passive VU meter
If you want to fit your loudspeaker enclosure(s) with a drive indicator, it is best touse a unit that does not need a powersupply. This article describes a passive VUunit for smaller amplifiers and loudnesses—see Fig. 1. It consists of a rectifier and six(may be fewer or more) identical stagesthat each comprises a current source, azener diode and a light emitting diode,LED. The current sources are built fromJFETs with interconnected gate-source terminals—here, Type BF256A. The saturationcurrent, IDSS, of these devices with adrain-source voltage, UDS = 15 V is about 5mA. This current is not exactly constant,but is perfectly all right for driving a lowcurrentLED and will not exceed the permissiblevalue of 7 mA. Networks R2-D2,R3-D4, and R4-D7 are protection circuits;they prevent the drain voltage of the relevantJFET rising above 30 V, which normallydestroys the transistor.
The rectifying circuit is formed by D1and capacitors C1, C2. Resistor R1 limitsthe peak current to about 1.5 A at a sourcevoltage, US, of 50 V. Since it is in series withC1 and C2, and thus in parallel with theamplifier output, it has no effect on thelevel of the input voltage. The peak outputvoltage of the rectifier is applied directly tothe single LED stages. The potential acrossC1 and C2 is not exactly equal to the instantaneouspeak voltage, but, because of timeconstant R1-C1-C2, is a good average of it.Consequently, the unit indicates briefly theinstantaneous peak voltage, and then themean of it.The coworking of the three parts of astage is easily understood by consideringthe following. If the rectified and smoothedvoltage rises a few volts over the level setby the zener diode, the current sourcecomes into action and causes the LED tolight.
Since the (input) voltage to the meteris directly proportional to the amplifier outputand the (assumed constant) impedanceof the loudspeaker, the indicated thresholdlevel (in watts) can be converted into azener voltage:P = URMS/R = (Us/√2)2/R = Us2/2R∴Us = UZENER = √2PR – ULEDwhere ULED is the starting voltage of theLED (and the voltage drop across the currentsource), which is equal to 2 V. Thus,for an indication of 100 W into an 8 loudspeaker,the zener voltage isUZENER = √(10028) – 2 = 38 V.The zener to be used should have the nextlower rating in the table (36 V), so that itlights brightly when the output is 100 W. Inthis way, the stages may be designed moreor less to individual requirements.In the most sensitive stage, T1-D2-D3,the zener diode is, strictly speaking, superfluoussince the indicated power is determinedentirely by the threshold values ofD1, T1 and D3.