VU Meters Vs Bargraph
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Volume Units and Decibels
My first peek at a Nakamichi was in Japan. Sparse in detail, no flashing lights, VU meters or fluorescent displays. Just so damned gorgeous. OK, then you have the PA-7, industrial strength, industrial looks and what a beauty. But if you one needs to really capture beautiful looking amplifiers, then look no further than the McIntosh range. Black in finish, blue lit VU meters.
So what to do if you need that really cool look and have a budget of a few ZAR? Unfortunately VU meters, really good VU meters are not cheap. The question about where to source comes up often on the web and besides eBay where you may not necessarily get exactly what you are looking for, Meter Sales or Instrument Meter Specialties in the USA may be your answer. In South Africa the Model 543 is going to land at about R1 800.00 ex duties and VAT. So make sure your amplifier is worth it.
The bargraph, long being favourite for many enthusiasts is your economical solution in most cases. Why have a VU anyway if a bargraph is a better solution in many ways? It all comes down to vintage. In the 50s, 60s, 70s and 80s most equipment was still analogue where the outputs were designed to be fed into an analogue recording device. Analogue recorders are a lot more resilient to input overloads, in some cases even done deliberately. Not so with digital devices where the results are painful to listen to. VU meters are very slow and don’t necessarily slot in well high with speed transients but they are brilliant for monitoring the average output amplitude.
VU meters with peak overload, monitoring the 0VU perimeter and making sure the VU meter is correctly calibrated is all part of your sound engineering experience.
Calibrating
As the VU meter is a result of the old (1942) ANSI C16.5-1942 standard, 0Vu is equal to +4dBU or 1.228V R.M.S. (0.707 x Vpk = 1.74V) dissipating 250mW across a 600 Ohm termination, an old telephone standard devised at Bell Labs.
Consumer grade VU meters require a 300mV drive (-10dBV).
A VU meter across an audio signal registers the “loudness” of a signal, a result of the inertia/mass of the needle. Mechanical damping further evens out the meter movement.
The meter design should reflect full scale or 99% of 0VU within 300ms with a test tone of 100kHz. The frequency response of the meter or circuitry and meter is important, usually 0.2dB variation 35Hz to 10kHz and 0.5dB variation 25Hz to 16kHz.
Now you know why a good VU meter and driver circuits are so expensive.
Use in power amplifiers.
Hoping that IMS don’t sue me for the use of their 43092 image.
The above meter requires 1.734V peak a.c. to fully deflect the needle. Knowing the internal resistance or what current flow this meter is required for FSD (full scale deflection) is critical. The meter can be driven through a high impedance input full bridge rectification circuit catering for the 300ms rise time. Knowing the internal resistances and/or uV needed to drive the meter to FSD is therefore a pre-requisite. What is just as important however is that as our hearing is logarithmic the approach to designing a VU meter driver should be such that the loudness, as registered by the meter, shows a loudness increment in sync with the volume we hear. Linear drivers will show a small movement until the output has been increased substantially which is not in line with perceived loudness.
If all else fails contact the manufacturer to get the optimal meter driver or circuit, one that uses op amplifiers.
Further Reading:
Objective Sounds – Driving VU and other AC Meters
Giangrandi – VU Meters
Rod Elliot – Project 55 – VU and PPM Audio Meters
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