Interesting. Mike. "Trim," in this case, does seem to stretch common
usage. Fostex recommends on page 60 of the owner's manual:
http://www.fostexusa.com/support/pdf/fostex/fr2le/fr2le_owners_manual.pdf
"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."
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. What mics/trim settings
do you typically use? Is there sufficient gain in quiet locations
when the trim is at the slightly lower setting--that is-- not fully
clock-wise? Rob D.
At 6:46 AM +0100 4/28/08, Michael Oates wrote:
>Rob,
>
>I think the use of the word trim is incorrect in this case, yes I
>know that's what Fostex
>call it. But better to call it gain, and the Recording level
>controls should be regarded
>as a fader.
>
>I set the "trims" as high as possible without introducing extra
>noise and leave them
>there. Any further adjustment has been done with the "Recording
>level control" and I have
>not had a problem. The noise is only introduced on the last few
>degrees of rotation, you
>just have to listen for it.
>
>Mike
>
>>Hi Mike--
>>I suppose there are exceptions to everything, but in all of the mic
>>preamp circuits I know of, "trim" is attenuation only. Fully
>>clock-wise or "off" can't add noise because its not adding
>>amplification. It could sound as if it is, I suppose,.. such as when
>>the headphone level is high or when a mic has exceptionally high
>>output. Given the very low-noise performance of this recorder's mic
>>pre, with the trim "off," it could be that you are hearing more of
>>the mics' self-noise.
>>
>>Some mic sensitivity/sound level combinations create circumstances
>>when one would want to routinely run with trim, but doing so to
>>improve noise performance isn't the way I understand mic pre circuits
>>to be designed.
>>
>>Here's a block diagram showing trim adjustment before amplification:
>><http://music.concordia.ca/Programs/Electroacoustics/M50_mixer/Mixer_Inpu=
t_Schematic.html>http://music.concordia.ca/Programs/Electroacoustics/M50_mi=
xer/Mixer_Input_Schematic.html
>>
>>Rob D.
>>
>>
>>At 10:54 PM +0100 4/27/08, Michael Oates wrote:
>>>The problem with turning those trim knobs fully clockwise is that
>>>the preamps get very
>>>noisy, you really need to reduce them a bit.
>>>
>>>Mike
>>>
>>>>At 2:48 PM +0100 4/27/08, James Huckle wrote:
>>>>>Hi All
>>>>>
>>>>>I appreciate this subject has gone around for a while now. My unit
>>>>>arrived on Friday and I've been researching what the mic trim does,
>>>>>generating tests tones to set mics etc.
>>>>>
>>>>>The missing piece of information I need is does the mic trim setting
>>>>>apply for given mic for *any* recording?
>>>>
>>>>Yes, except for rare, loud sounds and settings. ;-)
>>>>
>>>>Assuming the output levels of your stereo mics are matched to within
>>>>a decibel, for most quieter, natural environments and sounds, turn
>>>>the trim knobs fully-clockwise. This removes all "trim" from the mic
>>>>preamp circuit.
>>>>
>>>>One way to tell if the output levels of your mics are well-matched is
>>>>to turn the trim knobs fully clockwise, place the mics side by side
>>>>in a quiet setting, raise the master record level while listening
>>>>with a good pair of stereo headphones. As you raise the master
>>>>record level, the background "presence" (usually a dirty low
>>>>frequency "rumble") should increase in both ears at the same rate.
>>>>If one side fades in sooner (louder), that side may need trimming. If
>>>>the meter readings seem to show the same imbalance, go ahead and trim
>>>>the louder side until both sides fade in from "silence" at the same
> >>>rate. You can always adjust the stereo balance in post but sometimes
>>>>mic levels are so different its nice to match them more closely in
>>>>the field. Lower/trim just the louder side, not both.
>>>>
>>>>>I.e. for a given mic will
>>>>>the same trim setting be used for quiet ambiance as well as loud or
>>>>>will I need to trim for each situation?
>>>>
>>>>Its safest to use the trim knobs only when you cannot reduce the
>>>>master record level enough to get correct meter readings. Rob D.
>>>>
>>>>>
>>>>>Any advice appreciated. Many thanks
>>>>>
>>>>>James
>>>>>
>>>>>--
>>>>><<<http://www.phonography.org.uk/blog/>http://www.phonography.org.uk/b=
log/><http://www.phonography.org.uk/blog/>http://www.phonography.org.uk/blo=
g/><<http://www.phonogrphy.org.uk/blog/>http://www.phonography.org.uk/blog/=
><http://www.phonography.org.uk/blog/>http://www.phonography.org.uk/blog/
>>>>>
>>>>>
>>>
>>>
>
>
--
|