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Re: FR2-LE mic trim (again)

Subject: Re: FR2-LE mic trim (again)
From: "Michael Oates" msohooates
Date: Sun Apr 27, 2008 10:47 pm ((PDT))
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 shoul=
d be regarded
as a fader.

I set the "trims" as high as possible without introducing extra noise and l=
eave them
there. Any further adjustment has been done with the "Recording level contr=
ol" 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_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/blo=
g/><http://www.phonography.org.uk/blog/>http://www.phonography.org.uk/blog/
>>>>
>>>>
>>
>>




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