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Phase Me, Bro

July 1, 2009
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0.00gp0709_shop9923MY GUITAR PLAYER HAS BEEN JUMPERING THE INPUTS OF HIS blackface Fender Super Reverb by plugging his guitar into the Vibrato channel’s first input and then putting a patch cord from the second input to the Normal channel’s first input. Then to compound the situation, he runs a patch cord from the Normal channel’s second input to a tuner. Does all this cause any damage or reduction in performance?

I don’t think he will do any damage to his amplifier, but I doubt he’ll be winning any awards for thick tone either. To understand why, we must have a short discussion about phase relationships. A tube amplifier such as a Super Reverb has a preamp consisting of mostly 12AX7 preamp tubes. Depending on the number of gain stages found in each channel of the preamp section, the preamp’s output signal will either be in phase or outof- phase with the input. On a preamp with an even number of gain stages—such as the Normal channel of a Super Reverb— when the input receives a positive signal voltage, the amplified output is also a positive voltage. Conversely, on that same channel, when a negative signal voltage is applied to the input, the preamp’s output is a negative voltage—again much larger than the input, but still negative.

However, the Vibrato channel of the Super Reverb (as well as many other Fender amplifiers) has an extra gain stage that is used to “mix” the reverb. So with three stages of gain, when a positive signal is applied to the input of the channel, a negative amplified voltage comes out of the preamp. And when a negative signal voltage is applied, a positive output voltage occurs. Now, let’s say a positive signal voltage is sent to both channels simultaneously. The Normal channel’s preamp section puts out a positive amplified voltage while the Vibrato channel’s preamp section puts out a negative voltage. These two signal voltages are opposite and will cancel each other when both signals arrive in the power section of the amplifier. You will hear this as a loss of volume, and especially a loss of bottom-end. And when you vary the two volume controls, the phase of one channel will overwhelm the other channel, creating a thin, out-of-phase sound. Some amps, such as the earlier Marshalls, have the same number of gain stages in each channel and can be jumpered for an improvement in tone. Just remember, though, that when you jumper the inputs, the signal actually driving the amplifier is cut in half, resulting in slightly less gain per channel.

Phase is very important also when hooking up multiple amplifiers. When using two or more amplifiers, care must be taken so that all of the speakers are moving the same way at the same time. An easy test is to listen the sound of two amplifiers side by side. Then turn one of the amplifiers around facing backwards, but still side by side. Play and listen again to determine which way sounds better. If it turns out that the sound improves with one amp facing backwards, you can change the phase in one of the amps (it doesn’t matter which one) by reversing the leads that connect to the speakers. This will ensure that you’re getting the fullest sound when you face both amps forward onstage. —Gerald Weber, Kendrick Amplifiers

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