VU is a relative indication. Depending on where in the audio chain you stick the meter, you may want to use more or less resistance than the standard 3600 ohms. The idea of"bridging" is to prevent the meter from loading down the circuit or the audio line. Less resistance will make the meter more sensitive and vice versa, but if less resistance is used, it would be good to check the output level from the amplifier with a scope or a.c. voltmeter to see if the presence of the VU meter affects the output.
I put mine, which is a professional grade meter removed from some piece of discarded broadcast equipment, in the 500/600 line between the last speech amplifier stage and the input of the peak limiter. The idea of the VU meter is to give an indication whether or not the audio level is set correctly,
not to give a precise level reading. In broadcast gear, the VU meter often has an adjustable sensitivity control, normally a variable resistor or a set of switchable fixed resistors. You set the output of the amplifier to give the desired result: fully drive but not overdrive the recording device, fully modulate but not overmodulate the transmitter, fully drive a subsequent amplifier without sending it into distortion, etc., and then adjust the VU meter to best indicate 100% on audio peaks. That gives a
rough indication that the audio level is set correctly although, as the article says, it is a very poor indicator of short duration peaks from a program source like speech or music. The key to a good VU indicator, as pointed out in the article, is that the mechanical meter movement have the proper ballistic characteristics including damping.
Prior to the VU meter, "level meters" were used. The early ones were set to "Decibels", and the 0 dB reading was standardised to 6 milliwatts. Later, the standard for 0 dB was changed to 1 milliwatt ("dBm").
Special dB Scales
Over the years different segments of the radio and electronics industry have created special decibel scales for their own use...and the specific level used as a reference point. The standard reference voltage or power will be placed in the denominator of the equation, and is usually referred to as the "0 dB" reference level. This name comes from the fact that placing the same level in the
numerator produces a ratio of 1:1, or 0 dB. Several different special dB scales are listed below.
dBm. These units refer to decibels relative to one milliwatt (1 mW) of power
dissipated in a 50 ohm resistive impedance (defined as the 0 dBm reference level), and is calculated from either 10 LOG (PWATTS/0.001) or 10 LOG (PMW). The dBm scale is used in describing receivers and amplifiers. For example, an input signal or an output signal may be defined in terms of dBm. Similarly, the noise floor of the receiver may be given in dBm.
dBmV. This unit is used in television receiver systems in which the system
impedance is 75 ohms, rather than the 50 ohms normally used in other RF systems. It refers to the signal voltage, measured in decibels, with respect to a signal level of one millivolt (1 mV) across a 75 ohm resistance (0 dBmv). In many TV specs, 1 mV is the full quieting signal that produces no "snow" (i.e. noise) in the displayed picture.
dBmV. This unit refers to a signal voltage, measured in decibels, relative to one microvolt (1 mV) developed across a 50 ohm resistive impedance (0 dBmV).
dB (Old). An archaic dB unit used in the telephone industry prior to World War II used 6 milliwatts dissipated in a 500 ohm resistive load at the 0 dB reference level.
Volume Units (VU). This unit is used in audio work, and largely replaces the old dB scale given above. In the VU scale 0 VU is 1 milliwatt dissipated in a 600 ohm resistive load..
http://www.dxing.com/tnotes/tnote10.pdf The power rating of audio transformers is often stamped on the case or listed in the specs. It is important to make the distinction between "dB" and "dBm". Older transformers usually go by the "dB" standard (0 dB = 6 mw), while newer ones tend to go by the "dBm" standard (0 dB = 1 mw).
Unless otherwise indicated, if the rated level is given as "dB", assume the 6 mw standard. If it is given as "dBm", then by definition, they used the 1 mw standard.
