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Old 28th March 2010, 05:05 PM   #11
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Default PS schematic

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Old 28th March 2010, 07:46 PM   #12
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More annoying than the hum, which is bad enough, is the motorboating, really, more like a lawn mower... I have the xfrmr CT on the same ground point as the rest of the ground signals. Is that a problem?

You can hear the motorboating coming from the rectifier. I wonder if it's the snubber caps, maybe got damaged when the other rectifier burned? I'll try to connect without the caps.

Also, no music out when I connect the CD player to the amp. I checked the attenuator and it and all connections worked fine.
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Old 28th March 2010, 08:04 PM   #13
AndrewT is offline AndrewT  Scotland
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that schematic looks like it's from the National datasheet.
That's the stripped down version that is likely to misbehave when beginners and intermediates try to implement it.

You need the AC coupled version from the datasheet with all the optional parts fitted.
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Old 28th March 2010, 09:06 PM   #14
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Default motoroating

Thanks Andrew, I may indeed change layouts. I notice that when I relocate the rectifier, the motorboating either lessens or totally subsides. Is this a clue? Could the rectifier be too close to the star ground? the output signal wires?

Thanks again,

Luis
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Old 29th March 2010, 08:29 AM   #15
AndrewT is offline AndrewT  Scotland
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the motorboating may be an LF instability, but I have a feeling it is a protection circuit re-triggering due to detection of a fault.
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Old 29th March 2010, 12:47 PM   #16
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Quote:
Originally Posted by AndrewT View Post
the motorboating may be an LF instability, but I have a feeling it is a protection circuit re-triggering due to detection of a fault.
Andrew,

That makes more sense than anything else, as I have checked connections and moved things and nothing gets rid of it - also may explain why no output. So, if it's a fault, what could cause it on both channels? V levels are within specs.

Thanks for taking time to answer,

Luis
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Old 29th March 2010, 11:57 PM   #17
tomchr is offline tomchr  United States
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I wonder if a protection circuit gets cranky because the mute pin gets pulled low too fast. The data sheet calls for a 100 uF cap there. Also, what's the resistance of the mute resistor (Rm)? The current through Rm should be at least 0.5 mA. So calculate Rm from Rm = Vsupply/0.0005. Round down to the nearest standard value.

Your "amplifier board" circuit should work if Cm is fitted. You may have some offset on the output. The max input referred offset of the LM3886 is 10 mV. With a gain of 22k/680 = 32.4, the max voltage offset you should see on the output of your amp is 324 mV.

Note that all the grounds in your schematic should come together at one point - the star ground. The hum you are describing could be caused by a floating input ground.

~Tom
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Old 30th March 2010, 12:02 AM   #18
tomchr is offline tomchr  United States
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Quote:
Originally Posted by luis.jimenez View Post
Thanks Andrew, I may indeed change layouts. I notice that when I relocate the rectifier, the motorboating either lessens or totally subsides. Is this a clue? Could the rectifier be too close to the star ground? the output signal wires?
What do you mean by "relocate the rectifier"? If you physically move it around without disconnecting any of the wires, then I bet it's a cold solder joint somewhere around the connections to the rectifier.

The rectifier does not connect to the star ground so its location with respect to the star ground should not matter.

~Tom
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Old 30th March 2010, 12:22 AM   #19
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Tom, thanks for all the pointers and advise. I will fit CM and will verify Rm value. I will also check all solder connections.

Cheers,

Luis
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Old 10th April 2010, 12:04 AM   #20
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Hello again,

I corrected the ckt and got one channel to work, pretty quiet too. But, the other channel is still exhibiting some of the same symptoms, and it gets extremely hot right off the bat - the other channel doesn't. I checked everything and can't figure out what's causing it to get so hot. Could the chip be shorted inside? It amplifies the music, but very noisy and with a cadence, not like the motorboating earlier but like the protection ckt is kicking in and out, in and out, every half a second.

Any help is appreciated.

Thanks,

Luis
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