Empirical Labs RX Plus - EL/Rx-H DocDerr + EL500 1U Horizontal Rack

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RX Plus
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  • Empirical Labs RX Plus - EL/Rx-H DocDerr + EL500 1U Horizontal Rack
  • Empirical Labs RX Plus - EL/Rx-H DocDerr + EL500 1U Horizontal Rack
  • Empirical Labs RX Plus - EL/Rx-H DocDerr + EL500 1U Horizontal Rack
  • Empirical Labs RX Plus - EL/Rx-H DocDerr + EL500 1U Horizontal Rack


The RX Plus contains one 500 series EL/Rx-H DocDerr horizontal multi-purpose tone module 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

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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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

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