Earthworks, while best known for its catalog of super- accurate microphones, also produces a line of microphone preamps based on the philosophy of extremely-low distortion and fast-responding circuitry. Transformerless and fully differential amplifier designs with no electrolytic capacitors in the signal path ensure ultra-wide frequency response and extremely-low nonlinear distortion of less than 0.0001%. The late David Blackmer, Earthworks' founder and principal designer, who was also the db in dbx, dubbed these characteristics ZDT, or Zero Distortion Technology. The 521 ZDT preamp introduces Earthworks mic preamps into the 500-series format at an affordable price.
The 521 ZDT boasts a frequency response ranging from 2 Hz to 200 kHz with only 0.5 dB of variation - while up to 100 kHz, the frequency response is essentially ruler flat. The transformerless circuit provides an input impedance of 100 kΩ without phantom power and 10 kΩ with phantom power applied. Those very high input impedances provide a load that does not color the microphone's sound, further ensuring transparent amplification. The noise floor of the 521 ZDT lies well below virtually all commercial mic preamps, and below the self-noise of most microphones. These details, along with other design features, add up to a mic preamp that can faithfully amplify the microphone's signal with close to zero coloration and distortion.
The 521 ZDT provides one channel of microphone preamp in a single-space 500-series module. The module's matte-black front panel with simple white lettering matches the other Earthworks preamps and provides easily- legible legends for all its controls. A large, stepped rotary switch controls the preamp's gain from 5 dB to 60 dB in 12 steps. Three small toggle switches turn on phantom power, switch signal polarity, and mute the output, while very bright LEDs indicate the presence of mains power, phantom power, and signal clipping. Additionally, the front panel provides a 1/4'' TRS output jack with a companion rotary attenuator to feed balanced or unbalanced devices, from 0 dB to -20 dB with respect to the main gain control setting, while the main output, typically available in the back of 500 racks, provides a balanced output capable of driving long lines at levels up to +29 dBu.
Earthworks provided a pair of 521 ZDT preamps for evaluation along with a pair of their fantastic QTC40 condenser mics. I used the preamps in my six-space API lunchbox with the QTC40 condensers, as well as many other dynamic and condenser mics. When used with stereo pairs of mics, the ZDT preamps provide an extremely natural and 3-dimensional representation of the source, without any high-frequency smearing or crunchiness. Acoustic guitar, with mics placed at both the 12th fret and in front of the body of the guitar, sounded extremely natural and sat nicely in rock and pop productions. There is a sort of relaxed quality to recordings using the ZDT preamps. I could easily EQ in some extra top end or scoop out the lows and mids to fit the guitar into a mix without the phasey artifacts that sometimes haunt EQ'ed acoustic guitar tracks. Mics and preamps that accurately reproduce such an extremely wide frequency response provide a much more natural sound, even inside the limited frequency response of our typical productions. Drum overheads, especially when using the QTC mics, provided extremely natural dynamics and realistic transient response, which produced a very realistic and natural picture of the drum kit. Again, when using the ZDT preamps, I could EQ the kit without adding harshness or distortion to the cymbal overtones, and the toms sounded full and rich in the overheads. On hi-hat, a ribbon mic into the 521 ZDT provided a fantastic, clear sound without any splatter or crushed high-end that condenser mics and preamps with electrolytic caps sometimes bring forth. While there is no pad on the 521 ZDT, the 5 dB minimum gain setting allowed the preamp to be used in many instances where other preamps would require a pad. I also found the coarse gain steps to be fine enough that the variable output never seemed necessary. Perhaps the variable output would have been useful if I were recording a string ensemble and needed to precisely match the stereo image for two channels.
Overall, the 521 ZDT preamp provides extremely natural and accurate reproduction. The preamp doesn't color the microphone in any perceptible way, but maintains the size and imaging of the source. Recording with such transparent and uncolored preamps allows a certain amount of freedom to process or color the signal later without bringing out the limitations of the recording chain. It may take a moment to shift your ears and brain to appreciate the open sound that the preamp produces, but after working with those sounds, you will realize that the added depth and clarity ultimately benefit the instrument and production. In some cases, colored preamps may flatter the source, but sometimes the pure gain of the Earthworks 521 ZDT is just the ticket.