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The MP573 is a two stages, single ended class A preamp, based on the design that was originally used in the Neve 1290/1073 modules, that has been adapted to the '500' format of API.

Just install it in your API* compatible LunchboxTM rack and start recording.

The sound of the MP573, like its model is full, silky, detailed and musical, slightly larger than life, very complimentary to a digital recording environment.

The MP573 is great on vocals, acoustic instruments and generally all close miked sources that need to be pushed forward in the mix.




  • Single ended class A design
  • Carnhill/St. Ives transformers on input and output
  • 0 to 70 dB of gain
  • Switchable 300 & 1200 Ohms and line level input impedance
  • Switchable first stage
  • The output stage is biased in a rich Class A with 70mA of iddle current
  • Fet DI input, relay switched, the signal passes through the input transformer
  • Signal/Clip LED monitors clipping in both stages
  • 24V supply built from the +/-16V with a linear regulator
  • Controlled inrush current for lunchboxes with sensitive power supply
  • Golden contacts on relays and switches
  • High quality Grayhill gain switch
  • High precision aluminium knobs
  • Tantalum capacitors in the audio path (as in original)
  • High grade 105°C low ESR electrolytic capacitors
  • WIMA film capacitors
  • High grade double sided PCB with plated holes
  • Easy build, dense but straightforward
  • Available assembled or as a full kit (everything included)



1 Output level: Potentiometer that sets the signal level to the output stage

2 Signal/Clip LED: Flashes green when a signal is present and turns red 3 dB before clipping

3 Gain switch: In combination with the level potentiometer the overall gain can be set between 0 and +70dB. The 10 and 20 positions entirely bypass the input stage.

4 Input impedance: Center is the standard 1200 Ohms input impedance. Left sets a 300 Ohms low impedance, useful for ribbon mics that also adds 6 dB of gain. Right sets a 10 kOhm impedance input pad for line levels.

5 Polarity Sets the polarity (phase) of the output signal

6 Sends phantom power to the microphone

7 High impedance input (> 1 megohm) for instruments. The instrument signal goes through both transformers.

What's in the kit?

Absolutely everything!

  • Passive components (resistors, capacitors),
  • 2 Carnhill transformers
  • Active components (diodes, transistors, LED, ...),
  • Front panel elements (switches, potentiometers, knobs),
  • Connectors, relay,
  • PCB's,
  • Front panel and side panel,
  • All hardware


You bring the solder (use good quality solder, 1mm diameter max) and the Lunchbox.


Difficulty level

The MP573 kit is an easy build in spite of a rather large number of components.

The only setup is adjusting the bias current in the output stage. It requires a standard DC voltmeter.



Technical specifications

Measure Conditions Value
Iddle supply current No input signal

V+ : +92mA
V- : -92mA

Input impedance f=1kHz
300Ω, 1200Ω and 10kΩ
Maximum input level Line input unlimited
Maximum output level before clip f=1kHz +27dBu
Minimum gain   0dB
Maximum gain   +70dB (+76dB in 300 Ohms)
EIN Zin=0Ω
Frequency response Gain=+40dB
Deviation < 1dB
THD f=1kHz
< 0.008%
DI input impedance   > 1MΩ
DI gain range   -22dB to +48dB
DI max input level   +15dBu


Technical documents