The ADL 700 preamplifier runs on power rails of ±300V. Most off-the-shelf, op-amp-based designs run on power rails of 10V to 18V. Higher-voltage power rails deliver more headroom, deeper lows, smoother highs, and a richer overall sound.

We only use genuine transistors, resistors, and capacitors in the ADL 700 because this provides ultra-low noise and transparency. Op-amps add noise, coloration, and harshness to a signal. You won’t find them in the ADL 700.

The ADL700 employs large, high quality, custom-wound, shielded input and output transformers. Among other things, these securely decouple the preamp from the outside world (the mic and the A/D or following devices), which is very important in a high-voltage tube design such as this one.

The ADL 700’s Cinemag input transformer sets the stage for the preamp’s nearly ruler-flat/linear and detailed sound; the rest of the preamp amplifies what the Cinemag delivers. No other input transformer sounds like the Cinemag does—and we tried others.

The nickel/iron-core, large-scale output transformer delivers high headroom. It will begin to saturate in a pleasing (some would say “creamy”) manner at very high levels. This adds the bit of finish to the sound.

Most mic preamps have a fixed microphone-input impedance of between 1 and 2 kΩ. The ADL 600 provides variable microphone-input impedance, which can be set at 150, 300, 900, or 1500Ω. As the impedance on the ADL 600 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 addition, the ADL 700 provides variable mic-input gain, employing an 8-position, military-grade, rotary switch that provides 35 dB of gain in stepped, 5 dB increments. A Trim potentiometer (±30 dB) allows you to make fine adjustments to the final preamp stage of the ADL 700 input.

The output stage is controlled by a fully variable attenuator that delivers ±10 dB of fine-tune trim adjustment, enabling you to dial in the perfect gain structure for any application.

Broadly speaking, there are four types of analog compressor designs: those that use opto-isolators (optical), Field-Effect Transistors (FETs), voltage-controlled amplifiers (VCAs; an example is the PreSonus Studio Channel compressor), and variable-gain compressors. Each has its uses, but we’re going to focus on the two types you’re most likely to find on today’s high-end channel strips: optical and FET.

The majority of today’s high-end channel strips utilize optical compressor circuits. In general, the response time of the optical circuits in these compressors tend to soften the attack and release, which can smooth out uneven volume fluctuations. They often have a distinctive sonic signature, so they can be as much an effect as a gain-control device.

FET compressors use a special transistor to vary gain, emulating a triode tube’s operation and sound. Inherently high-impedance devices, FET compressors tend be very clean and reliable and provide a faster attack time and better repeatability than the optical compressors that are more commonly found in channel strips in this price class.

To some extent, which you prefer is a matter of taste. You might want the sound of an optical compressor for one application but prefer a FET-based compressor for another application.

However, aside from having a distinctive sound, optical compressors have drawbacks. Most notably, when the components in an optical compressor heat up or cool down, the resulting attack and release times can change considerably. That means you might not get consistent results—even on, say, multiple snare hits in the same song. In contrast, FET compressors are not susceptible to temperature fluctuations, so they provide consistent, repeatable results.