Dual Amping, Stereo, Phasing and its challenges

Most of us have seen those massive rigs of the pros. Multiple amp heads, big cabinets, 19“ racks full of magic tools and pedal boards the size of an office desk. Operating such a system often would need an education in flying a spaceship.
To get started with an easy setup consisting of 2 amps, you need: 2 amps. Easy. But how do you connect these 2 amps? There are many possibilities to send your signal to 2 amps. Maybe your delay, chorus or tuner pedal has a stereo output? Great! Try it. One output to amp A, second output to amp B. Does it work? Great! Does it hum? Thought so.

“I hum, so I am a bee!“

Since you connected the grounds of both the amps on the audio side, we created a “loop of grounds“ (the amps do have already a ground connection on/at the wall plugs). The ground connection through the third prong of the power cables and the shields of the audio cables are building a big loop, which work as an antenna for any kind of electromagnetic interference. The biggest interference is always the 50 or 60 Hz cycle hum from the net, which sounds like a deep hum.
So what could we do? A good way to start with, is the usage of a LEHLE P-SPLIT II or a LEHLE LITTLE DUAL. You can send in your signal and take two signals out. While one output is (and has to be always) grounded, the other output is the isolated one. There’s a built-in transformer, which lifts the ground and isolates the “hot“ audio signal via galvanic isolation.

“I have a stereo setup!“

I suppose you stopped reading here, because you already started jamming around in your room and kicked the other guitar player out of the band, since you can play 2 amps from now on. Playing 2 identical amps at the same time is so much fun and provides such a thick sound. Using 2 different amps can produce a way more massive sound, because the different characteristics of the preamps, power amps and speakers can complement each other. But is it stereo? No. Welcome back.
A true stereo sound can only be achieved if your 2 signals differ in:
A) level
B) time
C) pitch
D) give your ex-guitar-player a call and ask him if he wants to return

a) “These go to eleven!“

If you would stand in the middle of a big room and place one amp in the left corner and one in the right, you can produce a “stereo picture“ if you would turn down the volume of one amp (let’s say the right one) and the other to 10 (11), so your left ear gets more audio-information than your right ear.
But is this a stereo-sound we would like to have? Signal from one side only? Noooo! There are pedals called “Tremolo“ and sometimes they have a stereo output as well. These pedals are shaking, wobbling and panning the signal between your amps at your desired amount and speed.
Keep in mind that if you connect the stereo out of a pedal to the amps you again can get hum (as mentioned above). Since the P-SPLIT II doesn’t have a stereo input and output, you still can use it to isolate one side: take one output of your pedal, send it into the P-SPLIT II and take the ISO out to your amp. You have successfully isolated the amp again. If you take a look at the LITTLE DUAL or DUAL SGoS, you recognise that these two pedals have 2 ins and (at least) 2 outs. So you can send in the stereo signal coming out of your stereo pedal into the LITTLE DUAL or DUAL SGoS and send out the stereo signals to your amps.

b) “Lost in time!“

Next is time. Time is precious, I know, but sometimes shifting the time can help. And this is what a delay ……….does. It delays and can repeat the signal you send in …send in …send in. So, if your left amp would get the original signal first and the right amp the delayed signal a few milliseconds later, we speak of a stereo signal. Ping-Pong. Left-Right. If you play around with the time/speed-knob, mix-knob and turn up the feedback of your delay you can create deep sound layers, achieve crazy effects and Pink Floyd will love you. Again it’s possible that you need to isolate one amp because it hums.

c) “I’m in tune!“

Of course your instrument has to be in tune. But detuning a little bit has never hurt anyone. And of course we don’t use the tuners at your instrument but taking advantage of pedals, because we love them!
There’s a pedal called “Chorus“. Often these pedals have an input and 2 outputs. Stereo! A chorus pedal basically works like a delay: it doubles the signal, but delays the copy for a few milliseconds, pitches them up & down a few cents and merges them back together. If you play around with the knobs called “amount“, “modulation“ or “depth“ and “rate“ or “speed“, you delay and pitch the copy of your signal. By using this pedal and sending the signals to 2 amps, you really can create a wide stereo sound. If it hums, you already know what to do, because you’re clever.


If you are combining all these 3, level, time and pitch, you can create a beautiful hovering stereo sound. There are endless possibilities. But don’t use too much! The more amount of everything you are using, the more indirect your signal gets and you most likely get lost in space.

“It’s just a phase!“

Probably you have heard about “phasing issues“. So what are these issues exactly?
Your signal is made out of a positive and negative amplitude and moves forward on a time axis.

When it’s positive your speaker moves forward and when it’s negative the speaker moves backwards. This is how sound pressure is produced.
If you have two speakers (and 2 amps), of course the 2 speakers should move equally, or?

But what happens if one speaker’s moving forward and one backwards? This is when we are talking about phasing issues.
One speaker starts positive and one speaker negative. The result is cancellation. 1 + -1 = 0
Maths. Urgh.
Simply said: the air pushed by the one membrane is pulled back by the other one, so there’s no pressure reaching your ear.

But we don’t need maths for pedals, but only our hearing. Some of the LEHLE pedals have a phase switch. (P-SPLIT II, LITTLE DUAL, DUAL SGoS)
In case the signal coming out of one amp is out of phase (meaning it starts not with a positive, but a negative amplitude), you simply can press the phase button at the LEHLE pedal and the phase is flipped by 180°. Now both signals are starting in the same direction (depending if you flipped the positive or negative one).


How can we find out if 2 signals are out of phase? Truth: without a scope probably never. The good news are that with a little bit experience in hearing and the right tools I am sure you will get a sound which is satisfying. The easiest way is simply toggling the phase switch at the LEHLE pedal. And I guess you will find out in which position the signals seem to be in phase or out phase. Signals which are in phase have pressure, they are direct in your face. If signals are out of phase they sound often strange. Dizzy. I bet you will turn around your head, because your ears get confused. It often sounds as there’s a “hole“ in the sound range. If you are playing two high gain amps or having a very distorted sound it can happen that it sounds like there are no mids as well. Very scooped. ’90s metal. Toggle the phase switch again and you will hear the difference.
The bad news are: in 99% you won’t have 2 signals which are exactly 180° in or out of phase. This is a result of all the electronic stuff and simple acoustics.
 Take a look at this sketch.

Signal B is out of phase – unfortunately not 180°


Signal B is flipped by 180° – still not in phase.


If you flip the phase, the second signal is flipped by 180°. But a signal can also be shifted in time. If you’re in front of your 2 amps you should stand in an absolutely perfect triangle with your amps. Is this realisable? Probably not. Is it realisable that there’s no sound coming back from the walls or ceiling? No. And what happens if you are moving a little bit to the left? You are decreasing the distance to the left amp and increasing the distance to the right one. This means the signal of the left amp arrives earlier at your ear than the right one. So the signal is shifted on the time axis. And there are different acoustic reflections coming back from the room than before. I really recommend to try this. Move around in your room while you are playing both amps. It sounds like a… …Chorus? Flanger? Exactly. This is what basically those pedals do: Shifting the signal in time (and other crazy things) and merge it again.
I also heard about effects pedals which flip the phase if you activate them. Also within amps: clean channel sounds great with the other amp, overdrive channel is out of phase. The only chance to get this solved is to modify the pedal/amp (Good luck!) or simply use a P-SPLIT II and put it in a LITTLE LEHLE II or a D.LOOP SGoS and activate it at the right moment, so you take advantage of the LEHLE’s phase switch and flip the phase of the respective pedal/channel.
As mentioned there’s a lot electronic stuff you are using and they all do something with your signal. So it can happen that you have 2 signals which differ in their phase, but there’s a little chance they are exactly 180° flipped in their phase. You only can choose between dizzy and great, even it isn’t exactly 180° mathematically.

And I guess this is also the result of this excursion into Dual Amping, Stereo, Phasing and its challenges: trust your ears.
And don’t forget to call your ex-guitar-player!