# Pedals That Invert Phase



## Guest (Nov 22, 2011)

I read this thread about phase inversion, and I was hoping someone could shed some light on it for me.

http://www.thegearpage.net/board/showthread.php?t=796444


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## mhammer (Nov 30, 2007)

Phase inversion is only a problem if there are two or more "versions" of your signal running concurrently that are at risk of cancelling each other. 

Any individual pedal can end up producing an output which is inverted, relative to the input, without ill effect. Indeed, sometimes the design simplicity, and occasionally the deliberate "loading" effects, can justify a circuit that ends up inverting, such that the designer may elect to use an op-amp in inverting mode rather than non-inverting. For example, the MI Audio Crunch Box, discussed in another recent thread, uses two gain stages in "inverting mode" because that makes it possible for one control to simultaneously increase the gain in two stages, something you couldn't do with non-inverting stages. Inverting stages will have a lower input impedance, so you'll sacrifice a bit of high-end clarity, but since it is a distortion circuit, you'd actually _like_ to lose a bit of top end before the gain gets applied. As it happens in that particular instance, the "algebra" of inversion works to produce an output that is in-phase with the input.

If two signal paths are combined, and they are inverted, relative to each other at the point of combination, there will be cancellation of anything that is in common. So, let us say that I use a splitter and feed two sequences of pedals simultaneously. Path A consists of 3 pedals, and path B also consists of 3 pedals. If A consists of non-inverting, non-inverting, inverting, and B consists of all non-inverting pedals, then the two will cancel out at the mixing node. If B consisted of any combination that resulted in inversion at the end (same as A or invert+invert+invert), though, the two paths would be in-phase _with each other_ and no cancellation will result.

Think of it like speakers. If BOTH your speakers had flipped wiring so that hot went to ground and ground to hot, the speakers would both push when they should pull, but you wouldn't hear any cancellation or discrepancy between them. Of course, if you had something like a Bassman or Super Reverb or a Marshall 4 x 12 cab, and one of the 4 speakers was wired opposite phase to the others, you would hear a thin-ness to the sound.

There can be some unusual cancellations from "stereo" pedals. Some companies will produce a "stereo" output in modulation pedals by using sum/difference signals. So, a stereo chorus or phaser or flanger might add the phase-shifted or time-delayed signal to clean for one output, and subtract the modulated signal from clean for the other output. While those two outputs will be different, the problem is that the peaks and notches produced at each output are the mirror opposite of each other, such that if you plug into two amps, those signals cancel each other out "in air". The pedal itself is ostensibly "in-phase" with its input, since there is no inversion of the clean signal, but the intended stereo effect gets lost because of other means of cancellation.

The bottom line is that cancellation is the main issue in question. Sometimes it can be used productively, and sometimes it is a hindrance to one's tonal objectives. Being aware on inversion anywhere along the signal path is a good idea. That starts with wiring up your pickups properly, includes phase relationships in your pedalboard, but also includes any potential phase changes in multiple amp and multiple speaker setups. Having absolutely everything in-phase, up to the amp, and "doing a Jimi" by running a cable from one of the unused input jacks of your Marshall JTM45 to another JTM45, and wiring each of their respective cabs opposite phase to each other, will still get you cancellation in air.

make sense?


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## Guest (Nov 23, 2011)

Thank you Mark. In the linked thread they talk about mono setups being negatively affected by phase inverted pedals.


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## mhammer (Nov 30, 2007)

I honestly can't see any reason whatsoever why simple phase-inversion in a mono path would have any effect. Before attributing any stupidity to the writer, though, it may have simply been poorly explained. That happens.

It certainly didn't happen in the post from senior member cirrus, who said: "_Why does it matter? If you've got one signal chain going into one amp, it doesn't, because our ears are not sensitive to polarity: we'll hear the same sound either way.

But if you split your signal into two or more different amps, it becomes a problem if one amp is getting a positive signal and the other a negative one - the two amps end up fighting each other and certain frequencies can be cancelled out. If they're close together you'll hear a reduction in bass, if they're far apart you'll get a strange, hollow sound. You might have heard it before if you've ever wired up a hi fi incorrectly_."


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## Guest (Nov 23, 2011)

TGP is good for a laugh some times.


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## zurn (Oct 21, 2009)

I use a stereo setup, I noticed the the Dry and Wet outputs on a DDM are out of phase, so the "stereo" effect isn't really great. I had to use a Lehle dual to correct the phase. The problem with that is my other stereo effects in the chain that have a correct phase are now reversed...


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