Preamp with EF95 vacuum tubes.

The design is heavily inspired by Electronoobs design. I modified the power supply and adjusted some component values.

The amp is based on EF95 tubes in a Class A configuation (one tube per channel). The gain can be adjusted using two potentiometers.

To reduce noise in the audio signal it is recommended to use a 24V power supply with transformer (no switch-mode power supply).

schematic

pcb

case

I created the model using FreeCAD. You can download the files from Thingiverse using this Link: https://www.thingiverse.com/thing:6177183 (or from the section project resources)

final design

project resources

You can download all other project files here (including Fritzing-files and FreeCAD+stl files)

measurements and electrical characteristics

general parameters

• power consumption idle (dominated by the heating current of the tubes): 170 mA @ 24 V -> 4,1 W
• input-resistance: 59 kΩ (@1kHz Sinus)
• output-resistance: 2,9 kΩ (@1kHz Sinus)

frequency and phase response

distortion

FFT

(U_in_pp=350 mV)

sources/useful links

• main source: https://electronoobs.com/eng_circuitos_tut73.php
• other designs based on EF95-tubes: http://www.hifi-forum.de/viewthread-111-4306.html
• … and https://www.elexs.de/ef954.htm
• EF95 datasheet: https://frank.pocnet.net/sheets/129/e/EF95.pdf

• LT Spice Model LM358: https://www.mikrocontroller.net/topic/184648
• LT Spice Model EF95: https://www.tube-town.de/ttforum/index.php?topic=19590.0