Like everything with Disco Equipment there are Pro's and con's to this idea. Whilst on the surface it is a good idea to use more than one amplifier this is usually only done with larger sound systems and medium to high wattage amps.
For instance there is little to be gained from using two 200W + 200W amps driving 8 Ohm speakers, you may as well invest in a 2x 400W or 2x 500W amp to drive all 4 speakers. Just bear in mind that an Amp rated at 200W into 4 ohms per channel will actually only deliver around 125W when used with one pair of 8 ohm speakers!.
Usually those who have more than one amplifier are using their systems in Bi or Tri mode. This means using an amplifier for the bass bins, another amp for the mid range, and a smaller amp for the treble horns. Obviously these are the combinations that are used in high power systems, and there is no reason why any D.J should be investing in this level of hardware - unless you do large outdoor venues or very large events - this set up is usually more favoured by bands.
Most D.J's tend to use one medium or high power amplifier (Rated 250 - 1000W per channel) with good quality speakers and you then have the flexibility for using one or two pairs of loudspeakers depending on the size of venue. Rather than carrying four speakers around with you and two amps! - I mean that would be overkill for a Cricket club with say 30 guests and only 5ft of space to set up in!.
Yes, it is a good idea to always carry an extra (Spare) amplifier in the van, since faults can and do occur. Don't spend a fortune on this back up amp - a reliable second hand 150W + 150W Amp will usually see you through the night(s), until you get your main amp repaired, and it's a lot more professional then not being able to finish the gig
. The average punter will possibly not even notice that you've swapped your 250 + 250W amp for your 150W + 150W back up if you do have problems during the night
If you do still decide to "slave" two amps from the output of your mixer then the easiest way is to use the Slave In / Out sockets on the Amplifier. These are usually two of a similar type connector (usually 1/4" mono jack plugs) mounted next to each other and marked "link" or "Slave in / Out".
If your amplifier doesn't have this type of facility then you can use the record out on your mixer, although these can sometimes be a slightly lower level than the equivalent output of the master output.
Failing that, you need to split the output of both the "L" and "R" single outputs of your mixer. AVOID the use of the plug in type splitters which are bulky since the added weight of the connector and leads pulling down on the splitter can pull away the Phono connector from the pcb of the mixer resulting in loss or intermittant crackling through the P.A
A suitable Splitter lead can be obtain from MPS on (0870) 264 6000 Part number QN26D, and you'll need 2 - one for the left channel and one for the right. They are not cheap at £4.99 each but they are better than using the cheaper type splitter and risking damage to your mixer.
