Phase inverting, so what?

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Sonic
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Post by Sonic »

Ok I want to know what the big deal with phase inverting is, I mean, I have always heard that when building effect pedals, you should try and not invert the guitar singnal 180 degrees, but why, I'm a tech and knows that the the sound isn't affected by this, if there where two signals, it would make sense since they would phase each other out, Did I over-see something?

I know that LPB1 (or all pnp, npn one-tramsistor boosters), electra distortion, Little angel chorus ,and a lot of others pedals are phase inverting.

Ihave always build pedals with "0" degrees phase, and if it had phase inverting, I would build a buffer circuit to correct it back to "0" degrees, but I don't mind anymore since I can't see the problem in this.
So if you know about a problem I haven't heard of please let me know :)
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Hides-His-Eyes
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Post by Hides-His-Eyes »

For your own use it's no real problem. For a pedal to sell, you don't know if the end user will be using a two amp setup or something.
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Sonic
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Post by Sonic »

Hides-His-Eyes wrote:For your own use it's no real problem. For a pedal to sell, you don't know if the end user will be using a two amp setup or something.
Oh yes, didn't think of that, since I never use two amps.
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Ripdivot
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Post by Ripdivot »

I use two amps together all the time. One thing to remember is that not all amps are in phase with each other. I use a fender and a vox in stereo and I use an sho in front of the fender to get them in phase.
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Greg
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Post by Greg »

Sonic wrote:Ok I want to know what the big deal with phase inverting is, I mean, I have always heard that when building effect pedals, you should try and not invert the guitar singnal 180 degrees, but why, I'm a tech and knows that the the sound isn't affected by this, if there where two signals, it would make sense since they would phase each other out, Did I over-see something?

I know that LPB1 (or all pnp, npn one-tramsistor boosters), electra distortion, Little angel chorus ,and a lot of others pedals are phase inverting.

Ihave always build pedals with "0" degrees phase, and if it had phase inverting, I would build a buffer circuit to correct it back to "0" degrees, but I don't mind anymore since I can't see the problem in this.
So if you know about a problem I haven't heard of please let me know :)
I agree.. short of the 2 amp example I can't see that it matters.
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DrNomis
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Post by DrNomis »

Phase problems shouldn't cause much of an issue,but there are some occasions where phase issues can occur,for instance,in a recording scenario,sometimes there is what's called a "Phase-Invert" button on mixing desks which can sort these problems out.... :)


Sometimes a phasing problem can cause oscillations,particularly when a gain-stage is involved..... :)


Remember that there are two basic requirements for sustained oscillation..... :

Providing that there is a path from the system/circuit output to input...

1,Total phase shift of 0 degrees.

2,System,or circuit gain greater than unity,or 1.
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Hides-His-Eyes
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Post by Hides-His-Eyes »

So say I had a noisy circuit and my input and output traces/wires were parallel. Would I be better off leaving my pedal inverting? :)
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DrNomis
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Post by DrNomis »

Hides-His-Eyes wrote:So say I had a noisy circuit and my input and output traces/wires were parallel. Would I be better off leaving my pedal inverting? :)

Depends on how close together the input/output wires are,if they are very close together,there's a risk of the input picking up the signal from the output through capacitive coupling effects,note that the capacitance between the two wires would be very small,probably a few pFs,but it may be enough if the input happens to be a high impedance,personally,I would try and keep the input and output as far apart as physically possible to prevent the output being capacitively coupled into the input.... :)

Otherwise the circuit would have a tendency to oscillate at very high frequencies which our ears can't detect,the only way you could tell is that the circuit sounds unusually "dirty" since all the headroom would be taken up by the high-frequency oscillations.... :)


This actually caused me alot of headaches,trying to sort out a high-frequency oscillation issue in a pedal that I had designed and built about a year or so ago.... :)
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Post by Hides-His-Eyes »

Ah, but if the output's inverted, isn't that negative feedback? So there'd be no runaway at all?
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Post by JakeAC5253 »

To make matters worse, I've seen it happen where the amps themselves caused phase problems due to differing signal paths. I tried running a stereo rig once, tube amp next to a pre/power amp into same cab wired for stereo. Phase issues, flipped the phase on one of them 180º and there were still phase issues. Turns out it was the cable I was running between the pre and power units. And that isn't a quick and clean 180º flip either :?
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