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Gain Brain

Drawmer DL251 Spectral Compressor

This sophisticated compressor has a choice of hard- or soft-knee compression and has a built-in spectral enhancer.

Drawmer's DL251 offers both hard and soft-knee limiting, an independent acting peak limiter, and incorporates the newly developed Dynamic Spectral Enhancement system to prevent detail loss during heavy compression. Paul White checks it out.

Drawmer have produced several compressors over the past few years, including the excellent 1960 Tube Compressor, but it wasn't until the introduction of the DL241 last year that people started to really pay attention. Before that, Drawmer were still widely regarded as 'those guys who make gates' — a gross underestimation, in my view. In many ways the new DL251 is an extension of the DL241, though it should be made clear that it is not intended to replace the DL241 but to supplement the existing range. Whereas the DL241 offered a soft or progressive 'knee' characteristic, the DL251 is switchable between soft and hard-knee modes, while the expander section of the DL241 has gone to make way for the Dynamic Spectral Enhancement system. The highly effective full-wave peak limiter has been retained, along with the choice of manual or auto attack and release times. A sidechain insert point has also been provided.

Most of the terms will be familiar to anyone with any experience of compressors, with the possible exception of Dynamic Spectral Enhancement which, for the sake of brevity, I will refer to as Enhancement from here on. Most compressors follow the signal envelope of the incoming audio and use this information to apply variable gain reduction, in order to reduce the dynamic range of the signal being processed. In other words, when a loud signal is detected, it is used to control the compressor in such a way as to turn itself down.

The problem is that low-frequency sounds, such as bass guitars and kick drums, tend to have a much higher energy level than bright sounds like cymbals, acoustic guitars, and such like, so if a powerful bass sound causes the compressor to turn down the gain, then any bright sounds occurring at the same time will also be turned down — whether they need to be or not. On occasions when a high degree of compression is required, this can lead to a perceptible dulling of the overall sound, which is clearly not a good thing. The problem can be solved by using multi-band compressors that split the audio spectrum into two or more frequency bands and then treat each band separately, but these are costly to build and, unless very carefully designed, they can introduce phase distortions which compromise the subjective quality of the audio signal.

A less complex but nevertheless effective approach is to devise a circuit which will actively boost high-frequency detail during periods of heavy compression, and that is exactly what Drawmer appear to have done in the DL251. When no compression is taking place, the Dynamic Spectral Enhancement circuitry is inoperative and so no change is made to the signal. Equally, no noise is being added by the effects of high-frequency boosting on programme noise. However, as the signal level rises to a point where gain reduction is taking place, the Enhancement system kicks in applying dynamic boost which is directly related to the degree of gain reduction taking place. The degree of enhancement may also be controlled by the user, so there's no question of the machine applying dynamic boost on occasions when you don't want it to. Interestingly, the Enhance control has sufficient range to allow the unit to overcompensate quite noticeably, which opens up the additional possibility of creative processing.

The choice of hard or soft-knee action is becoming quite common on modern compressors, since the two systems do produce quite different results. As a rule, soft-knee designs manage to control the gain in a less obvious way, whereas hard-knee or straight ratio-type compressors exert a much more positive type of control, making them more suitable for creating compression 'effects' or for keeping tight control of a signal with unpredictable dynamics.

When the Soft option is selected, the compression ratio gradually increases as the input approaches the threshold. This happens over an input range of 10dB, settling at the value set by the user on the Ratio control. In other words, the compression is phased in more subtly, whereas in Hard mode, gain reduction is applied at the chosen ratio setting as soon as the threshold is exceeded.

The peak limiter is located at the end of the signal chain and after the Gain control, which means that it can be used to set an absolute level which the output cannot exceed. In operation, the DL251 limiter is quite ingenious because it reacts differently to short peaks than it does to long periods of overload. Very short peaks are first arrested by clipping diodes to prevent even the fastest transients sneaking through, but the gain is turned down sufficiently before any distortion caused by the clipping becomes audible, to allow the signal to pass through cleanly. When the input signal drops below the limiter threshold, the gain is restored over a period of around one second. If the limiter is deliberately pushed into action, audible gain pumping can be provoked, which may be useful as a creative effect. However, when sensibly set, the limiter is quite inconspicuous in operation.

Both the limiter and the compressor utilise the same dbx VCA chip, which has become very much an industry standard in gain control devices such as compressors, gates, and console automation systems. Used with properly designed circuitry, this chip is capable of very low noise and distortion, and the audio performance of this unit suggests that the Drawmer team have applied their usual high standards during the design of the DL251.

The Package

Physically, the DL251 is unmistakably Drawmer, with its black front panel, white legending, and yellow knob pointers. As with the other compressors in the Drawmer range, there are two audio channels which may be used independently or linked for stereo operation. A pushbutton on the rear panel presents the user with the option of Average or Peak characteristics in the linked mode. Some form of stereo linking is necessary when processing stereo signals, otherwise a loud event in~one channel will cause that channel to compress more than the other, making the stereo image appear to swing towards the channel being compressed the least. The most logical answer is to average the two channel levels so that the same compression is applied to both, hence the Average option, but some American engineers apparently prefer a system whereby the compressors respond to whichever channel is currently loudest. To satisfy this latter request, the Peak option has been provided.

Inputs and outputs to the unit are via electronically balanced XLRs wired pin 2 hot, though the unit can be used unbalanced by grounding pin 3 if required. A nice touch is the provision of level switches which configure the unit for either +4dBu or -10dBV operation, ensuring the optimum gain structure with either professional or semi-pro equipment. Mains is provided by a standard IEC connector, while a recessed voltage selector switch allows the unit to operate on either 240V or 120V supplies. Internally, the circuitry is housed on a large, double-sided glass-fibre PCB, with a smaller PCB carrying the meter LEDs. The transformer and power supply circuitry are located at one end of the unit, away from the audio circuitry, and connections between the two circuit boards are via hard-wired ribbon cable.


With the exception of the centrally-mounted Stereo Link switch and the mains switch, both channels of the unit are equipped with identical sets of controls. In linked mode, only the lefthand set of controls is operative and this serves as a master for the whole unit. During linked operation, only the output level meter readings are independent; all other knob and switch functions, including Bypass, are driven from the lefthand channel.

The controls themselves are set out in a logical sequence, starting with Threshold. This has a range of +20dB to -40dB and adjacent to it is the Hard/Soft switch, which has a red status LED for Hard mode and a green one for Soft. The Ratio control offers a particularly wide range from 1.2:1 right up to infinity:1, making the unit capable of hard limiting as well as the most subtle compression.

Separate controls are fitted for the Attack and Release times, the Attack being variable from 0.5ms right up to 100ms, affording plenty of scope for creative transient enhancement. Release is variable from 0.05 to 5s while the Auto switch (flagged by a green status LED) may be used to select programme-dependent attack/release characteristics. This type of auto function is invaluable when working on material with unpredictable or varying dynamics — such as vocals, slapped and pulled bass guitar, or complete mixes — as the characteristics are continually optimised. In fact, unless the compressor is to be used as an effect rather than as an unobtrusive means of gain control, I'd choose Auto every time.

Next along is the S/C Listen button, which routes the sidechain signal directly to the output for monitoring purposes. This is valuable when an external equaliser is patched into the sidechain insert point for de-essing, as it allows the engineer to home in on the frequency band causing the trouble. Because this is a locking switch, a red status LED is fitted to prevent it being left depressed inadvertently.

Because a compressor works by turning down the gain when high signal levels are present, some form of gain make-up is required. In the case of the DL251, the Gain control has a range of -20dB to +20dB, which should cover most situations comfortably. The channel Bypass provides a hard link directly from the input to output, which means the signal has a minimum signal path when the compressor is not being used; and, in true Drawmer tradition, a red LED is fitted to show when Bypass is active. A 9-segment LED meter continuously monitors the amount of gain reduction taking place, up to a maximum of 30dB, while the output level meter has 10 segments and reads from -20dB to +15dB. This follows the normal convention of green LEDs below 0VU, a yellow LED at 0VU, and red LEDs above 0VU.

Leaving the compressor controls, we come to the Enhance control, which is a single knob accompanied by an On switch. A green LED monitors the amount of Spectral Enhancement taking place and this is influenced both by the degree of compression being applied and the setting of the Enhance control.

Finally comes the Peak control, which sets the limiter threshold. This is calibrated from 0dB to +16dB and a red LED indicates when the limiter is active. As if the unit didn't have enough LEDs, a red status LED is also provided below the mains switch!

DL251 In Use

While setting up, it is best to first set the Peak threshold control fully clockwise, to prevent the limiter coming into play unexpectedly, and to turn the Enhance function off. This allows the compressor to be set up in isolation. In Soft mode, I found the unit behaves very much like the DL241, in that it provides very smooth, almost effortless gain control with very little of the transient dulling that the Enhance control is designed to rectify. The Auto function performed faultlessly on everything I threw at it, and as I've used a DL241 (which - behaves in exactly the same way in Soft mode) since they were first released, I can honestly say that I've been unable to catch it out yet. When working with vocals or complete mixes, this is the compressor I patch in first.

In Hard ratio mode, the firm, almost cheekily heavy-handed control is more evident, but in such a way as to creatively augment the material rather than spoil it. For example, rock vocals can be made to really thicken up with just a hint of pumping if required, to create the illusion of power and loudness. And if you really want to make it pump noticeably, you can bring down the Peak limiter threshold and let this join in! Likewise, bass guitars, electric guitars and even acoustic guitars are invested with a certain authority which they mostly lack in their uncompressed form.

So far, we haven't talked about the Enhance control, but don't worry, it really does work. On most material, a setting of around half-way provides more than enough lift to the high end detail, though the exact degree depends on how much gain reduction is applied by the compressor section. A longer release time will ensure continuing periods of gain reduction if the Enhance system is to be used as an effect rather than as a corrective tool. The effect is subtle but quite noticeable at the higher settings, and the result is not unlike that produced by an exciter. However, as the effect is dynamically related to the gain reduction being applied, it doesn't add any noise during pauses and quiet passages when little or no gain reduction is taking place.

If the Enhance function is deliberately over-applied, it can be used to good effect to brighten up cymbals, acoustic guitars, bright percussive sounds, and such like. This it does well without adding harshness or noise — its creative applications shouldn't be written off as a mere gimmick!

Finally, the Peak limiter is extraordinarily competent, and used normally to intervene only when unexpected peaks occur, it is quite unobtrusive. But if you crank up the Gain control or turn down the Peak threshold, then it starts to take over the show in no uncertain terms. Again, I foresee many engineers and producers actually using the limiter as a creative effect, especially on hard rock vocals.


I have a soft spot for Drawmer gear and really like the DL251, but at the same time acknowledge that compressors are nearly as personal as microphones and there are many excellent models already on the market. Even so, I feel that the flexibility provided by the Hard/Soft and Manual/Auto modes, combined with its utter predictability when setting up, make the DL251 a real winner. It has just enough character to be attractive while also having the ability to remain transparent when required for careful level control. Furthermore, the Enhance control really does fulfill its promise of keeping the detail intact while also presenting itself as a creative effect in its own right. The same is true of the limiter which, again, is identical to that in the DL241 — and I've had no complaints in that department over the past few months. The only thing I missed on the original DL241 was some means of sidechain access, allowing me to use the device as a de-esser by patching in an equaliser, but that has been redressed in the new DL251.

I suppose I like Drawmer gear because I'm inherently lazy and find it does just what expect it to do, and with the bare minimum of grief. The DL251 continues this admirable tradition and, considering its competitive price, wins my vote.


Input Impedance 22kOhm
Max Input Level +21dBu (+17dB ref. +4dBu)
Output Impedance 50 Ohm
Max Output Level +20dBu
Bandwidth <12Hz to 25kHz (-1dB)
Crosstalk @10kHz better than -80dB
@20kHz better than -70dB
Distortion (Enhance off):
Unity gain +4dBu input 100Hz <0.09%
1kHz <0.03%
10kHz <0.08%
Noise at unity gain (ref. +4dBu):
Wideband -86dB
IEC A-weighted -96dB

Further Information
Drawmer DL251 £558 including VAT.

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Modern Art

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Recording Musician - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.


Recording Musician - Jul 1992

Gear in this article:

Studio FX > Drawmer > DL251 Spectral Compressor

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Review by Paul White

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> At Home in the Studio

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