I WANT TO RUN THREE COMBO AMPS at the same time à la Stevie Ray Vaughan. What should I know to do this correctly?
—JH via email

While it is possible to get a great sound with multiple amplifiers, if done incorrectly, a multiple amp setup could sound worse than using only one amp. The essence of using two or more amps simultaneously is to have all the speakers in the system moving the same way at the same time. To produce sound, the speaker has a cone that moves back and forth. With multiple speakers, if one speaker is moving forward and compressing the air in front of it while a different speaker is moving backwards thus causing a small vacuum, then the vacuum created by one will be filled by the extra pressure from the other and neither speaker will make any significant sound for the listener. This condition of little or no sound is called “phase cancellation.” In this case, the speakers are fighting each other and said to be out of phase.

To further complicate matters, some amplifiers play “forward,” while others play “backward.” Take for example a Fender 5F6A Bassman. If you put a positive signal on the input of that amp, then a negative signal will appear on the speaker and therefore the amp is said to play backward. On the other hand, a blackface Fender with two channels has one channel that plays forward and another channel that plays backward.

First let’s check the speakers within each particular amplifier. Unplug the speakers from the amp or use the two wires that connect the speaker array to the amp. Using a 9-volt battery, touch the tip and sleeve of the 1/4" male plug that connects to the speakers (Fig. 1). You will hear a fairly loud pop, and the cone of each speaker will either move forward or backward. The key here is to have all the speakers that connect to that amp in phase with each other. Using a 4X10 Super Reverb amp as an example, I would look at each speaker while touching the battery to the 1/4" speaker plug. If I check and see all but one speaker moving the same direction, I would swap the two leads going to the offending speaker. Once you are sure all the speakers in that particular amp are moving together, check every amp you will be using to make sure the speakers inside the cabinet are moving together.

Next, we want to make sure the amplifiers’ speakers are moving together with each other. Start out by setting two combo amplifiers side-by-side and facing forward. Stand in front of the amps and play them together and listen carefully. Do not use any effects. Pay particular attention to the low notes because if the speakers are not in phase, you will hear it more easily in the lower frequencies.

Now, turn one amplifier around so it is facing backwards. Play the amps and listen carefully, especially to the low notes.

Notice that with one configuration, the amps sound thick and huge, and with the opposite configuration the lows were thin and weak. If facing both speakers forward sounds the best, you are good for now and ready to test a third amp. If facing one backwards and one forward sounds best, then the wires coming from the amplifier to the speaker array must be swapped on one of the amps. Before you decide which amp to swap, check a third amp with one of these two. From this you decide which amplifier didn’t match the other two and simply swap the leads going from the amplifier to the speaker array. Now each amplifier has its speakers moving together and all three amps are set where they are in phase with each other.

Warning! Shock hazards and/or annoying hum can be created by ground loops that may occur when you have multiple ground connections between the chassis of two or more pieces of electrical equipment.

Every amplifier or effects processor has the grounded side of its input jack connected directly to the device’s chassis. And also connected to the chassis is the AC cord’s third-pin ground. To prevent a ground loop, one of these two grounds from each device must be defeated. The best way to do this is to make special patch cords that have the shielding snipped on one end of each cable to use with any piece of gear that already has a third-pin AC plug ground (Fig. 2). You wouldn’t use this type of cable going from your guitar to the amp, but only on the connections to other amps or devices that have an AC cord ground. This is absolutely the safest way with respect to shock risk, although it can be inconvenient, because once you clip the ground lead on a cable, it can’t be used for any other application.