Power amplifiers are classified primarily by the design of the output stage and are designated Class A, B, AB, D, G, or H. In a Class A preamp, the output circuits are always on for the entire cycle of signal swing or the bias current flows at all times.

As a result, Class A preamps have the most linear design, with no crossover distortion, and they deliver purer, clearer, and more musical results than the Class AB designs that are found in many preamps.

Eureka contains a Class A discrete input buffer followed by a dual-servo gain stage. This yields ultra-low noise and wide gain control, allowing you to boost desirable signal without increasing unwanted background noise.

Most mic preamps have a fixed microphone-input impedance of between 1 and 2 kΩ. The Eureka provides variable microphone-input impedance, which can be set at 50Ω, 150Ω, 600Ω, 1.5 kΩ, or 2.5 kΩ.

As the impedance is lowered, a resistive load is put on the microphone. This will not damage the microphone but lowering or raising the impedance can create subtle coloring and filtering effects, enabling you to get a wider variety of sounds.

In general, a lower input impedance can add color to the mic to produce effects that simulate a “darker,” or more “closed-in” tone, or that change the mic’s apparent sensitivity. This effect is easier to notice on passive microphones (ribbon or moving coil) than on active microphones (condensers or active ribbons/dynamics).

In addition, some microphones are finicky about impedance; with variable impedance, you can make virtually any mic deliver its best performance.

The Eureka employs a high-quality transformer and features a Saturation control that adjusts the drain current on the input FET amplifier, altering the levels of even harmonics in the signal being passed. The Saturation control offers an adjustment range of 0% to 100%. The 0% position passes a pure signal. As the control is rotated to the 100% position, the signal’s even harmonics are boosted, giving the signal “warmth” very much like a vacuum tube or similar to the sound of analog-tape saturation.

The term “knee” refers to the way the compression curve bends at the threshold point when represented graphically.

With hard-knee compression, the gain reduction applied to the signal occurs as soon as the signal exceeds the level set by the threshold.

With soft-knee compression, the onset of gain reduction occurs gradually after the signal has exceeded the threshold, producing a more musical response (to some folks).

Sidechaining is useful for numerous applications, including removing sibilance from vocal tracks (de-essing), spectral processing, compression keying, and automatically ducking (lowering the level of) music tracks behind a narrator in a service, broadcast, or performance.

Sidechaining is often used for frequency-dependent gating, in which you put an equalizer or filter before the gate to, say, cut only the lower frequencies of a kick drum. The Eureka’s compressor sidechain includes a dedicated, variable (10 Hz to 10 kHz) high-pass filter for this purpose.

The Eureka provides a fully parametric equalizer—that is, you0 can independently control each band’s center frequency, gain, and Q. The frequency of the low band can be varied from 20 to 300 Hz, the mid band from 200 Hz to 3 kHz, and the high band from 2 kHz to 20 kHz.

Q is defined as the ratio of the center frequency to the bandwidth; a lower Q results in a wider frequency band. The Q on the Eureka ranges from 0.4 (wide) to 2.0 (narrow). In octaves the Q on the Eureka ranges from 3 octaves to 2/3 octaves.