PreSonus ADL 600 Stereo High Voltage Tube Microphone Preamp
The PreSonus ADL 600 is a high-voltage, 2-channel microphone preamp designed by world-renowned analog tube circuit designer Anthony DeMaria. It utilizes a Class A discrete design with 3 vacuum tubes per channel for maximum headroom and tonality.

Control and Flexibility
The ADL 600 is designed for supreme sonic performance and flexibility with XLR microphone/line inputs and 1/4" instrument inputs. The input selector allows for selection between Hi-Z instrument input, line input, and 4 different mic impedances for a number of applications. The rotary High Pass Filter switch has 3 settings (40Hz, 80Hz, and 120Hz @ 6dB per octave) for controlling room rumble and low-frequency noise. The input gain attenuator delivers 5dB-step increments and a total gain of over 72dB. The output stage is controlled by a variable conductive plastic attenuator that delivers +/-10dB of fine tune trim adjustment to dial-in the perfect gain structure for the application at hand. The ADL 600 also includes 48V phantom power and a -20dB pad.

Under the Hood
There are no op-amps or integrated circuits (IC's) in the signal path to ensure the purest sound possible. The dual-transformer design guarantees ultralow-noise operation with maximum common mode rejection.

Metering
The ADL 600 is loaded with both fast-acting 8-segment LED meters for accurately detecting fast transients and peaks, as well as large analog VU meters for metering output level.


Components
The overall approach to the ADL 600's design was to keep the signal path as simple as possible by keeping the component count to a minimum. The few components that are in the signal path were chosen for their sonic qualities: custom audio transformers, polyfilm pulse capacitors, 1% metal film resistors, and conductive plastic potentiometers. Also, rotary switches and fixed 1% metal film resistors are used in the preamp gain circuit to keep noise to a minimum and to prevent any coloration that may be caused by a potentiometer.

The custom-designed input transformer is fully encased in a Mumetal enclosure, yielding excellent noise characteristics and magnetic shielding. The transformer interfaces the balanced mic/line inputs to the tube preamp circuits. On the primary side of the transformer are circuits for +48V phantom power, microphone impedance, 20dB pad, and line input. All of these are switched by sealed relays for reliability. Also, the relays are located at the point where the switching is required. This minimizes the signal path length and helps to keep noise low.

The combination of a 12AT7A tube and a 6922 tube form the principle gain stage, giving the ADL 600 its open and airy sound. This is primarily due to the circuit topology and the wide power supply rails applied to this stage--+/-320VDC. A high endurance Grayhill rotary switch and 1% metal film resistors are used to set the gain in 5dB steps. Polyfilm pulse capacitors are used for interstage coupling to maintain stunning clarity. The preamp stage drives the High Pass filter circuit and the Output trim. The High Pass filter can be bypassed or set to 40Hz, 80Hz, or 120Hz. Polyfilm capacitors are used for the filters, which are first-order and provide 6dB/octave roll-off. The Output trim potentiometer feeds the input to the second 6922 tube, which drives the Mumetal shielded output transformer. A DPDT relay allows for output phase reversal at the primary side of the output transformer.