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µ-Amp Guitar Preamplifier

This LearnTronics presentation mirrors the established FJD1 project page while retaining the LearnTronics navigation and visual style.

Circuit Design

This µ-amp began as a hand-drawn analog circuit and was rebuilt in KiCad as a compact, through-hole printed circuit board. Explore both the original and updated boards in 3D, then continue below for the circuit description, response charts, specifications, testimonial, frequently asked questions, and ordering information.

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Original µ-amp PCB — 3D Viewer Drag to rotate · wheel to zoom · right-drag to pan
Loading the embedded VRML model…
The original self-contained KiCad VRML export is embedded directly in this page. Its three resistors now show their value bands: R3 is 120kΩ, while R1 and R2 are 100kΩ. This earlier board did not yet include the later 0Ω jumper. Internet access is needed only to load the Three.js viewer library.
Updated µ-amp PCB — Phoenix terminal-block version Drag to rotate · wheel to zoom · middle-drag to pan

This browser could not start the updated VRML viewer.
Loading the updated VRML model…
The updated model loads the main WRL file together with its linked component models from /circuits/shapes3D/. Value-specific 0Ω, 100kΩ, and 120kΩ resistor models now show their color bands. Two-sided rendering prevents cutaway faces, and the same Isometric, Top, Bottom, Reset, and Auto-rotate controls are provided as on the original model.
Single-sided routing All copper traces were routed on the underside of the board for a straightforward build.
Hand-soldering friendly Large wire pads, through-hole parts, and substantial traces support practical assembly and repair.
KiCad workflow Schematic capture, ERC, PCB layout, DRC, Gerbers, drill files, and the 3D model were produced together.
Onboard Guitar Preamp & Character Drive · Single Op-Amp · Battery Powered

µ-Amp: Boost, Bite, and Musical Compression

A compact, battery-powered onboard preamp designed for magnetic pickups. It can run clean as a boost, or push into a guitarist-friendly “edge” with a unique gain/feedback control that players describe as lively and touch-responsive.

Magnetic pickup input DPDT true bypass 9V (optionally 18V) Built around NE5534

What It Does

  • Clean boost to wake up long cable runs, pedal chains, or a lazy amp input.
  • Musical drive that can feel like “sweet grit” rather than harsh fuzz.
  • Touch compression that responds to pick attack—great for lead lines and sustained chords.
  • True bypass via DPDT switch—straight pickup-to-jack when off.

Why It Feels Different

The gain control is not a typical “volume knob.” The input is injected at the pot’s wiper while the pot also participates in the feedback path. Turning it changes more than gain: it changes loop behavior, impedance at the summing node, and how the op-amp approaches its limits, often perceived as “alive” and expressive.

Best use: installed inside the guitar to drive a stage rig confidently—especially where cable length and live dynamics matter. Micro-Amp circuit board Circuit board.

How It’s Wired (Plain English)

Signal Path

  • Pickup input → series resistor (about 120kΩ) → 1µF coupling capacitor.
  • That signal goes to the wiper of a 500k potentiometer.
  • The pot participates in feedback between the op-amp output and the inverting input (pin 2).
  • Output is AC-coupled through a 1µF capacitor to the output jack.
  • A stereo jack can disconnect the battery negative when unplugged.

Bias and “Feel”

  • Non-inverting input (pin 3) is biased at mid-supply with a 100k/100k divider.
  • A 0.1µF cap decouples that bias node to ground for stability.
  • Output includes a 0.1µF cap to ground (a strong contributor to the circuit’s character).
  • Optional small compensation cap at the op-amp comp pin may be present (value depends on build).
Note: The 0.1µF output-to-ground capacitor is a large capacitive load and can contribute to compression/edge. Layout and wiring quality matter.

Specifications (Typical / Adjustable)

Power9V battery (optional 18V with two batteries, more headroom / cleaner boost)
BypassDPDT true bypass (pickup straight to output when off)
Operational amplifierNE5534 introduced by Signetics in 1979
InputMagnetic pickup (passive)
Gain RangeApprox. 0× to ~4.2× (about +12 dB), depending on pot wiring and component tolerances
OutputAC-coupled to output jack; stereo jack can act as battery disconnect
Cable CompatibilityDesigned to comfortably drive typical 10–20 ft guitar cables when engaged

These are practical, musician-focused specs. Exact response depends on pickup, cable, amp input, and control position.

Installation Notes (Tech-Friendly)

  • Keep leads short and routing tidy—this circuit can be sensitive to layout because of capacitive loading.
  • Recommended: add a small series resistor on the op-amp output (e.g., 47–220Ω) if you want extra stability without losing character.
  • Use a stereo output jack for battery negative switching (battery disconnect on unplug).

Testimonial

Metaldude Metaldude

The µ-amp is unique for some of the material aspects, but mostly the 5534.

The slew rate of the 5534 was 48k which allowed full frequency headroom in the audio range. The electrolytic to ground were filters for squeal and noise.

I must admit there is perceived compression with increased volume, but no limiting roll off like real compressors.

The audio is clean replication of the original pickups, behaving with the tonal and harmonic characters of a Les Paul 1970 windings.

No distorted clipping at all when the amp was in clean mode. I’ve experienced only one pedal with these characteristics, but it was a clipping compressing OD with many tone controlling aspects with many choices, and that’s not always good. The µ-amp is mono dimensional: hit the switch+ volume on 10! I played hundreds of hours with our preamp and it is a fond memory.

Tube amps loved the input voltage and capacitance with a 20' cable!

Metaldude Metaldude
Olympia Conservatory of Music

Ordering Information — Send Check To

Delphi O.E.M. Co
5348 216th Ave SW
Centralia, WA 98531