VU-Meter with EM84

This is a retro project I built in 2014. I was looking for a nice VU-Meter with tubes but didn’t find any project in the internet that fulfilled my requirements. So I had to develop a circuit by myself with the “magic bar” tube EM84. The front page picture of this blog shows the finished project in operation.

For the tubes I used a cheaper and NOS replacement from Russia, 6Е3П. The control electronics, made with four operational amplifiers of a TL064, does the signal amplification, logarithmic function, peak detection and an analog adder to adjust the zero point. An N-FET current source is used to realize the linear drop. For better thermal stability of the calibrated circuit the TL064 can be replaced by a LT1014 precision Op Amp.

There are two switching regulators to create all needed voltages from a single 12V supply. The first regulator with a TLC555 as controller creates the anode voltage of approximately 240V. From that source four other voltage potentials for the operational amplifiers are derived with a chain of Zener diodes. The second switching regulator is a modern one, LM2575T-ADJ. It is used to efficiently create the heater voltage of 6.3V.

The sum of all features makes this project unique and most professional compared to a lot of similar circuitry found in the internet.

project goals:

• single power supply of 12V
• logarithmic scale
• constant current source for linear drop
• multiple adjustment possibilities to match unequal tubes
• aesthetic scene

The schematics is available for download here, the bill of materials you can find here.

VU-meter-EM84-schematics

 

setting instructions:

• let the circuitry warm up for at least one hour
• shorten jumpers JP1 and JP2 and adjust with P4/P8 for equal brightness
• remove jumpers
• adjust zero point with P3/P7, that only just the green bar disappears
• a signal generator is useful for this step: connect signal with maximum level to the input and adjust maximum (gain) with P1/P5
• disconnect signal, shorten jumpers again
• remove both jumpers synchronously and adjust drop timebase as desired with P2/P6