500 Series Multi-Purpose Tone Module

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. The DocDerr’s versatile EQ, low-noise preamp, and unique dynamics section allows for shaping the tone of any track in any situation. Whether its on the road, in the studio, or in your guitar rig, the DocDerr has got you covered! The 500 Series format allows for any sticky situation and can clean up any thin guitar sounds or over-bearing vocal chains.

Multi-Purpose Tone Module

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 unparalleled 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.


  • 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 – between .0006% and 10% at 1KHz, depending on mode and settings.
  • I/O – Maximum Input before hard clipping – +22dBm, “ Line In” mode. Maximum Output (Mix at 0) – +28.5dBm
  • Impedance – 120k Ohm, Instrument In Mode, output impedance is less than 75 ohms.  20K input impedance in balanced Line in mode.
  • Compressor 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.


Yes, indeed! There is a unique soft clipper in the DocDerr that can be used heavily by disabling the compressor. Press the two outside buttons (LF and HF) to toggle the compressor on and off. Then turn Mix 100% and adjust saturation with input knob.

The ratio is about 5:1 but has a huge 5dB knee. With an attack under 1mS, it could be considered a “limiter”. It has been compared to an LN1176.

Yes, but it is not on by default. There is a jumper on the circuit board of the unit that needs to be switched. Refer to the user manual for the location of this jumper because there are a few!


Magazine Reviews


“The DocDerr is another piece of gear from Empirical that I could not image not having, especially while I'm mixing. It's a super musical eq and the compression/saturation circuit is so fat and punchy sounding. I always use it on on my electric guitar sub mix and because it has the blend knobs to do the parallel thing, it's a complete mixing tool. It is a must have for me.”
Jacquire King
Arctic Monkeys, James Bay, Modest Mouse, etc.

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