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DBX 904 - 900 Series Noise Gate

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This used DBX 904 Noise Gate will ship in an anti-static bag.

This gate is in good condition with normal wear, has been fully tested, and works great. Please see pictures for details on item condition. Only what is pictured is included. Item will be shipped in an anti-static bag.

Product Details:
The basic purpose of a unit like the dbx 904 is to remove unwanted background sounds in the spaces between desired foreground sounds. Note that there has to be some real level distance between the unwanted and wanted material- at least a few dB -in order for the 904 to "get its foot in the door." If levels are too much the same (because of compression, for example), the downward-expansion efforts of the unit will go for naught. Therefore, use the 904 before any compressors.

For example, a 904 can be used to prevent or reduce leakage among microphones in live-sound reinforcement and during panel discussions. Placing a unit on each mike and setting its THRESHOLD below the level of the music or speech will achieve this. Similarly, broadcasters can employ 904s to tighten land-line feeds or noisy ENG audio7 gating out low-level trash on the lines by having a unit set as just discussed on the output from the feed or ENG equipment.

ATTN LIMIT. This is useful if you don't have total control over the signal and the input level and/or you don't want to attenuate the signal severely and make it sound unnatural or dead. It permits much gentler effects to be · achieved. As suggested, try damping the reverb of a note or chord in various amounts so you can sense the effect; fine-tune the sonic tail down perhaps 6-8 dB and see how it suits you. Full gating (greater than 60 dB of attenuation) often sounds too dry to be believable. This control is especially useful in damping a particular track's reverberance/ambience.

The ATTN LIMIT knob, by the way, is the last circuit element in the CV-processing path. It is therefore best set after everything else is adjusted. If you start with the limit at maximum (60 dB nominal), it is easier to hear the effects of the other controls 

As noted, the actual· amount that the signal is attenuated is a function of both the THRESHOLD setting and the RATIO control. The amount of attenuation can be readily calculated by determining how far the signal is below the threshold and what the actual ratio is: for example, if the signal is 10 dB below THRESHOLD setting and the ratio is 5, then the signal will be attenuated 50 dB (unless the attenuation has been limited).

RATIO.  To repeat, a mild setting (low ratios) typically will do for reverb control and general dynamic-range expansion - subtle effects, to be achieved by altering a signal's dynamics through downward expansion. Low ratios also are good when the distance between the signal and the noise is little and you don't want the signal "torn up" as it crosses the threshold. When the signal ends sharply or there is a lot of S/N distance, high ratios are possible without harming (tearing up) the sound. Again, particularly when combined with high attenuation, high ratios are appropriate for tightening percussion sound (for example) and eliminating background noise. This essentially turns off all signal during quiet parts.

ATTACK. Since this determines the speed at which a signal returns to normal after being attenuated, a fast setting lets the natural attack of virtually any signal pass through unaltered, and slower ones control the initial transient for those applications where smoothness is called for. Likewise RELEASE, which determines the speed of the 904's attenuation: faster settings on percussive (or similar) sounds gate out background noises, and slower settings provide smoother transitions for signals with long decay.

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