The DerrEsser ELDS-V from Empirical Labs is a vertical multi-function dynamic filtering device, designed for API 500-Series enclosures.
 
In its basic DS mode, it is an effective, level-insensitive de-esser, a device that will adjust the high-frequency gain on overly treble-y or spiky vocals or other sources in an extremely musical manner. However, unlike other de-essers, the device allows for multiple uses of its circuitry. As a de-esser in the DS mode (HF Limit OFF), the threshold can be set 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. The DS threshold is not level dependant, and compares the amount of high-frequency content to the low-frequency content, so adjusting the overall level does not affect the threshold. It features VCA technology for glitch-free, distortion-free, and noise-free performance. The corner frequency is user adjustable with a high-performance active crossover.

In HF Limit mode, the DerrEsser essentially acts 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 is more of a general high-frequency smoothing device. In high-pass mode and while using the LISTEN function, the user can employ the 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 with the frequency knob. In low-pass mode and 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 with the frequency knob.
High-frequency limiting with a tailored soft knee.
 
The Features
- The Dynamic Section can be used for de-essing or smooth high-frequency softening.
- Hard-Clip Indicator Light This Bad-Clip LED indicates when the sound level is within 1 dB of hard clipping and monitors several sections within the DerrEsser for possible clipping
- Unity Gain  6 dB of gain on inputs and outputs available
- API-500 Series rack compatible
- True bypass
- Military-grade switches and relays