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SL4KE01 REV B

Interactive Assembly, Calibration Guide & Bill of Materials for the 500 / 51X Series Preamp Kit

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1. Introduction

Welcome to the build manual for the SL4K-E01 Rev B Preamp Module.

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The SL4K-E01 Rev B is a high-performance analog microphone preamplifier engineered for the popular 500 / 51X series rack format (+16V / -16V rails). Inspired by the legendary 4000E console dynamics and preamp circuitry (E01 architecture), it combines classic British punch with modern utility and expansive saturation options.

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In this Rev B revision, the module features an upgraded 4-layer PCB design for superior shielding, significantly reduced noise floor, integrated variable High Pass (Low Cut) and Low Pass (High Cut) filters, as well as full compatibility with the DIYRE Colour Module ecosystem for custom analog saturation and character.

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Key Features of the SL4K-E01 Rev B:

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  • Classic Console Preamplifier Circuitry: Based on the iconic 4000E-E01 topology with discrete stepped Gain control.

  • Integrated Variable Filters: Sweepable Low Cut / HPF (10 Hz to 250 Hz) and High Cut / LPF (3.5 kHz to 20 kHz) filter sections with dedicated enable switches.

  • DIYRE Colour Module Slot: Integrated socket and drive control for inserting custom analog saturation and saturation color modules.

  • 4-Layer Low-Noise PCB: Rev B layout optimization with ground planes for ultra-low noise floor and shielding.

  • Output Fader & Trim: Output attenuation/boost control ranging from -100 dB to +12 dB.

  • Flexible Peak & Metering Display: Dedicated Peak LED (set to +18 dBu post-preamp) and multi-segment LED ladder with jumper selection for PRE- or POST-FADER metering.

  • Professional Interfacing: High-grade input transformer architecture paired with a THAT 1646 balanced driver output stage.

  • Utility Features: Selectable +48V Phantom Power, Phase Invert (POL), and 20 dB Attenuation Pad (or optional Line-Level Mod).

 

This manual will guide you step-by-step through component selection, board assembly, initial voltage checks, and precision alignment. Please take your time, read all instructions carefully before soldering, and double-check component polarities at each stage.

Finished Module: SL4KE01 500-series preamp module featuring black aluminum front panel

2. Line Level Conversion (PAD Mod Option)

All resistors included in the standard kit configure the 20 dB PAD switch according to the original 4000E console schematic. If you wish to convert the PAD switch into a Line-to-Mic Level Switch for routing line-level interface signals through the preamp at unity gain (at a preamp gain setting of 42 dB), apply the following resistor modifications:

Line Mod Resistor Values:

  • R_PAD1: 5.1 kOhm (5k1)

  • R_PAD3: 5.1 kOhm (5k1)

  • R_PAD2: 56 Ohm (56R)

  • R_PAD4: Omit Component (Do not install)

3. Step-by-Step Assembly Guide

3.1 Mainboard Resistors and Diodes

  • Place and solder all low-profile resistors on the mainboard according to the Bill of Materials.

  • Colour Module LED Configuration (Choose one option before soldering):

    • Option A: Using an RGB LED for Colour Indication

      • Add a small solder bridge on the back of the PCB near the Colour Module connector.

      • Do NOT install the 6K81 resistor at position R_C_RGB1 (leave open).

    • Option B: Using a Yellow LED for Colour Indication

      • Do NOT bridge the solder pads on the back.

      • Install the supplied 6K81 resistor at position R_C_RGB1.

  • Install all signal diodes.

  • Important: Double-check orientation and polarity (cathode stripe matching PCB silkscreen) of all diodes before soldering.

Main PCB with color-coded resistors and diodes positioned and soldered
Sub PCB with color-coded resistors and diodes positioned and soldered

3.2 IC Sockets, Small Capacitors & LED Meter Board

  • Install all IC sockets on the mainboard and LED Meter subboard (ensure notch orientation matches the PCB silkscreen).

  • Solder low-profile ceramic capacitors (yellow 100nF decoupling caps, etc.).

  • On the LED Meter Board: Populate IC sockets, capacitors, WIMA film caps, and board headers. Trim all leads on the underside as short as humanly possible to avoid shorting against adjacent 500-series modules.

PCB assembly showing IC sockets and low profile ceramic capacitors
PCB assembly showing IC sockets and low profile ceramic capacitors
PCB assembly showing WIMA capacitors, connectors and bridge rectifier
PCB assembly showing WIMA capacitors, connectors and bridge rectifier

3.3 Film Capacitors, Voltage Regulators & Electrolytics

  • Place and solder all WIMA film capacitors and electrolytic capacitors on the mainboard.

  • Bend and install the LM317 voltage regulator as shown in reference photos. Ensure its metal tab does not touch adjacent resistors.

  • Install the input transformer on the mainboard.

  • Install all inter-board connectors.

PCB assembly showing electrolytic capacitors, relais and potentiometers
PCB assembly showing electrolytic capacitors, relais and potentiometers

3.4 Board Cleaning

  • Clean the mainboard and LED meter subboard thoroughly using Isopropyl Alcohol or flux remover to clear away all solder residue. Allow to dry completely.

  • Warning: Do NOT clean the PCB with liquids AFTER trimpots, potentiometers, Grayhill switches, or pushbuttons are installed. Solvents will wash away internal conductive grease and ruin electrical contact permanently.

4. Potentiometers, Switches & LED Meter Assembly

4.1 Potentiometer Allocation Guide

Check potentiometer markings before soldering to ensure correct placement:

  • C50K: Output Fader

  • B10K: Low Cut (HPF) Frequency

  • B20K: High Cut (LPF) Frequency

  • A10K: Colour Module Drive

4.2 Mainboard & Subboard Component Placement

  • LED Meter Board Pots: Insert the three potentiometers into their designated positions on the LED Meter subboard. Solder only one pin per pot first to allow for mechanical alignment. Do not solder remaining pins yet.

  • Grayhill Gain Switch: Insert the Grayhill switch into the mainboard. Insert the mechanical stop pin at the 12 o'clock position and secure it with the provided sticker so it cannot move.

  • Mainboard Switches & Pots: Insert the Grayhill switch, potentiometers, and pushbuttons onto the mainboard. Solder only one pin per component first.

  • Verify alignment against the printed silkscreen outlines, attach the mounting bracket to the mainboard, and adjust component positions if needed.

PCB assembly showing electrolytic capacitors, relais and potentiometers
PCB assembly showing electrolytic capacitors, relais and potentiometers
PCB assembly showing Grayhill switch and Jensen transformer
PCB assembly showing Grayhill switch and Jensen transformer

4.3 Front Panel Fitment & LED Soldering

LED Leg Trimming Reference:

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  • Mainboard LEDs: Trim long leg (Anode, +) to 8 – 10 mm.

    • 1x Red, 2x Green, 2x Yellow, 1x RGB (or Yellow for Colour slot).

  • LED Meter Board LEDs: Trim long leg (Anode, +) to 12 – 14 mm.

    • 2x Red, 6x Green, 4x Yellow.

  • Tip: Keep the ground leg slightly longer on RGB LEDs to easily identify it during placement.

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Front Panel Assembly Steps:

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  • Attach the front panel temporarily to the mainboard using spacers and hardware.

  • Insert pushbuttons and align switch bodies so they sit perfectly centered in the front panel cutouts. Solder switch pins completely, then trim leads short.

  • Apply paper masking tape over the front panel LED cutouts to establish a flat height stop.

  • Mainboard LEDs: Insert LEDs into position flush against the tape.

    • Yellow/RGB Colour LED: If using a Yellow LED, solder the long leg to R/Y and the short leg to the unlabeled pad. If using RGB, follow the dedicated pinout guide.

    • Solder one leg first, verify depth/alignment, then solder remaining legs.

  • Disassemble the front panel. Mount the LED Meter Board onto the mainboard using 25 mm spacers and screws. Re-attach the front panel and tighten bracket nuts.

  • Final Potentiometer Alignment: With the front panel locked in place, solder all remaining pins of the LED Meter board potentiometers (they are now perfectly centered).

  • Meter LEDs Placement: The LED Meter board pads feature long dashes (+) for long legs and short dashes (-) for short legs. Insert LEDs step-by-step from the front, solder them temporarily from the top side, then remove the front panel to complete soldering from the back side.​

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5. Final Assembly and Voltage Check

5.1 Pre-Flight Power On and Voltage Check

  • Ensure NO ICs are inserted into any sockets.

  • Connect the mainboard and subboard using ribbon cables and power the unit via your 500-series rack test rig.

  • Verify relay switching operation and switch LED activity.

  • Note on Metering: Without the LM339 comparator inserted, all meter LEDs should light up.

  • Set multimeter to DC Volts and verify supply voltages at each IC socket relative to ground:

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NE5534A - Pin 7 (+15.3V) - Pin 4 (-15.3V)

THAT 1646 - Pin 6 (+15.3V) - Pin 5 (-15.3V)

LM339 - Pin 3 (+15.3V) - Pin 12 (-15.3V)

TL074 - Pin 4 (+15.3V) - Pin 11 (-15.3V)

NE5532A - Pin 8 (+15.3V) - Pin 4 (-15.3V)

5.2 IC Installation and Hardware Assembly

  • Populate all IC sockets (NE5534A, THAT 1646, LM339, TL074, NE5532A). Pay strict attention to Pin 1 notch orientation and verify each IC is in its designated position.

  • Re-attach the front panel using 3 mm hardware, spacers, and potentiometer nuts.

  • Set the LED Meter Mode Jumper (located next to the meter ribbon connector) to PRE-FADER or POST-FADER mode based on preference.

  • Fit color-coded knob caps:

    • 15 mm Blue Cap: Mic Gain Control

    • 15 mm Red Cap: Output Fader

    • 11 mm Blue Caps: Low Cut (HPF) and High Cut (LPF) Controls

    • 11 mm Red Cap: Colour Module Drive Control

6. Calibration Guide

Calibration requires passing pure sine waves through the module while measuring AC voltages at test points or monitoring DAW input levels (e.g., via REW - Room EQ Wizard).

 

Test Requirements:

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  • Headroom: Maintain at least 10 dB headroom on your audio interface converter to prevent clipping during testing.

  • Reference Levels:

  • 0 dBu Reference: 0.775 V RMS (AC)

  • +6 dBu Reference: 1.545 V RMS (AC)

  • Preamp Calibration Input Signal: -40 dBu (0.007 V RMS / 7 mV AC) between Pins 2 & 3 of XLR input.

 

Recommended Resources:

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Initial Check:

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  1. Set initial controls:

    • All Pushbuttons: OFF (Disengaged)

    • Mic Gain: Lowest setting

    • Output Fader: 0 dB position

  2. Inject a White Noise signal at -40 dBu into the microphone input.

  3. Increase Mic Gain to a comfortable monitoring level.

  4. Verify filter operations (sweep Low Cut and High Cut), PAD engagement, POL phase inversion, and Colour Module drive.

  5. Verify +48V Phantom Power voltage using a phantom-powered microphone or active DI box.

  6. Once functional verification is complete, proceed to precision calibration using a 1 kHz Sine Wave at -40 dBu.

 

Output Fader Calibration (0dB_TRIM)

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  1. Set controls: All switches OFF, Mic Gain at lowest setting, Output Fader at 0 dB.

  2. Inject a 1 kHz Sine Wave at -40 dBu (0.007 V AC) into the XLR input.

  3. Adjust the Mic Gain stepped switch (and generator level if needed) until your multimeter measures exactly 0.775 V AC (0 dBu) between test points TP2 (PREAMP_OUT) and TP1 (GND).

  4. Measure AC voltage between test points TP3 (OUTPUT FADER) and TP1 (GND).

  5. Adjust trimpot 0dB_TRIM until TP3 reads exactly 0.775 V AC (matching preamp output level).

 

LED Meter Calibration (R_meter1)

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  1. Maintain setup from Step 2 (0.775 V AC measured at TP2).

  2. Observe the LED meter ladder on the front panel.

  3. Adjust trimpot R_meter1 until all Green LEDs on the meter ladder just illuminate.

 

Low Cut (HPF) Filter Calibration (R_LC_TRIM)

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  1. Maintain setup from Step 2 (0.775 V AC measured at TP2).

  2. Engage the Low Cut (HPF) switch on the front panel.

  3. Measure AC voltage between test points TP3 and TP1 (or monitor DAW input level).

  4. Adjust trimpot R_LC_TRIM until TP3 reads exactly 0.775 V AC (unity gain pass-through at 1 kHz).

 

High Cut (LPF) Filter Calibration (R_HC_TRIM)

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  • Maintain setup from Step 2 (0.775 V AC measured at TP2).

  • Disengage Low Cut and engage the High Cut (LPF) switch on the front panel.

  • Measure AC voltage between test points TP3 and TP1 (or monitor DAW input level).

  • Adjust trimpot R_HC_TRIM until TP3 reads exactly 0.775 V AC (unity gain pass-through at 1 kHz).

  • Once calibration is complete, your SL4K-E01 Rev B preamplifier module is fully aligned, quiet, and ready for recording!

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7. Interactive Bom

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