SoundMeister warning

[dempson@swell.actrix.gen.nz (David Empson)]

I've just had a disturbing experience with my SoundMeister card.

I was setting the computer up to demonstrate the card to several IIgs
members in my user group.  I plugged the cable into the SoundMeister's
line-level output, and one of the channels sounded very strange - the
computer's internal fan was clearly audible through the stereo, along
with "bus noise" (all very loud). No standard audio was coming out of
that channel.  The other channel appeared to be fine.

The same noise was output through the amplified output (which I had
connected to headphones), but the IIgs's standard sound output was fine.

It appears that I managed to damage the SoundMeister's line-level
output stage while plugging in the cable.  The stereo was powered on at
the time, and isn't earthed.  My theory: a large voltage spike may have
been sent down the cable into the SoundMeister when the plug first made
contact, damaging the output amplifier.

I had a closer look at the problem when I got home, and in the process
somehow managed to destroy my headphones and the other channel of the
SoundMeister!

Since I live in New Zealand, I didn't fancy waiting about a month
while the card was sent back to Econ for repair, so I decided to find
the problem and fix it myself (voiding the warranty in the process).

With help from a friend (thanks, Kris!) I worked out which chip had
been affected, and we replaced it.  In the process, I analysed enough
of the circuitry of the SoundMeister to see how it worked.

The card now works fine.  The damaged chip was a TL072 dual op-amp,
labelled "U3" on the card.


The warning: the SoundMeister card has no protection against DC
voltages applied to its amplified or line-level outputs.  As a safety
precaution, I would recommend turning off any separately powered
device before plugging it into the SoundMeister (or unplugging it).


In my case, it seems that plugging the cable in caused some kind of
voltage spike which damaged the output stage of the TL072, and caused
it to short its output to the 12 volt rail (the internal fan is
connected to 12V, which explains why it could be heard through the
output).  When I subsequently plugged in my headphones, the 12V DC
signal was fed through the headphones, destroying one of the
"speakers".  (I could smell something burning!)
I suspect the other channel of the SoundMeister was damaged because the
12V signal was briefly fed back into the other channel while I was
plugging in the headphones.

The TL072 labelled "U3" provides the line-level output signals for both
channels (each op-amp provides one of the channels).

There is a second TL072 on the board ("U5") which is used to buffer
the signals from the line-level outputs before they pass through a
second amplification stage for the amplified (high-level) outputs.


If anyone has any further questions about this, please E-Mail me, or
post a follow-up to this article.
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand