I see that my dog has brought back a crow he found on the road. I
should be joining him for lunch.
I apologize. I was pretentiously assuming that I have a grasp of the
basic circuitry when I should have been asking questions along with
James. Thanks, Mike, Dan and Lou for speaking up and for your
clarifications. Sorry for muddying-up your advice, Marc. I've not
used a mic pre/recorder where any degree of "trim" was the better
option in a quiet setting.
I remain interested in understanding how to best utilize the trim
knobs in the FR-2LE. Assuming a variable feedback resistor circuit
(until we get a schematic of the FR-2LE), if one uses a mic with
25mV/Pa sensitivity (about -30dBU, right?) I can predict that the
trim knob setting will be closer to the clockwise (-50dBU) end of the
range. But, as to finding the trim setting "sweet spot," for this
mic, the calibration procedure isn't quite clear to me working from
Marc's directions or the Fostex's manual. Can someone describe this
procedure again and perhaps some of the principle(s) involved?
Once determined, is this trim knob position essentially,"unity" or
_the_ optimal setting for the particular mic no matter the sound
levels encountered (as I believe Marc was alluding to)? Rob D.
At 8:49 AM -0700 4/28/08, Dan Dugan wrote:
> > "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>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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