> "Using the [MIC TRIM] control, you can adjust the input gain
> appropriately to accept a signal between -50 dBu and -10 dBu. At the
> fully counterclockwise position, you can get the proper input gain
> for a -10 dBu signal. At the fully clockwise position, you can get
> the proper input gain for a -50 dBu signal."
Clearly, the trim control is a variable feedback resistor in the mic
preamp. It sets the gain of the preamp. It would be impossible for an
attenuator to make the preamp accommodate the stated gain range.
Another good clue that it is feedback rather than a fader is the fact
that it doesn't go "off" in the minimum-gain position.
Most mic preamp trims that I have encountered are made like this. It's
the best way--you "trim" the gain of the preamp so you only have as
much gain as you need. Optimizes both noise and overload level. Try
this:
<http://mysite.du.edu/~etuttle/electron/elect3.htm>
About half-way down the page, there's an illustration "gain control"
on the right that shows the typical circuit.
> The "max" trim knob setting could be wired to introduce a gain stage.
> What you describe as the final few degrees of knob rotation sounds
> somewhat like the 10dB gain "bump" one typically finds just before
> the highest setting on a record level knob.
Not typical of a fader-type record level, but very common in feedback
gain trim pots. They simply don't make pots with the necessary taper
to perform this function smoothly. Even very expensive preamps usually
have a huge gain increase in the last 10% of rotation.
The big boost in perceived noise is the result of a big boost in gain.
That could be mic noise or preamp noise--either way you'll hear more
when you turn up the gain!
-Dan Dugan
I'm not an engineer, but I play one at work.
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