Empirical Labs DocDerrEsser - EL/Rx-H DocDerr + ELDS-H DerrEsser & EL500 1U Horizontal Rack

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  • Empirical Labs DocDerrEsser - EL/Rx-H DocDerr + ELDS-H DerrEsser & EL500 1U Horizontal Rack
  • Empirical Labs DocDerrEsser - EL/Rx-H DocDerr + ELDS-H DerrEsser & EL500 1U Horizontal Rack
  • Empirical Labs DocDerrEsser - EL/Rx-H DocDerr + ELDS-H DerrEsser & EL500 1U Horizontal Rack
  • Empirical Labs DocDerrEsser - EL/Rx-H DocDerr + ELDS-H DerrEsser & EL500 1U Horizontal Rack
  • Empirical Labs DocDerrEsser - EL/Rx-H DocDerr + ELDS-H DerrEsser & EL500 1U Horizontal Rack
  • Empirical Labs DocDerrEsser - EL/Rx-H DocDerr + ELDS-H DerrEsser & EL500 1U Horizontal Rack


We understand that there are people interested in getting our 500 series modules that want the best enclosure/power supply possible for them, so we are glad to pre-load and bundle the DocDerrEsser with one EL/Rx-H DocDerr 500 series horizontal multi-purpose tone module and one ELDS-H DerrEsser 500 series horizontal dynamic high frequency fixer fitted into EL500 1U horizontal rack.

DocDerr 500 Series Multi-Purpose Tone Module

One of the newer members of the Empirical Labs family is the DocDerr, a 500 series multi-purpose tone enhancement module that houses six different sections of digitally controlled analog processing.  With over two years of meticulous development and testing behind it, the DocDerr provides an unprecedented combination of power and musicality in a compact, easy to use format. Empirical Labs' first compressor in the API 500 format.

An extremely low-noise preamp capable of working with line level or instrument level signals, is followed by four sections of EQ combining a two position high pass filter with three parametric bands, each with its own set of carefully tuned frequency presets.  An internal jumper can turn the DocDerr into an unparralled instrument preamp with 16dB of gain, ideal for acoustic guitar, bass, synths, exotic plucked instruments, etc.

The versatile EQ is followed by a unique dynamics section which offers renowned Empirical Labs compression at an unprecedented price, and an extremely musical tape emulation circuit that softens and tames high frequencies and transient spikes.  We believe it’s the best single knob “knee” compression available in this format.

One of the pivotal features of the DocDerr is the “Mix” knob which allows the user to blend the uncompressed EQ’s signal with the compressed and saturated signal.  This parallel compression technique, used by top recording and mixing engineers for many years unlocks a whole world of sonic possibilities.

Like all ELI 500 series modules, the EL-Rx is available with either horizontal or vertically oriented front panel configurations, and is “Linkable” for using our famous compression on stereo sources.

The DocDerr is ideal for tracking, for fine-tuning tracks during mixing, or as a killer solution for the gigging "acoustic" musician in search of studio quality tone control.


  • Super low noise line input, or instrument preamp, with famous Empirical Labs compression. Six digitally controlled analog sections of processing
  • Incredibly dense processing module available for the API 500 series format
  • Four sections of Equalization – One High Pass and 3 parametric bands.
  • Empirical Labs first compressor for the API 500 format. It is an aggressive compressor that follows the EQ, tailored to acoustic instruments, and plucked attacks, but good on just about anything. A must try on vocals and bass!
  • Complex compressor envelope generator. Fast, yet musically saturates at high gain reduction. Program dependant release
  • Saturation circuitry that scrunches the highs and allows hotter levels like tape. Uses filtering combined with germanium clip circuitry to soften loud high frequencies
  • Mix control to allow user to blend an un-compressed EQ’d signal back in with the compressed saturated signal. When on 0, only the EQ’d signal is put out
  • Adapter is provided for free, allowing a guitar, bass, etc to be plugged straight into the rear of your API powered rack
  • DC Coupled Input and Outputs
  • Selectable single ended or differential output. Differential out adds 6dB gain
  • Traditional and digitally controlled analog combined in new topology
  • A “BAD!” Hard Clip Indicator lights – A “Bad” Clip LED that lets user know he is within 1dB of hard clipping. It monitors most every section for internal clipping
  • Highest quality Components. All metal film and Roederstein resistors in the audio path - top quality components, most being high temperature military specparts
  • User modifiable elements, including the Compressor. Lots of future options!
  • Three year unlimited warranty. Built and calibrated in USA

DerrEsser 500 Series Dynamic High Frequency Fixer

The first entry by ELI into the 500 series format, the DerrEsser is a multi function dynamic filtering device. Available in vertical or horizontal versions.  A perfect fit for our EL500 powered rack. In its basic “DS” mode, it is an effective, level insensitive “de-esser” - a device that will adjust the high frequency gain on overly trebly or spiky vocals or other sources in an extremely musical manner.  However, unlike other de-essers, the device allows for multiple use of its powerful 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.


  • Possibly the highest performance analog De-esser in existence
  • HF Lim Mode, as well as standard De-Ess – High Frequency limiting with a tailored soft knee. The Dynamic Section can be used for De-essing or smooth high frequency softening
  • A “BAD!” Hard Clip Indicator light – This “Bad” Clip LED lets user know he is within 1dB of hard clipping. It monitors several sections within the DerrEsser for possible clipping
  • Unity Gain in and out – 6dB of gain available
  • Foolproof operation - it's almost impossible to clip or get a bad sound
  • Industry standard API 500 Series Rack compatible
  • True bypass - Know what it's really doing
  • High reliability parts and military grade switches and relays
  • Three year limited warranty
  • Interface and features found nowhere else
  • Hand built and calibrated in USA. Shipping weight 1 Lbs

EL500 Rack

As far as value, it’s hard to beat 500 Series compatible modules.  Their smaller physical size let you get a lot of performance out of a smaller space, they tend to cost less than their larger counterparts, and – the best part – you can customize your own combinations of them.

When ELI started making our own 500 series products, we looked at the power supplies on the market and couldn’t help but notice a need for a better single height dual module rack unit.  On other single height rack units, the modules protrude from the front panel, and don’t look sturdy or attractive. Power supplies were often noisy, frequently reducing the performance of any module mounted in them.  Also, only XLR ins and outs were provided to the user, limiting the device’s applications.

The Empirical Labs EL500 rack eliminates these obstacles.  At the heart of the unit is a super quiet power supply - over 30 dB quieter than some supplies we’ve seen - that will ensure optimal performance from any compatible device.

All 500 series modules will flush mount cleanly in the EL500, providing a much more professional look and feel. The EL500 offers both XLR and phone rear panel inputs and outputs to allow it to interface perfectly with any system.

In addition, a ¼” instrument preamp/direct box input is provided on the front panel, which will automatically switch the left 500 module’s input to this front phone jack once something is plugged into it – a feature we’re sure will prove to be extremely useful. In fact, it has become the first call bass DI for many engineers.  Switches to link compressors and engage a 48V phantom supply are also provided.

Whether you’re already invested in 500 series modules or they’re something you’re thinking about adding to your arsenal, the EL500 rack will make any 500 series compatible module perform ideally and interface effortlessly into just about any audio application.


  • Unique Flush Mount system for durability & looks
  • Super Quiet ± 16Volt Supplies. Perhaps quietest available
  • More than 280mA available on both supplies
  • 48 volt Phantom supply. 35 mA of current
  • Link Switch to Stereo Link two Compressors
  • Built in “Inst In” (Instrument In) for high impedance inputs built into front panel. Direct box built right in!
  • “Inst In” THD is less than .0006%
  • A performing musician can use the INST IN combined with his favorite modules, using the super quality preamp on this front panel jack
  • Auto-Sense on “Inst In” jack
  • XLR & Phone Plug Inputs and Outputs. A total of 9 input and output connectors
  • Front panel Link Switch for Stereo linking dynamic devices such as compressors/limiters/gates
  • Power Consumption – 14 Watts max. 9 Watts Typical with modules installed
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DocDerr 500 Series Multi-Purpose Tone Module 

Processing - Six digitally controlled sections of analog processing
Frequency response - 3 Hz to 180 kHz on eq section (-3dB points)
Dynamic range - 120 dB from maximum output (.5%THD soft   clipping) to minimum output.   Noise floor with Mix on 0 is typically   – 91dBu
Distortion range - between .0006% and 10% at 1KHz, depending   on mode and settings
Maximum Input before hard clipping - +22dBm,  “ Line In” mode.   Maximum Output (Mix at 0) - +28.5dBm
Compressor Dynamics Time Constants - Attack time is around 1mS.  Release .2 sec, fixed internally. Stereo Linkable for stereo tracks.  With the EL500 powered rack, the push of a button enables linking
Input Impedance - 120k Ohm, Instrument In Mode, output impedance is less than 75 ohms.  20K input impedance in balanced Line in mode
Guitar adaptor cable included
NOTE: With all the processing, the DocDerr draws slightly more current than the API 500 spec, but has worked perfectly in all racks tested. Six have been tested in the API Lunchbox and other racks, without overloading the power supply

DerrEsser 500 Series Dynamic High Frequency Fixer

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
High quality audio caps used internally
Input Balanced, Output single ended but with balanced impedance
Pin 2 hot
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

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