The PreSonus ADL 600 is a high voltage Class A, two channel tube microphone preamplifier designed by world-renown tube circuit designer Anthony DeMaria. The ADL 600 is packed with six high-voltage vacuum tubes (three per channel), military-grade switched attenuators, analog VU and LED metering as well as instrument input, line input and selectable microphone input impedance. The ADL 600 utilizes extremely high voltage power rails (600V peak to peak) delivering 75dB of gain, over 30dB of headroom, and extremely wide frequency range of 5Hz to 80kHz within 0.5dB. The combination of Anthony DeMaria’s preamplifier design and PreSonus’ state-of-the-art manufacturing process achieves a new standard in hand-made, high-voltage tube preamplifiers.
Control and Flexibility
The ADL 600 is designed for ultimate sonic performance, however its ease of use and flexibility allowing for many types of input sources further sets it above all other preamplifiers. The input selector allows for selection between Hi-Z instrument input, line input and four different microphone impedances for a wide variety of sounds and applications. The High Pass Filter is a rotary switch with three filter settings (40Hz, 80Hz, and 120Hz @ 6dB per octave) for passively and unobtrusively controlling room rumble and low frequency noise. The input gain attenuator is a military-grade rotary switch delivering 5dB step increments and a total preamplifier gain of over 72dB. The output stage is controlled by a fully variable conductive plastic attenuator delivering +/-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 both controlled by big sturdy military grade switches.
Under the Hood
The ADL 600 is built with the finest components available in the world. The ADL 600 is a fully discrete transformer input and output Class A design incorporating three military-grade vacuum tubes per channel operating at voltages of +/-300V power rails for maximum tube tonality and headroom. There are no op-amps or integrated circuits (IC's) in the signal path to ensure the purest sound possible. The dual transformer design also ensures ultra low noise operation with maximum common mode rejection.
The ADL 600 is loaded with both fast-acting eight-segment LED meters for accurately detecting fast transients and peaks, as well as large high quality analog VU meters for metering output level.
The ADL 600 sets a new standard in tonatily for tube preamplifiers. Vocals become big, smooth and articulate, bass guitars are deep and tight, and mixes become richer than ever. The ADL 600 is breathtaking both in terms of appearance and in sound performance. The ADL 600 delivers a sound like no other.
PreSonus ADL600 Features
- High-Voltage Class A Dual Transformer Design
- >73dB Gain
- Selectable Microphone Input Impedance
- Switched Gain and Variable Fine Trim Controls
- Microphone, Instrument and Line Inputs
- Ultra Low Noise (-100dB S/N ratio)
- Variable High Pass Filter, 48V, -20dB Pad, Polarity Invert
- Analog VU and Fast-Acting LED Metering
ADL600 Design Overview
The primary goal at the onset of this project was to manufacture a wide dynamic range/low noise tube microphone preamplifier with large headroom. Not long after the start of the project, PreSonus was presented with a proven design from Anthony DeMaria Labs that exceeded the original goals. The topology is a three dual triode vacuum tube design utilizing high-voltage Class A operation. New features were added to the original design to make the product a better fit for the target market. These include adjustable microphone load impedance, high pass filters, and line input. The end result is the ADL600 stereo microphone preamplifier. The "ADL" portion of the product name gives credit to Anthony DeMaria of ADL for providing a solid design to build upon. The ADL600 features wide dynamic range, low noise, large headroom, and low distortion.
Controls and I/O
The ADL600 front panel is an attractive blend of old and new styling. The black extruded aluminum face plate is accented with a combination of large chrome toggle switches and brushed aluminum knobs. At the center of the panel is a large brushed aluminum bezel containing an analog Vu meter and an eight segment LED meter for each channel. The toggle switches are for +48V phantom, Phase, 20dB Pad, and High Pass Filter enable. There are also rotary switches for Input selection (includes microphone impedance), High Pass Filter frequency, and preamp Gain. A potentiometer is provided for Output trim. Two more toggle switches are for power and Vu meter sensitivity. Also located on the front panel are 1/4" Instrument inputs for each channel.
XLR connectors for Microphone input, Line input and preamp Output are located on the rear of the chassis. There is a two position terminal strip for Signal/Earth (chassis) grounding. The grounds can be isolated by removing the shorting clip.
The overall approach to the 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, poly film pulse capacitors, 1% metal film resistors, and conductive plastic potentiometers are used. 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.
At the input to the preamplifier is a custom designed input transformer. The transformer is fully encased in a Mu-metal enclosure yielding excellent noise characteristics and magnetic shielding. The transformer interfaces the balanced inputs from the Microphone or 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 triode and a 6922 triode form the principle gain stage of the preamplifier. This stage gives the ADL600 its "open and airy" sound. This is primarily due to the circuit topology and the wide power supply rails applied to the this stage, +/-320VDC. A high endurance Grayhill rotary switch and 1% metal film resistors are used to set the gain of this stage in 5dB steps. Poly film pulse capacitors are used for inter-stage coupling to maintain the highest sonic 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. Poly film capacitors are used for the filters also. The filters are 1st-order and provide 6dB/octave roll-off. The Output trim potentiometer feeds the input to the second 6922 triode. This is the output driver and drives the Mu-metal shielded output transformer. A DPDT relay allows for output phase reversal at the primary side of the output transformer.
A DPDT relay switches in the Instrument jack at the input to the 12AT7A triode bypassing the input transformer.
At the heart of all high quality microphone preamplifiers is the power supply and the one in the ADL600 is exceptional. It starts with a filtered A.C. input receptacle and ends with a power supply that far exceeds the load requirement. Fast switching diodes are used instead of standard recovery 60Hz diodes in the regulated high voltage supplies. The custom designed Mu-metal shielded toroid power transformer was designed to be over-rated by a factor of four. This ensures superior load regulation and minimizes the transformer flux leakage. All of these factors yield very low noise at all the supply outputs and very good load regulation.
ADL 600 History and Partnership
Several years ago, Anthony DeMaria, President of Anthony DeMaria Labs and Jim Odom, President of PreSonus met at an AES show in New York. Jim noticed a prototype microphone preamplifier lurking in the back of the ADL booth and was intrigued. After chatting with Anthony about the design - three tubes per channel, dual input/output transformers, 600 volt power rails - he had to hear it. Totally floored by the mammoth sound and complete absence of noise, Jim was convinced this was a match made in sonic nirvana. Designed by Anthony DeMaria, engineered and manufactured by PreSonus in the USA, the ADL 600 is a microphone preamplifier that has a sonic character like no other.