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Robert Wong (rwong@direct.ca) and antekl (antekl@hacktic.nl) remarked:
Some modems will make a hissing noice coming from the modem's speaker.  It's 
not very loud, yet loud enough to be heard in a quiet room by a person sitting
nearby the modem.  This sound is still heard, despite the ATM0, ATL0 and ATN0
register settings.
 
Michael Schuster (schuster@panix.com) wrote:
The early boards had three control resistors on the audio amp which were
not quite the right value.  As a result the speaker was never completely
off.  That was ages ago ... well before the Plus models were designed.
On the older board you either replaced three 2.2K resistors with something
higher (3.3K worked for me as I remember) or else stuff some paper toweling
between the speaker and the case.

Some parts © 2009 Advameg, Inc.