Electrical Audio EApreq500 2024

The EApreq500 is a custom, transformer-based microphone preamp and equalizer built for Steve Albini's Electrical Audio.  The preamp is designed to allow the subtle character of the input transformer to color the sound with very little noise or amplifier distortion.  The shelf EQs are boost only, to broadly add bass and/or treble when desired.  A design premium was placed on long term reliability, and flexible application.

The Basics

•   Low distortion Lundahl input Transformer.  Provides slight coloring as input level gets higher.
•   16-position, gold contact, rotary gain switch ranging from 20 to 65dB of gain, 3dB per step.
•   -20dB input pad switch - sealed gold contact relay
•   Polarity reverse switch - sealed gold contact relay across the output for quietest operation
•   48v phantom power switch - standard application
•   Low and High frequency shelf boost EQ. controlled with a solid multi-section gold contact switch, 5 gain positions.  A sealed relay bypasses the EQ circuit when not in use.
•   DC servo for both preamp gain and shelf EQ make-up gain amplifiers.  This provides long term predictable operation and sonics, avoiding the gradual decline that comes with electrolytic capacitor-coupled circuitry
•   Ground-sensing solid state balanced output amplifier capable of driving 600Ω loads with ease, balanced or unbalanced.
•   Clear green LEDs indicating which shelf band is engaged.
•   Bright, amber colored “peak” LED warn just before clipping.
•   The performance of the preamp, and shelf EQ are closely matched across all EApreq500s.  

TECHNICAL SPECIFICATIONS

FREQUENCY RESPONSE

± .5dB from 7 Hz to 102 kHz (3dB down @120kHz)

MAXIMUM OUTPUT LEVEL=

+27dBu Balanced 10kΩ Termination (typical modern equipment inputs)

+26.5dBu Balanced 600Ω Termination (“vintage” input impedance standard)

+21dBu Unbalanced 10kΩ Termination

NOISE AND DISTORTION

THD+N (1kHz, 35dB gain) ≤ .003%

THD+N (30Hz, 35dB gain) ≤ .012%

IMD (50Hz/7KHz) ≤ .0015%

Noise floor (22- 22kHz unweighted, lowest gain setting) ≤ -99dBu

Noise floor (22- 22kHz unweighted, EQ engaged) ≤ -86dBu

Equivalent Input Noise = (44dB gain, 150Ω input source, no EQ) ≤ -128dB

Power Consumption is typically < 50mA per 16 volt rail, and < 100mA per rail in the extreme (1kHz sine wave @26dBu into a 600Ω load, all relays on).