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Review: Empirical Labs DocDerr EL-Rx

The DocDerr provides uncompromised equalization and compression within a single 500 series-compatible module.

Dave Derr has done it again! The new Empirical Labs DocDerr EL-Rx 500-series module is a digitally controlled analog instrument (not mic!) preamp, equalizer, compressor and saturation device and, like everything else in the Empirical Labs line, it’s feature-packed and amazingly flexible. But most importantly, it sounds incredible.


The DocDerr tone enhancement module includes six stages of digitally controlled analog processing.

Empirical Labs classifies the DocDerr as a tone enhancement module, and it includes six stages of digitally controlled analog processing. The module features DC-coupled inputs and outputs with a frequency response of 3 Hz to 180 kHz and dynamic range of -120 dB from maximum output (.5 percent THD soft clipping) to minimum output. The input impedance is 120K Ohm in instrument mode and 20K Ohm inline input mode. When utilized with a 16-volt power supply, the DocDerr requires a maximum of 5 watts, but it typically utilizes 3 watts; it is more power hungry than the majority of the 500-series-compatible modules available today. This isn’t an issue when using one of the more robust supplies available, but make sure your supply will handle these power needs before you fill it with DocDerr modules. The DocDerr is available as either a vertical or horizontal module. Both versions are physically identical except for appropriate labeling on the front panel.

While it has grown into a much broader product, the DocDerr was initially developed with the live performer in mind who needed an all-in-one device to shape the sound of their acoustic guitar, bass or other direct instrument. The first stage in the DocDerr’s circuit is a low-noise preamp that can be configured for either line-level or instrument-level input via a jumper on the circuit board. An additional 16 dB of gain is provided when configured for instrument input. Some 500 series-compatible power supplies (such as the Radial Workhorse) include quarter-inch inputs, but most don’t. For that reason, Empirical Labs has included a short quarter-inch female to M-XLR cable making it easy for anyone to use a direct instrument output with the DocDerr regardless of which 500-series-compatible frame they are utilizing.

The next stage is the equalizer, which includes a selectable high-pass filter and three bands of parametric EQ. The DocDerr’s EQ is based on the Lil FrEQ circuitry. The high-pass filter is 18 dB per octave (3rd order) and can be set to 70 Hz or 100 Hz. There are seven frequency options for each of the three bands of EQ (LF: 80, 125, 160, 200, 300, 400 and 500; MF: 250, 500, 750, 1K, 2K, 3K and 4K; HF: 3.5K, 5.2K, 6.5K, 8K, 10K, 12K and 15K) and the Q on the mid-band can be toggled between broad and narrow.

The DocDerr’s dynamics section provides legendary Empirical Labs compression as well as a tape-emulation circuit that softens high frequencies and eliminates clipping. The compressor has an approximate 5:1 ratio with the input knob being the compressor’s primary control. An internal jumper allows the compressor’s detector to be set to flat or for the low frequencies going to the detector circuit to be optionally rolled off. The Input control follows the EQ and pushes signal into the compressor decreasing the dynamic range. Dave Derr selected the compressor’s time constants and curve to be as aggressive as possible while still retaining complete musicality.

The Mix knob allows the uncompressed but EQ’d signal to be blended with the compressed and saturated signal providing built-in parallel compression control. An internal jumper (the third and final) allows the DocDerr’s output to be switched between single ended and differential, which provides an additional 6 dB of output gain. The “Bad!” hard-clip indicator illuminates when a section of the circuit clips.

In Use

The DocDerr is more button and knob populated than any 500 series module I’ve encountered. Realizing that this could potentially be a bad thing, when I used the DocDerr for the first time, I purposely avoided reading the manual so I could see how intuitive the plethora of controls would actually be. I was quite surprised at how quickly I adapted to the module. Pressing specific buttons or button combinations, all of which are clearly labeled on the front panel, switches the HP filter and equalizer frequencies and toggles between Q settings.

The only exception is the compressor bypass function that is activated by simultaneously pressing the LF and HF buttons. (This is the only feature that required a look at the manual.) Bypassing the compressor allows the soft limiter in the DocDerr to be pushed harder, resulting in quite convincing- sounding analog tape emulation that works well on everything from drums to vocals to acoustic guitars.

The DocDerr works superbly with both male and female vocals. When pushed harder, the compressor takes on a tubecircuit character as the harmonics become more prevalent. During normal operation, the compressor behaves somewhat like, and is sonically reminiscent of, an 1176 that has the optimum ratio and attack and release times already dialed in. I had good results driving the circuit to nearly 20 dB of compression on the vocals and then using the mix function to dial in some of the uncompressed signal, which resulted in a signal that had significantly reduced dynamics but still sounded quite natural. This approach worked well with electric guitars, too.

I also had great results using the DocDerr on kick and snare. I like the sound of the compressor on the snare but on the kick I prefer to bypass the compressor and drive the circuit a bit harder to utilize the tapesaturation simulation.

I set the internal jumper to instrument and recorded bass and acoustic guitar direct through the DocDerr. Bass is killer through this box! I used the HF section of the EQ to boost 3.5K to add some definition, I pulled some 250 Hz out on the MF band to decrease the tubbiness, I squashed the signal so my maximum reduction was occasionally hitting 12 dB, and I set the blend to 7 and the result was a thick, defined bass tone with more sustain; it improved the note detail without sounding at all overcompressed.

The box is equally adept at capturing direct acoustic guitar. While I’m not much of a fan of recording acoustic guitar direct, musicians who play acoustic in live settings will fall in love with the DocDerr; it will greatly improve their instrument’s sound quality.

Often with equalizers with preselected frequencies, I find myself wishing I had a frequency option right between two of the preselected frequencies. This was surprisingly never the case with the DocDerr. Dave Derr designs gear with an artist’s mastery and has done an amazing job at selecting the most musical frequencies for virtually any situation. This is true with the compressor as well; I would typically stray from a modern compressor with fixed attack and release times, but the DocDerr’s compression circuit sounded good on every sound source I tried.

The DocDerr’s manual is well written and thoroughly explains all of the unit’s functionality. There are several common scenarios described with typical DocDerr settings making it easy for even the most novice engineers to get up and running with the DocDerr.

I only have one real complaint with the near-perfect DocDerr module: there is no bypass button. It’s so feature-packed that I’m not sure where it would go, but I’d like to have one so it would be easy to compare the processed signal with the unprocessed signal.


The Empirical Labs DocDerr EL-Rx offers performance beyond anything I’ve encountered in a 500-series module. Using the DocDerr feels like having a Lil FrEQ and a Distressor at my fingertips with only slightly less control and a substantially smaller price tag.

Price: $799

Contact: Empirical Labs Incorporated |

Russ Long is a Nashville-based producer, engineer and mixer as well as senior contributor to PAR.