DIY SynthesizersInterfaces and sub-systemsAudio & Effects
Go to the HomepageWhat's up Doc ?Go to the Linkpagem.bareille

True RMS Plasma Vu-meter



The Plasma VU-meter display audio levels on Russian bargraph gaz discharge tubes
IN-13 or IN9









First prototypes

Here are pictures and informations about my Plasma VU-meter prototypes most of them were powered by well regulated external hi-voltage power supply.
The true- RMS converter stage is inspired from a diy project published into the french electronic magazine Radio Plans (now disapeared since many years), based on the THAT2252 rms detector chip.

A THAT2252 chip convert audio levels ( dBs ) into a linear control voltage ( Volts DC ) to drive an IN-13 (or IN-9 ) gaz discharge bargraph tubes ... The THAT2252 is set to provide almost 0V on the output at -20dBu input level and 2.50V at +20dBu.

An adapted RC time constant set the up/down raise time to imitate classic vu-meter balistics, The IN-13/IN-9 driver stage is based on a common LM358 and a hi-voltage power transistor. The full range display is set by a trimmer.


Download the Plasma VU-Meter schematic v1.0 (pdf )

Here are pictures of the VU-meter v1.0 fitted with IN-9 ( purple ) tubes,
both powered with an external hi-voltage psu

bellow it is the same... but with IN-13 tubes.
Much more stable and it require at least 2 time less current on the hv-psu

Clic on pictures to view in big



Basic Plasma VU meter +/-20dBU - THAT2252

Features :

    • Balanced/unbalanced input preamp
    • Cal/Uncal levels ( +4dBU/-10dBV)
    • -20dBU to +20dBU (If calibrated to 0dB)
    • 40 dB range over 10Hz up to 25kHz bandpass
    • True RMS Levels
    • CV output to use as an enveloppe follower
    • Designed to hold IN-13 or IN-9 tubes
    • One trimmer for full range display , hardware calibration
    • Stackable pcb for stereo or multi-track displays
    • Integrated hi-voltage transformless PSU
    • Module powered by +/-15V or +/-12Vdc


Download the Plasma VU-Meter +/-20dB -That 2252 BOM v2.1 (txt )

Download the Plasma VU-Meter +/-20dB -That2252 schematic v2.1 (pdf )



The revision 2.1 pcb is perfect to build mono , stereo or multitrack displays

Here is a stereo Vu-meter with IN-13 made with two rev 2.1 boards stacked...Ready to work !


The stereo version with IN-13 and revision 2.1 pcbs in action on YouTube !



closer view ...

Plasma Vu meter +/-20dB stereo module in 5U MOTM format
with engraved plexiglass scale and wood front panel /tube holder.


Wide Range 120dB Plasma VU meter - THAT4301


The RMS detector of the VUmeter is based on THAT Corp. design note 119 .
The IN13 driver and Hi voltage PSU are still the same.

Please notice that the Wide Range Vu-meter is not an 'upgrade' of the previous one. They do not have the same use in audio reccording .

The display range is 3 times wider but the db/mm resolution is also 3 times lower ! Not counting the compression factor of 2x of the RMS analyser stage.

It is just a different tool to display audio levels another way .

Features :

  • unbalanced input
  • -100dBU to +20dBU range
  • True RMS Levels
  • Adjustable 0dB point
  • CV output
  • Integrated hi-voltage transformless PSU 150V/8mA
  • Module powered by +/-15V or +/-12
  • Small pcb with 0.100" connectors

The THAT 4301 chip contribute to reduce the number of ICs ,so the pcb is smaller too.

Download the Wide Range 120dB Plasma VU-Meter - final schematic (pdf )

Download the Wide Range 120dB Plasma VU-Meter -BOM (pdf )



Picture of the first prototype

Here is a scale for this vu meter ...Not yet sure it is exact ;)


2x Stereo Wide Range Vu meter front panel made with teak wood and carnauba oil finish.
Format rack 19"-1U . Vu meters pcbs ( in construction ) are at the bottom left of the picture ...more to come soon....




Multiband VU Meter projects


todo, coming soon :)




-All audio circuits are powered under +/-15V . To simplify the integration of the VU meter into various devices, a high voltage DC to DC converter has been included. It transform the +15Vdc into a clean +150Vdc at approx 8mA. The high voltage converter is a minimalistic switched psu based on a 555 and a mosfet transistor. This cheap and secure solution provide an independant power supply for each tubes. It is ideal for multiples Vu-meters displays.

Please notice the plasma VU-meter pcb v2.1 integrated hi-voltage psu is only to drive IN-13 tubes. This psu is not powerful enough to drive IN-9 tubes !

-For IN-9 afficionados : The IN-9 require at least 12mA but many tubes need up to 50mA for tube pre-polarisation during a short time period to be able to reach the full range at 12mA ...However it is still possible to use the RMS detector+ tube driver holded by the pcb v2.1and to supply the IN-9 tube with a more powerful external hi-voltage psu. Be aware that 50mA is for one IN-9 tube so for a stereo version you will need at least 100mA under 140V dc ...

-To build an accurate VU-Meter the IN-13 is the best choice :

  1. It is very stabe
  2. it start glowing at the bottom of the tube
  3. it consume less curent : 5mA full range, ideal for multi-tracks displays
  4. it have increased length ...

-In normal use the IN13 tube do not heat



Plasma VU-Meter pcbs +construction manual
are available on order


Please be aware this schematic work with
potentialy lethal DC voltages ! Use at your own risk !!!

I am not responsible in any cases or situations of any damages or destructions of devices or humans resulting of the use of informations provided into this page.

Last revision 01/2012
© Marc BAREILLE 2007