The variable resistor is to ensure an impedance balance between the positive and negative inputs, and should be adjusted until any hum is at a minimum.
Balanced lines are also the most effective means of reducing or eliminating RF or ‘radio frequency interference’. Long unbalanced cable runs are likely to pick up radio signals. While these will be at a low level, they can be easily audible once they are amplified through a gain stage. This also applies to inputs to your mixer, so use balanced inputs wherever possible.
Losses Due to Cable Runs
To connect the speakers to the amplifier, some type of cable must be used. This cable must be able to carry the signal some distance with the minimum amount of degradation along the way. Unlike earlier stages in the audio chain, the signal is at a relatively high level, therefore it is relatively immune to noise, RF interference and hum. However, power losses and a reduction in damping factor (the ability of the amp to control the speaker accurately) can occur, and these depend on a number of factors:
∙cable gauge - the higher the gauge, the smaller the cross section of each conductor in the wire. This gives the signal more resistance and therefore results in more power loss and a reduction in damping factor. For best performance use 8, 10 or 12 gauge cables as opposed to 16 or 18 gauge.
∙cable length - the longer the cable, the higher the losses. For best performance use the shortest possible cable (i.e. don’t use a 100ft cable to connect to an amp that is only 20ft from the speakers).
∙speaker load - a 4 ohm load will have double the power loss associated with an 8 ohm load and half the damping factor.
The difference that these factors can make is significant. For example, a Numark amp with 5ft of 12 gauge cable and an 8 ohm load will have a power loss of 0.2% and a damping factor of 142. The same load with a 40ft, 10 gauge cable will have a power loss of 1% and a
damping factor of 67. Increasing the cable length to 320ft of 8 gauge, the losses are 5% and the damping factor is just 18.2.
Overload
It may seem an obvious point to make, but overloading speakers can permanently damage them, so care should be taken to select speakers capable of handling the rated power output and peak output of the amp. If the system uses a crossover, ensure that the high frequency ‘tweeter’ cabinets are not driven with signals of a lower frequency than they are designed for, either by wrongly setting the crossover, or by hum which appears in the amp or anywhere between the crossover and the amp.
Another overload which should be avoided is that of the human ear. Observe health and safety regulations where necessary regarding levels and exposure times.
Bridged Mono Operation
The amp comes factory configured for stereo operation. In this mode, each channel drives a separate speaker load with a separate signal. However, in some circumstances, it may be necessary or desirable to drive a single load at a higher level. This can be achieved using the amp in bridged mono mode.
What this essentially does is uses one signal in positive polarity through one terminal and negative through the other. The combined voltage across the terminals is then twice what it would be between a positive terminal and speaker ground (0 V). This is like two cars driving towards each other at 50mph; the relative speed between the two is actually 100mph.
Note that in this way, a load of twice the impedance can be driven at twice the power compared to stereo operation (see specifications).
In order to configure your Numark amplifier for bridged mono operation, follow the instructions listed under bridged mono, 10 in each amplifier’s description (front half of manual).
CAUTION 
The speaker outputs of the Performance Series can be high enough to constitute a shock hazard (especially in bridged mono mode). Always make speaker connections with the power off, and ensure that there are no loose strands to cause short circuits.
©1997 Numark Industries | 11 | http://www.numark.com |
