500 Series Dynamic High Frequency Fixer

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.

The DerrEsser Has Four Modes

DS MODE – (Bypass Button IN, all other buttons OUT). 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.  Easy to set and forget. 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 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, and is probably the first carefully tailored soft knee HF limiter in existence.

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.

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.


  • Frequency Response – 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.
  • I/O – DC Coupled input and outputs. Input Balanced, Output single ended but with balanced impedance. Pin 2 hot.
  • Impedance – 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.
  • Power Consumption – 8 Watts max. 4 Watts Typical.


Yes it is! We based the de-essing circuit in the DerrEsser off of the DS section in the Lil FrEQ.

If there is just an occasional peaky note, the normal DS mode can be tame specific notes, taking the harsh frequencies out surgically. You can also use the DS dynamic HF limiter to get a very tape like effect if used judiciously. We have found it to be very useful on overly crunchy plucky guitars, and “spitty” vocals alike.

Bandlimiting the top end can tighten up and quiet the hiss of electric guitars, especially “heavy” ones. Put the DerrEsser in LP Mode and close down the top end to 8 – 10KHz. You will usually find you don’t miss these frequencies on heavy distorted guitars at all. It lets the cymbals and vocals etc have more clarity up there.


Magazine Reviews


“We were primarily interested in the DeEssing, but were pleasantly surprised at the HF limiting. As it turns out, we’re using them for the limiting way more than in De-Ess mode. Who’d have thunk? Excellent job, man. The DerrEsser is an amazing new piece of gear. We came for the De-essers, but stayed for the HF Limiter.”
Bob Weston
Chicago Mastering Service

Related Articles

Quest for the Invisible De-Esser

Empirical Labs uses an unusual De-Esser and HF (high frequency) limiter circuit in two of their products, the
Read More