RCA cable to the input. I have built power amps using TDA 2030 and 2050, both don't have such problem, only LM3886 got these problem. After connect the RCA cable from Pre-amp to the power amp input, I can hear humming sound near the speaker, not very loud, but audible when near the speaker. anyone out there can help me? |
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| sagarverma |
shzzzzzzz:::::its called 'pop'.
for hummmmm::use zobel at output. |
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| Matthewong |
| just now I've do the zobel network, but It doesn't work. the humming still there. I use a 0.1 uF to connect from speaker output positive to speaker output negative. |
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| tja |
| quote: | Originally posted by Matthewong
just now I've do the zobel network, but It doesn't work. the humming still there. I use a 0.1 uF to connect from speaker output positive to speaker output negative. |
Shouldn't there be a resistor in the Zobel (in series with the cap) as well?
Take |
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| Matthewong |
| I don't think it will work even I add a resistor to the 0.1 uF in series. right now I'm using 2 seperate boards for left and right channel. Will the problem coming from there? But I don't understand why humming become louder when I didn't connect the RCA from pre-amp? and the left channel generate weird sound after 3 to 4 seconds. |
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| tiltedhalo |
| The wierd sound is input variance oscillations, caused by input ground loops causing the feedback network to slowly ring at frequencys above 1k. Make sure the input is referenced to ground with a resistor, if your input is 1k resistor use a 10k to 22k resistor to ground. Your zobel should be a 2.7ohm and .1uf cap also put a .74uh indutor inseries with a 10 ohm resistor to unload capactive coupling. Check your grounding really close!! |
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| peranders |
| Matthewong, what kind of schematic have you used? If you short each input do you still get a hum? |
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| Matthewong |
| Ok, I short the input positive and negative in the left channel and the humming is gone. If i disconnect the the RCA from pre-amp, not power amp, that's mean the power amp still connecting the RCA cable. The power amp is in good condition, no weird sound and no humming, but when I disconnect the the RCA cable on the left channel, the weird sound come and humming also very loud. |
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| Matthewong |
| Hi, I'm back. I've done everything that you guys told to do, and now my power amp is nearly complete. But there is still have some problems. First is my power amp is still come out very low hum sound when I connect to preamp. Without connection to preamp, the humming gone. I've tried quite lot of preamp, all also same. Even when I just connect to preamp but preamp remain switch off, still come out hum. What is the problem there? Another problem is when I turn the volume up till almost 11 o'clock, one side of my poweramp output become distorted, seems like not enough power to drive the speaker. Before I modify, this problem never occur even I turn the volume up till 2-3 o'clock. Please help me! |
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| Matthewong |
| By the way, now I'm using a transformer with 16.5 - 0 - 16.5 A.C output @ 8A. When I change to DC, it would be around 23v - 0v - 23v. I know this voltage isn't enough for LM3886 i.c and the worse is I use it for stereo. I would like to know which is best AC voltage for this I.C because I want to replace a new transformer for my poweramp. Can anyone help me? |
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| Matthewong |
| Does anyone can give me some advice? Please! |
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| sagarverma |
| +-30v dc output.its safe and good. |
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| Matthewong |
| +-30vdc Can I load it with 4ohm loudspeaker? I still wondering can LM3886 make to bridge mode and drive 4 ohms speaker? |
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| sagarverma |
4 ohm can be comfortably driven.
caution!:att'n:
in bridge mode,the amp 'sees' half the impedance of the speakers u r using.so 4 ohm will look like 2 ohm.which is bad for this ic. |
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| Matthewong |
| how about if I lower the input voltage to around +-25vdc or +-22vdc? |
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| sagarverma |
| 25v low.u have to consider clipping also. |
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| Matthewong |
| ya, forgotten the clipping. So there is no way to bridge LM3886 unless drive only 8 ohms speaker. |
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| Nordic |
Might get shot down in flames, but the longer I spend on these chips the more seems to sink in...
If memory serves me correctly, for the 3886, the recommended amount of power the chip can handle without protection being activated (due to heat dissipation) is;
24V into 4 ohm load
35V into 8 ohm load
Have you looked at parallel layouts maybe? |
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| sagarverma |
| quote: | Originally posted by Matthewong
ya, forgotten the clipping. So there is no way to bridge LM3886 unless drive only 8 ohms speaker. |
try it out.if anything goes wrong,the thermal shutdown circuitary will step in:) |
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| Matthewong |
| does anyone of you have the bridged-mode schematic for LM3886, I think i'll give it a try. |
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| Matthewong |
| Thank you for the links. After reading some of the links, one problem is on my mind. What is the different between use a conventional amplifier(stereo amplifier) parallel the both channel together, and built a parallel poweramp, such as the schematic in the AN-1192? |
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| Nordic |
Oi, sorry its 28V into 4ohms, ta for the link PA.
"The other way to determine IC power dissipation capabilities
is to analyze the output power specification in the datasheet.
In the case of the LM3886T, there are two output power
specification guarantees: 60W (min) into a 4Ù load using
±28V supplies and 50W(typ) into an 8Ù load from ±35V
supplies....
....the IC can handle a maximum of ±40W of continuous power dissipation without SPiKe Protection being turned on under continuous sinusoidal input with proper heat sinking. The same theory applies to other Overture ICs as well, like the LM3876T, which is capable of dissipating 31W with proper heat sinking."
P.A. can you just confirm my understanding that this basicaly means that even if the chip is guaranteed to reach 60W, everything exceeding the initial 40W, will incur a big hit on THD due to Spike? |
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| Matthewong |
| so do you all think that bridge the LM3886 is an ideal way to have more output power or this is a very bad ideal? If this is not the good way, do you all have any ideal to get a more output power chip amp? |
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| Narcisse91 |
| If bridging was a bad idea (or parallel, for that matter), it wouldn't be in one of their app notes... |
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| Matthewong |
| I still have problem to understand in between bridging and parallel. What's the different in between bridging and parallel? |
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| peranders |
Bridging = Double output voltage, load not connected to ground (not suitable for 4 ohms load)
Parallel = Double output current, load connected to ground |
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| Matthewong |
| And which one is better? Bridging will have louder output and parallel is still the same right? In terms of perfomance and output quality, which one is better? |
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| peranders |
This dependent of your load and your power supply voltage.
I'll recommend that you check the Excel file above. |
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| Matthewong |
| Can I parallel more than 2 ICs in a time, such as 4 ICs parallel together? Before this never I never do any of these electronic thing, so I really have no idea what it is all about. Hope you all can help me. |
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| peranders |
You can connect as many you like in parallel but I'll that you'll need more than two for a 4 ohms load.
Have you checked the design guide (the excel file)? If not, do that! :nod: |
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| Bazukaz |
You don't need any zobel networks for LM3886.
Hum may be because of poorly designed schematic diagram , or incorrect connection of components.
You only need a carefully designed schematic , as stated in its datasheet.Its layout is critical for IC amps.
It is important that all ground points are connected to one ground point.Otherwise , noises may pass to input , as only tiny amounts of current are required.
All connections should be as short, as possible. |
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| Matthewong |
| I really want to show you the schematic that I followed, but I can't find it in my PC, don't know where it is gone. It has little bit different to the National one. So you are meaning following the schematic that given by National will be better? If you are saying that LM3886 no need zobel network, then maybe my poweramp has distorted output when I turn the volume to 11 o'clock because I added the zobel network to my poweramp already. Will it be? Regarding to the parallel the ICs, when I parallel the IC, it won't get louder than single IC, am I right? |
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| Bazukaz |
I don't know your schematic diagram. What is the purpose of zobel network ?
If you want to parralell your ICs , you need :
1) Very well matched feedback components(0.1 % tolerance preffered)
2) Some resistors on output (like 0.5 ohm or so , 5 W).
By paralelling ICs you can get a bit more output if you rise power supply voltage(Remember , LM3886 can handle AT MOST +/-42 V no load!).Power output will not double , though.
Also , you can google "BPA 200". It is a schematic of amp , based on 4 LM 3886 chips , delivering 300W RMS.
LM3886 must be absolutely quet even at max volume .I made an amp without a volume control very succesfully, by using National's preferred schematic.
Did you read Lm3886 datasheet carefully ? It has lots of explanations. |
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| peranders |
| quote: | Originally posted by Matthewong
I really want to show you the schematic that I followed, but I can't find it in my PC, don't know where it is gone. |
Have you used the file search feature? |
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| peranders |
| quote: | Originally posted by Matthewong
Regarding to the parallel the ICs, when I parallel the IC, it won't get louder than single IC, am I right? |
The voltage, possibility to more current out.
Seriously, you must set up the specifications of you project, then choose topology. Put those specs in the Excel file I mentioned and from it get facts. Much of you questions are based on that you don't have tried the Excel file nor read the AN-1192 anf the datasheet. |
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| the-paradox |
| quote: | Originally posted by Matthewong
... I'm facing humming problem. First the humming is very loud ..., something like shzzzzzzz, .... |
Hi,
Are you still suffering from the hum?
Almost every hum problem is caused by incorrect ground loops, not only within an amp but it can also exist between an amp an a pre-amp.
You can also measure the voltage potential between the (not connected) RCA-ground of the amp and pre-amp: does it have any ac component?
About your other question, bridging is only advisable when you need more output power.
Connecting 2 (or more) amps in parallel is desired when you need to drive low impedance loads. |
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