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FET Focus Deep Dive: Tape Op JFET Mic/Guitar Preamplifier (Scott Hampton Design

Updated: Aug 25

In July 2002, Tape Op featured Scott Hampton’s ultra-simple, ultra-musical DIY JFET Mic Preamp, a solid-state marvel emulating tube-like warmth without the hassles of high-voltage supplies. Hampton called it a "solid‑state tube"—a nod to how closely a JFET can mimic the behavior and sonic character of a triode. (Tape Op)


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Figure 1 – The “Solid‑State Tube”

Figure 1 juxtaposes a classic N-channel JFET class-A stage with a triode tube amplifier. They’re functionally identical: both offer high input impedance, soft saturation, and effortless gain. In Blind A/B comparisons, many listeners preferred the JFET version, praising its clean highs and tight bass—even alongside the tube circuit. (Tape Op)


Why it matters: It visually demonstrates why we love using JFETs like LSK189/389 in audio—organic response without the complexity of tube gear.


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Figure 2 – The Build‑Ready JFET Module

Figure 2 transitions from theory to application. It’s the complete circuit:


  • JFET (e.g. 2N5457, or better—LSK189/LSK389)

  • 15 kΩ drain resistor to 9–24 V supply

  • 50 Ω unbypassed source resistor + 470 µF bypass—cuts 3rd harmonic by ~20 dB

  • NPN Darlington buffer (MPSA14 / ZTX653) for low-impedance output

  • Coupling capacitor on output to block DC offset

  • Approximately +26 dB gain, with high input impedance (~1 MΩ) (diyAudio, Tape Op)


This design gives a clean gain stage with a mild but useful amount of JFET-driven harmonic coloration. The voltage-follower stage ensures you can drive downstream gear with confidence.


Applications & Advantages


1. Microphone Preamp

Stack two of these modules, add a mic transformer and gain pot—voilà, a rich, transformer-balanced mic preamp. The high input impedance works wonders with ribbon, electret, and piezo mics.


2. Guitar/Instrument Preamp

Just one stage with output pot transforms a guitar pickup into an amp-ready signal—class-A mojo in pedal-friendly format.


3. Sonic Character & Distortion Control

Staging multiple modules or subtly increasing gain introduces pleasing saturation. The 50 Ω unbypassed Rs tempers highs, taming harsh harmonics while retaining clarity. Many DIYers build dual-stage versions with level controls on each for saturation orchestration. (Tape Op, GroupDIY Audio Forum)


Supercharge With Linear Systems JFETs


  • Replace generic FETs with LSK189 (single) or LSK389 (matched dual) for exceptional performance: low noise (~0.9 nV/√Hz), low leakage, and superb frequency response.

  • Why switch? The stock 2N5457 offers a great starting point, but LSK-series JFETs bring studio-grade sound, especially beneficial for ribbon mics and piezo pickups.


💡 Learn More & Resources


  • Original Tape Op 2002 Article detailing Figures 1 & 2 and timestamped examples: “DIY JFET Mic Pre: Build Your Own Studio Preamp” by Scott Hampton. (Gearspace, Tape Op)

  • Hamptone kits (HJFP1/HJFP2) – built around this JFET module: learn via Ralf Kleemann’s build log. (kleeworld.de)

  • Reviews – from gearspaces like Gearspace/GroupDIY: users praise its mojo and clean-to-gritty transition. (Gearspace)

  • Linear Systems JFET catalog – explore datasheets for LSK189 and LSK389 at linearsystems.com




 
 
 

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