Electronics with passion: amplifiers and power supplies, FET's and vacuum tubes..

HotFET Pre: J-FET audio preamplifier parameters and Q&A

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Most of these questions and answers are coming from discussions with friends. Hence the style.

Want to see the scope traces, square wave if possible
100KHz, 8V peak-to-peak at the input of the amplifier. Attenuator was set at -3dB.
Input signal connected to CH2 (blue). Output is shown on CH1 (yellow).
No load at the pre-amplifier's output:

Signal slopes magnified:

The amplifier's load was 4.7 kOhm there - conventional situation:

470 Ohm load connected to the output - preamp is heavily loaded:

Handling 47 Ohm load - extreme, unrealistic load:

Measured output impedance was around 180 Ohm.

Q: Where are all those Q&A;?
A: There is much more to tell and explain about HotFET-Pre(c). Thus more to come - stay tuned!


  1. Pingback: HotFET Pre (c) в вопросах и ответах, или зачем там резистор последовательно с диодом | MyElectrons.ru

  2. Aren van Waarde

    Hallo Serge,

    Today I discovered your electronics website: Great work! I consider both the audio and measuring instrument pages very interesting. I have run a simulation of the HotFET Pre schematic, and it appears to behave very nice indeed. There is something about the circuit that I do not understand (I am not an electronics engineer, but a biologist and electronics hobbyist). Can I ask you a question? For R2 and R3, you specify 91K resistors and, based on my simulation, that value seems correct. But 182k series resistance means that very little current can flow through the LEDs (probably <= 0.2 mA). Yet, in the photograph of your prototype, the LEDs seem to glow a bit. Will the voltage stabilizing action of the LEDs be OK at so little current? And is it true that the LEDs are starving?

    Best regards from the Netherlands,

    Aren van Waarde (Groningen)

    • Serge

      Hallo Aren van Waarde, welcom op de MyElectrons website!

      I had very same worries too – would that be enough current for LEDs to keep a steady voltage drop. But after trying this solution out I noticed no problems whatsoever: stable voltage drop, no substantial noise and such. This will depend on the particular LED of course.

      To be honest the whole schematic looks a bit too bulky. The solution is rather simple – using depletion MOSFETs in the cascode’s “top” position instead. For years I’ve been using DN2540N5 in that position happily.

  3. Klaus Hollinetz

    Can you please submit the upgraded schematics for the use with depletion mode mosfets.
    Thank you


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