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Re: FR2-LE mic trim (again)

Subject: Re: FR2-LE mic trim (again)
From: "Rob Danielson" danielson_audio
Date: Sun Apr 27, 2008 9:33 pm ((PDT))
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_S=
chematic.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/blog=
/><http://www.phonography.org.uk/blog/>http://www.phonography.org.uk/blog/
>>>
>>>
>
>


--






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