MP73

The MP73 is a three stages, single ended class A preamp based on the 1290/1073 from the world famous vintage British consoles of the 70's. The sound of the MP73, like its model is full, silky, detailed and musical, slightly larger than life, very complimentary to a digital recording environment. The MP73 is great on vocals, acoustic instruments and generally all close miked sources that need to be pushed forward in the mix.

Highlights

• Single ended class A design
• Carnhill/St. Ives transformers on input and output
• 70 dB of gain
• Switchable first stage
• Signal/Clip LED monitors clipping in the 3 stages with 3 full wave rectifiers
• All low level signals switched by relays
• Golden contacts on relays and switches
• Jumper selectable input impedance : 300-1200 ohms.
• Local voltage regulator for better channel separation
• Neutrik XLR connectors
• Nicely damped quality conductive plastic potentiometers
• 1% metal film resistors
• Tantalum capacitors in the audio path
• High grade 105°C low ESR electrolytic capacitors
• WIMA film capacitors
• High grade double sided PCB with plated holes

More details...

The preamp is based on two BA283 NV gain stage and one BA283AM output amplifier.

The phantom power is applied to the XLR pins through 1‰ matched resistors to keep the CMRR high. The 48V voltage is filtered by a double RC network to eliminate any residual noise.

The original design rotary gain switch has been replaced by the combination of a potentiometer and a 3 positions gain switch, Lo, Mid and High. The first stage is inserted in the signal path when the gain switch is in the “Hi gain” position. The Mid and Lo gain positions use only one BA283NV with two different gain settings. The 3 positions give gains of approximately 40dB, 55dB and 70dB.
This configuration has several advantages : a greater flexibility in choosing when to engage the first gain stage, no pops in medium gain adjustments, no big switch prone to hum picking and a smaller cost.
From an electronic point of view, the circuit remains identical to the original.

The output stage is biased in a rich Class A with 70mA of iddle current.

The DI, “Hi gain” and Phase switching is done by relays which are feeded by a constant current source without any return through the ground. This is to eliminate all switching pops in the audio.

The clip LED if fed by three full wave rectifiers from the three amplification stages. It will flash green with audio levels of 40dB below clipping giving a visual feedback of the preamp activity. It will flash red if anyone of the three stages reaches the clip level, which we use to set to 3dB below the actual clipping.

The audio supply voltage is regulated on board, in addition to being regulated in the external PSU. This ensures a very stable supply and an excellent channel separation. The relays and LED supplies are taken pre-regulator. The ground return is split into two paths, Signal and Power, giving a very clean ground reference to all the critical stages.