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EAE DIY HD130 Preamp Builder's Manual

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HD130 Preamp
Builder's Guide
Version 1
April 11, 2024
John W Snyder

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Summary of Contents for EAE DIY HD130 Preamp

  • Page 1 HD130 Preamp Builder’s Guide Version 1 April 11, 2024 John W Snyder...

  • Page 2: Table Of Contents

    Revision History Version Changes Draft Copy Contents 1 Introduction 1.1 Background ........1.2 Operation .

  • Page 3: Introduction

    1 Introduction This document contains a guide for assembling the HD130 Preamp by Electronic Audio Experiments. This PCB is based on a schematic that we originally shared in 2017 as an open source project. This version was also available in a fully assem- bled form thanks to our friend Brad at Nerd Knuckle Effects.

  • Page 4: Operation

    1.2 Operation The controls are as follows: Volume Self explanatory. Can get quite loud. Equivalent to the master volume on the original amp. Gain Sets the gain of the circuit near the input. Low settings are very clean, higher settings can get pretty aggressive. Equivalent to the volume control on the original amp.

  • Page 5: Before You Start

    Appendix. 1.5 Disclaimers and Licensing Caution: EAE is not responsible for the outcome of your build and cannot offer direct support or troubleshooting beyond this document. This document and all associated materials are released under the Creative Com- monce BY-NC-SA License.

  • Page 6: Parts List

    2 Parts List The following parts list assumes a build in the stock configuration. Parts marked with an asterisk (*) have additional information in Section 7, where you can also find suggested mods. We recommend reading this section before you begin. Resistors, Potentiometers, Trimpots Value Ref #s...
  • Page 7 Capacitors Value Ref #s Description 3.3n Capacitor, Box Film 5mm LS 220n Capacitor, Box Film 5mm LS 470n Capacitor, Box Film 5mm LS C14, C16 Capacitor, Box Film 5mm LS 100n C6, C9, C12 Capacitor, Box Film 5mm LS Capacitor, Box Film 5mm LS 100u C2, C5 Capacitor, Elec 6.3mm dia...
  • Page 8 Off-Board Parts Value Ref #s Description Enclosure 125B or equivalent 1/4” Mono Jack IN, OUT Switchcraft 111x or equiva- lent DC Jack 2.1mm barrel connector 3PDT 3PDT Footswitch, solder lug Knob 1/4” shaft w/ set screw 3PDT Daughter- Included w/ PCB board Any 3mm or 5mm LED Ribbon Cable...

  • Page 9: Wiring

    3 Wiring The wiring diagram is shown in Figure 1. For overall assembly instructions please refer to the Appendix. Wire pads on the board are listed in Table 1. For convenience, pads make connections to hardware located in close proximity. Note that the LED is mounted on the reverse side of the footswitch PCB, or can be wired off-board using a bezel.
  • Page 10 Function Input Jack Tip Output Jack Tip G1, G2 Jack Sleeve Connections (Ground) 9V input (DC jack) DC jack ground Bypass footswitch connection to input jack LED + to footswitch LED/Footswitch ground Send/Input to effect Return from effect Bypass footswitch connection to output jack Table 1: List of wire pads and functions.

  • Page 11: Drill Template

    4 Drill Template Print this page out using 1:1 settings on your printer and fold to form the drill template. We recommend drilling pilot holes first to check alignment.

  • Page 12: Schematic

    5 Schematic...

  • Page 13: Circuit Analysis

    6 Circuit Analysis 6.1 Signal Path The HD130 preamp consists of three cascaded op amp gain stages with passive at- tenuation and filtering in between. What distinguishes this circuit is the unusual approach to gain adjustment. In most op amp preamp/drive circuits, variable neg- ative feedback is used to set the gain based on ratios of resistors.
  • Page 14 Figure 3: Some representative frequency sweeps of the tone stack. of 330k /4 . 7k 70, or about 37dB. This stage contributes the bulk of the gain and distortion. With a gain of 70, even very small signals are going to drive the op amp into clipping.

  • Page 15: Brief Notes On The Power Supply

    places a treble bleed on this pot, which boosts high frequencies at low gain. This gives the low gain sounds a more sparkly, jangly sound. For a comparison between the normal and bright cap modes at various Gain settings, see Figure 4. In the clipping stage (IC2), the op amp selection has a significant impact on the sound.
  • Page 16 The TC7660SCPA and similar specify a higher switching frequency for this purpose. The actual charge pump switching lines (in this case, going to C4) should be short PCB traces to keep inductances low and prevent ringing. C4 itself is a low ESR, non-polarized multilayer ceramic capacitor which eliminates the polarity concerns and limited lifetime of electrolytic capacitors.

  • Page 17: Modifications

    7 Modifications Caution: Modifications should not be attempted unless you have the means to troubleshoot changes beyond the base configuration. 7.1 Swapping Op Amps Because this circuit relies on op amp clipping, swapping op amps will have a signif- icant impact on the drive character. The original HD130 uses the LM307, which is a slow BJT op amp that is akin to the LM301, but with an internal 30pF compensa- tion capacitor.

  • Page 18: Bright Cap

    Finally, the resistors R4 and R8 provide additional opportunities for adjusting the gain of the other two fixed-gain stages. If you get excessive clipping at low gain, try R4 = 47k . 7.4 Bright Cap We have included pads for a bright cap (CX1) and accompanying series resistor (RX1).

  • Page 19: A Generic Assembly Guide

    A Generic Assembly Guide This is a fully general pedal assembly procedure that applies to all EAE open source projects. Note that minor details can vary between PCBs. But this will serve as a quick reference for those already familiar with this type of assembly.

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