April 2010 \ Tech Tips \ Tech Tales \ Getting Back In-Phase

Getting Back In-Phase

Chad Weaver

What's in-phase and out-of-phase, and how do you get your amps back in-phase? Chad explains.
Premier Guitar April 2010
When I was 15 years old, I tried using two amplifiers at the same time. There was tons o' noise and it had a honky sound to it. In the years after my ill-fated attempt at a multi-amp setup, I learned a bit more about how two amps can be in phase or out of phase with each other when used at the same time. I'm sure you've heard these terms before, but if you haven't, or if you're a little sketchy on the subject, I’ll go over them quickly before giving you a few tricks to make it easier next time you want use a two-amp rig. $0 $0 Some Background$0 When an amp is in phase, the speakers will push—literally. I'm sure you've watched your speakers when you’re playing and seen them move back and forth. The first tone that is emitted through them will make the speaker basket push toward the grill cloth of the speaker cabinet. That is in phase. When the speaker is out of phase, the first tone will make it draw back toward the speaker's magnet. $0 $0 The reason an amp pushes or pulls is that every gain stage your amp has will flip the phase of the signal 180 degrees. For example, the process might start in the 12AX7 tubes in the preamp section of your amplifier. Unlike a power tube, every 12AX7 has two sides, or triodes, to it. It's basically like having two tubes inside that little glass bottle. On a blackface Fender, the normal channel will use both sides of the 12AX7 in the V1 position (tubes are numbered V1, V2, etc, beginning closest to the input jack). This gives you two gain stages. On a plexi Marshall, the normal channel will use one side of V1 and both sides of V2 for a total of three gain stages, which of course is 180 degrees, or out of phase with the Fender. That's the basic idea, and it’s enough to cover what we're trying to achieve here. $0 $0 In the past with Brad I've used up to six amps in the same rig, with anywhere from two to four on at the same time. Getting all of them in phase with each other in their various combinations is a trick to say the least. When amp 1 is in phase with amp 2, it might not be when used with amp 3, but 3 has to also be in phase when it's turned on with amp 4. Confusing? Yes, especially when any of those amps could be switched out from day to day. But, there's no need to break out the physics books here—we’ll just be discussing a quick reference, so you can tech your own rig if you decide you want to run multiple amps. $0 $0 How you split your signal between the two amplifiers is up to you. Keeley's Framptone amp switcher works great. Axess Electronics’ BS-2 buffer/switcher is another good one. Both are a single input with dual outs, and have the ability to flip the phase on one of the outputs. There are tons of these on the market, so finding one is really easy. Of course, any stereo output pedal works as well. $0 $0 Getting In-Phase$0 I tend to check my speaker cabinets first, so a quick test can be done with a 9V battery and a speaker cable. Plug your cable into the cabinet’s input and touch the 9V to the cable's 1/4" end—ground to the sleeve and hot to the tip. You'll hear a pop and see the speakers move. Just like when a guitar is played through them, if the speaker cabinet is in phase, the speaker (or speakers) will push toward the grill cloth. This is what we want—all speakers moving the same direction all the time. $0 $0 When you play your guitar between the two amps you may hear a hollow type of sound between them. This happens when the speakers are moving in opposites directions. It's almost like there's a hole there and the tone will have an excess amount of midrange to it. If you're using a pedal like the Framptone, you should be able to cure the problem with a flip of the phase switch. If you’re using a stereo effects pedal, you'll have to flip-flop the speaker wires coming out of one of the amps. Just switch the hot and the ground wires on the speaker terminals. If your amp has two speakers in it, you'll want to switch only the two wires that are coming directly from the amp. Remember, there are dangerous high voltages inside your amp, so if you do not feel comfortable, consult a technician. $0 $0 Turning on certain compressor pedals will also flip the phase. Just like the amps, the compressor has a gain stage, so the signal will be turned 180 as it passes through. Not all compressors on the market today do this—some specifically state that they won’t change the phase. If you have one that does, try putting it on just the out-of-phase amp. It's kind of a cool effect to compress one amp in a two-amp rig to give a little more variance between them, so using one in this application yields two results—three if you count not having to rewire your cabinet. $0 $0 One final note for using a two-amp rig: sometimes ground noise can be an issue when using several pieces of gear. A couple of ground lift adapters on hand should handle anything that might pop up. Take your time and lift each amp and your pedalboard, one at a time, until they're all quiet.

     

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Comments

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UsernameComment
CapnZilog
on 05/20/2010 		Graeme, if you're thinking about phase in the EE sense, then you're right. But in the audio world "flipping the phase" is really not about flipping phase at all (indeed, that would have to be frequency dependent) - it's about inverting adio polarity. And in that case, the author is correct. The popular nomenclature, however, is murky just like the way Leo Fender called his whammy bar "tremolo" when it's technically vibrato.
john bohlinger
on 03/27/2010 		Thanks Chad. I gave up on two amps years ago because of phasing issues. I'm inspired to try it again.
DR.Z
on 03/23/2010 		Great article Chad.
Made me realize about pedals inverting inputs to amps.
I normally concentrate on amps mostly not thinking about there inputs.
I don't think you simplified the subject as implied by earlier comment. Multiple amp setups can have a great sonic presence, but can also cause a subtle to extreme loss in tone. This can be controled easier in the studio , but can be a PIA in a live venue.
Through out the years the use Ground Lifts as a troubleshooting tool to solve rack hum problems has been standard procedure. To date not one death has has been reported to this technique.
Tom Wellbrook
on 03/23/2010 		The internet never stops amazing me. No good deed ever goes unpunished it seems. Phase inverter? Phase splitter? Yes, these generally produce the maxima and minima of the output waveform but this sinewave can be in phase or out of phase with another wave from a second source. It has nothing to do with what Chad is talking about in the phase of two amps. As far as frequency - ok Mr. lack-o-data or perhaps Mr. too-deep-in-thought ... Frequency does not cause 180 degree swings but think about the slight phase shift that a tone control pot does to a signal coming into the pot and the associated loss. One reason the Bible of Rock makes the point ... 'if your amp has more than six knobs you are one of them'. One problem with the internet ... too many people are there to argue when they are armed with too little real education or are too educated. Thank you Chad for a well written article which will be helpful to many folks that spend a few seconds to implement your advice rather than complain and argue. As a side note from personal experience, most of the detractors here only have one amp anyway :)
Ron Baker
on 03/23/2010 		Hello Chad, I have a phasing problem when i use the drive channel on my Lonestar in a 2 amp rig.In the clean channel it uses 3 stages.(In phase with my other amps)When I use the Drive channel,it adds 1 more gain stage and I lose all low end.The rectos use 3 stages clean and 5 stages drive and do not have this problem.This is a very common problem. Thanks for bringing it up.I remember you atJJ's.
Pete Dixon
on 03/22/2010 		How I long for the days of grinding out ear-splitting riffs in the confines of the dark, damp basement whilst chaining every amplifier in my possession to random assortments of speaker cabs, homemade and otherwise. I'm sure the concept of "phasing" never entered my mind. Thanks for the tips! GREAT series and be safe out there!!
Greaeme
on 03/21/2010 		Sorry - love most of your articles but this one is appalling.

You imply that 2 amps alongside each other need to be in phase - agree, but be aware that phase varies with frequency. Low notes might be on one phase which varies to opposite phase for higher string harmonics. Even identical amps with different tone settings will not have all frequencies in phase. Also, should you switch to an overdrive channel on one amp (or both) there's no guarantee this won't flip the phase. It's just not as simple as you make it sound.

Further, you suggest that because a pedal has a gain stage that it flips the phase - completely wrong - it might, it might flip it twice (back in phase) or it might deliver gain without phase change. You do acknowledge this might be the case, so it's all really quite irrelevant.

Finally you talk about lifting the ground on amplifiers. This is not the right way to solve earth loop problems and quite frankly it's dangerous and removes double safety protection - a single fault could be fatal. Don't do this.
zaphod_phil
on 03/20/2010 		Bruce Clement, instead of lifting the ground of one amp, it's safer to use an A/B/Y or splitter pedal that has ground isolation.
zaphod_phil
on 03/20/2010 		@confused, the phase invertor (also called a phase splitter) splits out two signals which are in opposite phase to each other. These are amplified in a push-pull power amp and recombined to the same phase. To make things more confusing several amp companies, especially Vox, Matchless and Bad Cat, falsely call their Class AB amps, "Class A". At the end of the day, there's no point worrying how many stages your amp has, what class it is, and so on. If a pair of amps sound wrong when you play them together from a single guitar, then you need to flip the speaker connections of one of them.
confused
on 03/20/2010 		I'm confused; don't class AB amps like Marshall Plexis and most Fenders have phase inverters that sends a signal 180 degrees out of phase with the original sound? Wouldn't this make the phase of the previous tubes irrelevant?



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