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Old 23rd June 2005, 06:36 AM   #1
Fizx is offline Fizx  New Zealand
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Default Help with a LM3876 amp! (nOOb)

Hail to all

I have put together a kitset amp from jaycar, it uses the LM3867 chip.

The problem I have right now is that there seems to be some parts in songs (mainly bass) thats not being played, I'm not too good at the whole frequancy range thing but I have found that the begining of Metallicas St. Anger and Unnamed Felling is missing completely.

I also find that my bass distorts badly if I put the volume up just on the bass. I had a 12V amp that did a better job at bass! (I reversed the polarity on that amp and blew the IC up >

I plan on building a stero amp so I have a 160VA tordial with 4700uF caps on it, my powersupply works, I have checked that all the components are in there right places! I am stumped!

Can some one please point me in the right direction? Also, where can I find amp theory? Like why you put caps and resistors on the audio input path and the theory behind inductors and resistors-cum-inductors.
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Old 23rd June 2005, 07:16 AM   #2
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Hi Fizx,

first have a look at Nationals datasheet.

Maybe you have the wrong input caps. Try bigger values to lower the entry frequence.

Also the Overture Design Guide may help to select matching power supply:

Be sure to add lots of elko capacity if you don't have a regulated powersupply, better for tighter bass. If you want to rebuild your supply check the LM338 regulated psu thread...
There are also some infos about snubbering the National chips.

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Old 23rd June 2005, 07:33 AM   #3
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Location: Sydney/Australia
hi Fizx,

You should post a link to the Jaycar amp in question.

Have you got it mounted on a heatsink? The Jaycar chipamp I looked at said "Heatsink Optional", which is incorrect.
Greg Erskine
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Old 24th June 2005, 07:22 PM   #4
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There is lots that could be happening. The two inputs so far are very good. Depending on the schematic there could be several capacitor values that need adjustment. Your power supply may be drooping under heavy bass causing clipping at much lower power, the IC may be too hot if not heat sinked properly which will greatly increase distortion when the protection circuits come on. Do as SlackMan say, read through the datasheet (the LM4780 has the most information which applies to the LM3876 as well), and try the Design Guide.

If you can provide a schematic that will really tell all.
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