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Drawmer DF330

Universal Noise Filter By Paul White
Published April 2002

DRAWMER DF330 universal noise filter.

We test this two-channel analogue dynamic noise filter, expander and HF enhancer.

The 1U DF330 is an updated version of Drawmer's original DF320 dual-channel noise filter and is designed to reduce broadband 'hiss' noise by means of dynamic filtering and low-level expansion. All-analogue in operation, it features both balanced XLR (+4dBu) and unbalanced jack (-10dBv) I/O, where each channel combines a separate expander and tracking low-pass filter. Unlike Dolby or Dbx processes, it does not rely on encoding, and it works on most types of source material, whether they be single tracks or complete mixes.

The reason dynamic filters of this type are so effective is that they use psychoacoustic masking to their advantage. When sounds are loud, the noise is masked, so no processing is necessary, while at low levels where noise can be a problem, the human ear expects less high frequency content, and so more high-cut filtering can be applied. The filter cutoff frequency automatically tracks the input signal level and closes progressively as the sound level decreases, according to the user threshold setting. As the filter closes, the hiss level is reduced.

In addition to the manual 'level threshold' mode, the DF330 also features an Auto mode that tracks the frequency content rather than the level of the input signal. Auto mode can sound somewhat overprocessed on some material, but tends to work well with electric guitars or basses, with the Manual setting being better for mixes. A meter shows the filter's cutoff frequency during operation, and both the filter and expander have separate bypass switches. A switch links both channels for stereo operation.

The expander section, though sporting exactly the same controls as the original, has been redesigned along the lines of the expander used in the Drawmer DL241, which provides a smoother operation due, in part, to its sophisticated automatic attack-time function. The expander only comes into play at very low levels to clean up pauses and has switchable 1:2, 1:5 and 1:20 ratios along with variable threshold and release settings. A bar-graph meter shows the gain reduction being applied and a 50Hz low-cut filter can be switched in to attenuate low-frequency noise, such as hum. One refinement of the DF330 is the inclusion of a preset enhancer circuit in the filter section to compensate for the loss of high-frequency harmonics incurred by the processing.

DRAWMER DF330 rear panel.

How Well Does It Work?

Setting up any dynamic-filter noise reduction is always a compromise between removing enough hiss and causing audible dulling of the material. For moderate noise contamination, the system works very well, but severely degraded material may be impossible to process satisfactorily without incurring some unwanted tonal change during quiet passages. As the filter removes high frequencies, the enhancer partially replaces them so as to produce less of a subjective tonal change, and in cases where minimal processing is being applied, the output can end up sounding slightly brighter than the input. You still have to listen very critically to the processed material to ensure that low-level detail isn't being blurred or that reverb decays are not being choked, but the enhancer means more processing can be applied before tonal changes become noticeable. When processing noisy material, it's sometimes possible to hear the background noise rising and falling in level with the programme material, but this usually only happens where the noise contamination is quite severe in the first place or where there are no high frequencies present to mask the noise.

The DF330 is great for combating guitar amp noise, tape hiss and background noise from old hardware, though it's less effective against problems such as hum and buzz, except where the impressively smooth expander comes into play. It is also not much use against clicks and pops. That said, the DF330 definitely works that little bit better than the DF320, and is probably the most useful of the analogue noise filters I've tried to date.


  • Easy to use.
  • Can be used on any mono or stereo source material.
  • Effective on light to moderate noise contamination.


  • Noise breathing and tonal changes may be audible when applying heavy processing to heavily contaminated material.


A worthy successor to the DF320, offering worthwhile improvements in performance.


£499.38 including VAT.

Drawmer Distribution +44 (0)1924 378669.