Subject: Re: Problem with LM1877 audio amp I.C.?
From: Al Clark
Organization: Danville Signal Processing, Inc.
NNTP-Posting-Date: Mon, 23 Sep 2002 09:35:26 CDT
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Date: Mon, 23 Sep 2002 14:35:26 GMT
firstname.lastname@example.org (Richard) wrote in
> I have been using this device (DIP version, LM1877N-9A) for years and
> occasionally have a problem which bugs me. I have just used it in a
> new design and the problem has become more prevalent.
> I'm using a single supply configuration, with a 47uF from the bias pin
> to ground and 1M resistors to the inputs, exactly as detailed in the
> National app. note. Here is the problem - every now and then, when the
> device is powered up, the output will go straight to Vcc and obviously
> no audio is forthcoming. A power reset will correc the fault. Some
> chips are worse than others - some never faulter, while some do it
> almost always and are throwaways.
> In my various designs, the supply has varied from about 5V to 12V. It's
> a good clean supply with an electrolytic and ceramic bypass. Sometimes
> regulated, sometimes not, but seems not to make a difference as far as
> I can recall. I vary the gain from about 10 to 20. Apart from that,
> the topolgy is the same, but some designs have never been problematic
> and others, very much so.
> A web search has brough up nothing and I'd be surprised if nobody else
> has had similar problems. I know it has been used in arcade games, so
> maybe somebody here has some experience?
The part appears to be latching. I would look very close at the bypassing
and the ground return to the part. You might try soldering a bypass cap
directly on top of the IC from Vcc to Gnd to see if it has any effect.
I have never had a problem with the LM1877 and the other similar National
parts (LM2877) but I always use them in an inverting configuration.
Danville Signal Processing, Inc.
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