User manual ELECTRO-VOICE 668 DATASHEET

[. . . ] Use of a mains separator transformer is mandatory during maintenance while the appliance is opened, needs to be operated and is connected to the mains 3. Switch off the power before retrofitting any extensions, changing the mains voltage or the output voltage. The minimum distance between parts carrying mains voltage and any accessible metal piece (metal enclosure), respectively between the mains poles has to be 3 mm and needs to be minded at all times. The minimum distance between parts carrying mains voltage and any switches or breakers that are not connected to the mains (secondary parts) has to be 6 mm and needs to be minded at all times. [. . . ] Additionally, special circuitry prevents the output-stage transistors from being damaged by Back-EMF. During soft start, delayed switching of the power outputs is accomplished via relays and a limiter controls the initial current inrush, preventing the mains fuse from being blown during the power-on operation. The mechanical construction as well is carried out following the highest precision standards of the industry. The robust steel chassis provides extreme rigidity and it is meant to live through any hard wearing condition of a touring application. Thermal stability is guaranteed by two 4-Mode silently running fans that offer the possibility to also use the amplifiers in a studio environment. The extensive comparator circuitry constantly monitors the input and output signals and activates the internal limiters whenever a non-linear operational state is encountered. This provides reliable protection of the connected loudspeaker systems against overload and clipping. The sound quality of the CONTRACTOR PRECISON SERIES power amplifiers is superb. Using comprehensive dimensioned power supply units with low-interference toroidal transformers gains a headroom that exceeds the nominal power handling capacity by far. No V/I-Foldback-Limiter circuits are employed within the power amplifiers, making it possible to operate the amps on complex loads up to ±90° phase angles without a problem. The input facilities are carried out as balanced XLRF-type sockets while the Direct-Outs ­ on which the carried-through signals are present ­ come as XLRM-type connectors. If the LED flashes briefly, this is not a cause for concern. If this LED is lit permanently, the volume should be reduced to avoid overload damages to the connected loudspeaker systems. PROTECT When this LED lights up during operation, one of the protection circuits against over-temperature, overload, shorted outputs, radio frequency interference or DC faults has been triggered. a shorted loudspeaker line must be remedied. In case of overheating, wait a little until the amplifier switches back to operating mode itself. POWER Switch The unit is switched on via the power switch. The loudspeaker outputs are switched on via delayed relays so that no startup transients are audible. A current limiter prevents startup peaks on the mains line and prevents the mains fuse from blowing. 10 REAR PANEL POWER AMPLIFIER INPUTS Parallel to the XLR-type inputs, output connectors are provided, offering the opportunity to feed additional power amplifiers with the same signal. The inputs are electrically balanced with pin-assignment according to the IEC 268 standard. Pin-assignment of the XLR-type input connectors: PIN 1: PIN 2: PIN 3: SHIELD a, + b, - Input sensitivity is factory set to 0 dBu. The power amplifier can be set internally to +6dBu input sensitivity or to +26dBu gain. INPUT ROUTING PARALLEL MONAURAL Setting the selector switch to the PARALLEL/MONO position puts the channel A and channel B inputs in electrically direct parallel configuration, leaving you still the possibility to control the channels' level settings separately, using the output controls A and B. [. . . ] The phases of Channel A and B are in opposite and therefore the individual channels must not be used as loudspeaker outputs. HI-LO-CUT FILTER This filter attenuates subsonic and high frequency signals so that the power amplifiers are not modulated with these signals. This switch should normally always be in position ON. The OFF position is only for applications where an upstream unit, e. g. [. . . ]