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Desser / Dynamic Section from the Lil FrEQ as a module for API 500 Series, with Vertical faceplate.

The DerrEsser is a multi function dynamic filtering device, in an API 500 series module format. In its basic “DS” mode, the DerrEsser is an effective, level insensitive de-esser, which is a device that will adjust the high frequency gain on overly trebly or spiky vocal, or other source. By using a crossover technique, and dividing the audio up into a high frequency range and a low frequency range, the DerrEsser can use its Voltage Controlled Amp to turn down comparatively high or harsh high frequencies in an extremely musical manner. However, unlike other de-essers, the device is not limited to just de-essing, and allows for multi use of its circuitry

Besides Bypass, there are essentially four modes of the DerrEsser:

DS Mode – (Bypass Button IN, all other buttons OUT). As a de-esser in the DS mode (HF Limit OFF), one can set the threshold to tame harsh S’s (sibilants) that result from an overly bright singer or from EQ’ing to make a vocal sound aggressive and clear. In this mode, the dyamic action is “level insensitive”, as the detector circuitry compares the High Frequency content to the low frequency content, Overall signal level doesn’t matter at all. When there is enough HF compared to LF, the threshold is exceeded and the DerrEsser starts to turn down the frequencies above the crossover point set by the Frequency control on the front panel. Again, great care was taken to ensure this DS section meets the Empirical Labs reputation for originality and ease of use. We use the latest technology VCA, specially selected for glitch free, distortion free, and noise free performance. The time constants were carefully selected and the corner frequency is user adjustable with a high performance active crossover.

HF LIMIT – (Bypass Button IN, HF LIM IN, all other buttons OUT). In this mode, the DerrEsser looks at the general level of the High Frequencies only. Think of it as a high frequency compressor. If there is enough high frequency content to exceed the threshold, then gain reduction occurs. It IS level sensitive, but responds much more to high frequencies. This mode can also act as a de-esser but is more of a general high frequency smoothing device.The HF limiter is probably the first carefully tailored soft knee HF limiter in existence. We believe both the DS and HF LIM modes offer the highest performance of any similar device

HIGHPASS – (Bypass Button IN, Listen IN, HP/LP OUT, HF LIM doesn’t matter). By using the “LISTEN” Function, the user can employ the superb filter circuitry inside the DerrEsser to process their audio. With the LISTEN button IN, and the HP/LP switch OUT, the user will be hearing the high frequencies that are being controlled with the dynamic/compression circuits. The corner frequency is adjustable by the Frequency knob. Once the LISTEN switch is engaged, all bargraph LED’s should light as indication.

LOWPASS – (Bypass Button IN, Listen IN, HP/LP IN, HF LIM doesn’t matter). With the LISTEN button IN, and the HP/LP button IN, the user is now listening to the low frequencies, those not affected by the VCA and dynamic/compression circuits. This is a 12 dB per octave filter whose corner frequency can again, be adjusted by the Frequency knob.

One of the coolest and most useful tools ever invented for the 500 Series mounting format is the Empirical Labs “DerrEsser” (a.k.a. the “EL-DS”). This seemingly small package combines a LOT of punch and control.

There are two main modes in the “DerrEsser”. The first is the “de-esser”, and the second (not necessarily in this order) is the “high frequency limiter”. While kind of similar in thought, the two modes are rather different in operation. Additionally to “de-essing” and “high frequency limiting” the “DerrEsser” can also act as a “HPF” ( High Pass Filter) or “LPF” ( Low Pass Filter).

When the “DerrEsser” is used as a “de-esser” the unit compares the quantity of high frequency material as it relates to the low frequency content of a sound and adjusts the high frequencies accordingly so no matter the level the relationship between the high frequency and low frequency information remains balanced.

The “corner frequency” for the action of the “de-esser” is controlled by the frequency selection knob at the bottom of the module. This corner frequency selection is made quite easy with the “LISTEN” function engaged. When “LISTEN” is engaged it becomes amazingly quick and easy to identify any problem frequencies (finger squeaks from acoustic guitars, “ssss” or “whistle” problems within a vocal, etc.), while you employ the “de-essing” function to control these problems / issues and move on with your day.

The “HF Limiter” is an entirely different, and as far as we know, an entirely unique device that solves all kinds of problems while making your day remarkably easier. The “HF Limiter” works like most limiters, but only on the high frequency content. It has a “knee”, and drops the level of the information that crosses the “threshold” so the limiter will tame the volume of the high frequency signals that has crossed the user determined threshold. However, what makes this unit unique is the shape of the limiting curve, with its smooth unobtrusive knee and its dependence on frequency as determined from the frequency selection knob on the front of the unit. Unlike “de-esser” mode, when the “HF Limiter” is engaged it is only interested in the overall volume of the high frequency information on a “stand alone” basis – the “HF Limiter” does NOT compare the high frequency information to the low frequency information as is done in “de-esser” mode.


If you’re a fan of “pop music” from yesteryear you have undoubtedly heard the brilliant, bright, ringing and expressive piano sounds on Elton John or Bruce Hornsby recordings. You may have marveled at the “airy” backing vocals on Def Lepard or later CARS recordings. Well now you can achieve VERY similar results in the privacy of your own home!!

One of the high profile users of the “EL-DS” discovered that if you engage the “HPF” with the “LISTEN” function, the result (which can be controlled through the frequency selection knob) is that only the “tippy-top” frequencies of the signal make it through the unit. That user found that by using a pair of “Derr-Esser”s with a pair of “EL-8X” units that they could achieve this effect with ease. This has apparently become part of that particular user’s mixing regimen where there is ALWAYS a “stereo aux” send set up with a pair of “EL-DS”s and an “EL-8XS” set that return on a fixed level “aux return”.

  • Multi-purpose tone enhancement module for 500 Series racks
  • Super low noise line input, or instrument preamp
  • Six digitally controlled sections of analog processing
  • Four sections of Equalization - One High Pass and 3 parametric bands
  • Renowned Empirical Labs compression
  • Saturation circuitry that scrunches the highs and allows hotter levels like tape
  • De-Ess, HF limiter, HP and LP filter modes
  • Freq. Response is 3 Hz to 120 kHz (-3dB points).
  • Dynamic Range - 115 dB from maximum output (.5%THD soft clipping) to minimum output.
  • Transformerless noise floor typically – 93dBu.
  • Distortion Ranges between .0035% and .01% depending on mode and settings.
  • DC coupled Input and Outputs
  • Input Impedance is 20k Ohm. Output impedance is less than 51 ohms.
  • D-S Dynamics Time Constants - Attack time is less than 1mS. Release .04 sec, fixed internally.
  • Input Balanced. Output single ended or differential, selected with jumper.