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Dave Platt Dave Platt is offline
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Default Mixing 4 audio channels to 3?


In article ,
DaveC newsgroups wrote:

Only a single supply (in the amplified speakers) is available to power this
circuit. I can tap this supply for my circuit:


One cautionary note with regards to this circuit, as drawn: be careful
when you wire up R15. If you use a standard three-terminal
potentiometer, make sure that you wire both the wiper, and one of the
two ends to U3's inverting input. Don't just wire up the wiper!

The reason: pots occasionally go "open" due to dirt or wear. If you
have only the wiper connected, and it goes open, you'll have no
feedback path around U3, and it'll immediately and enthusiastically
slam its output against one of the rails (or both in rapid succession
if there's a signal present). This will let out a really unholy
THWOMP from your subwoofer, and may pop the cone out of the cabinet or
at last shove the voice coil out of the gap. Expensive damage.

With a three-terminal wire-up, the resistance in this part of the
feedback loop will never be more than the bulk value of the pot (i.e.
open wiper == wiper all the way at one end) and this will limit the
maximum subwoofer volume. You can choose the maximum loudness by
setting the value of the pot.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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