The DocDerr ELRX-V from Empirical Labs is a vertical multi-purpose tone enhancement module that houses six different sections of digitally controlled analog processingand designed for any API 500- Series enclosure.
 
The DocDerr features a low-noise preamp capable of working with line level or instrument level signals, 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 16 dB of gain, great for acoustic guitar, bass, synths, exotic plucked instruments, and more.

The EQ is followed by a dynamics section which offers compression and a musical tape emulation circuit that softens and tames high frequencies and transient spikes. The DocDerr features the Mix knob, which allows the user to blend the uncompressed EQ's signal with the compressed and saturated signal. The DocDerr is great for tracking, fine-tuning tracks during mixing, and studio quality tone control.
 
The Features
- Low noise line input, or instrument preamp, with compression and six digitally controlled analog sections of processing
- Four sections of equalization including one high pass and three parametric bands
- Complex compressor envelope generator that is fast, yet musically saturates at high gain reduction and features a program dependent release
- Saturation circuitry that scrunches the highs, allows hotter levels, and uses filtering combined with germanium clip circuitry to soften loud high frequencies
- Mix control to allow user to blend an uncompressed equalized signal back in with the compressed saturated signal (when on 0, only the equalized signal is put out)
- DC coupled input and outputs
- Selectable single ended or differential output (adds 6 dB gain)
- Traditional and digitally controlled analog
- Hard clip indicator lights illuminate when the sound level is within 1 dB of hard clipping and monitors many sections for internal clipping
- All metal film and Roederstein resistors in the audio path