Dual Source - EQ Conflicts?

[AndyC]

In a dual source system, it would seem to make sense that if you had seperate EQ control over each source that should be a good thing. (using something like a Grace Design Felix for example).

But what about when you blend the sources into one? Is there then a possible issue with phase cancellation affecting the sound? Or would the phase invert ability on most preamps resolve this problem?

I've heard an argument that says it is better to blend the two signals and then apply EQ to the single blended output.

Comments appreciated.

[tadmania]

No, phase cancellation will not be a problem when EQ is applied discreetly to each channel. After all, EQ is merely a set of incremental volume knobs, right? The way I approach this (I have two Headway preamps) is to 'assemble' the overall sound from what each source does best and blend them post-EQ to a mono output. I can easily switch to true stereo when need arises, but that is rare.

[guitaniac]

Phase cancellation can definitely affect the blend, so its best to be able to reverse the phase of one of the signals to see if that improves (or worsens) the blend.

In past discussions its even been mentioned that there are devices which can shift the phase of a signal to any needed degree in order top achieve the best possible blend.

At the very least, it helps to have the capability to invert the phase of one of the source signals. Its best to try both phase positions to see which position yields the best result.

Its interesting to note that even the Fishman Aura Spectrum preamp is a blender of sorts (blending the processed signal with the dry signal) and that the processed signal's phase should be inverted (as a test) to determine which phase position yields the best result.

My Zoom A3 has a phase inversion capability on the mic channel (which can also be used for an active pickup system), and even the inexpensive iRig Acoustic Stage preamp, which has an Aux In and can be used as a blender, has a phase inversion switch on the Aux In to accommodate for a better blend. (I'm told that the phase inversion capability applies to the MEMS mic if the Aux In input is unused.)


I hadn't heard the argument that its better to EQ post blending. The conventional wisdom seems to be that its best to EQ the individual signals before blending. If one of the sources tends to be more feedback prone, its often preferable to get most of the low end from the other source.

[tadmania]

Don't misunderstand the phase of input signals with that of sound waves. It is common to selectively reverse input phase settings to achieve certain results, but it doesn't cancel out signal from other sources in the same mixer/preamp.

[guitaniac]

Originally Posted by AndyC

In a dual source system, it would seem to make sense that if you had seperate EQ control over each source that should be a good thing. (using something like a Grace Design Felix for example).

But what about when you blend the sources into one? Is there then a possible issue with phase cancellation affecting the sound? Or would the phase invert ability on most preamps resolve this problem?

I've heard an argument that says it is better to blend the two signals and then apply EQ to the single blended output.

Comments appreciated.

Perhaps I'm not understanding this question correctly. A post blend phase inversion is sometimes useful to get the blended signal less in phase (to inhibit feedback) or more in phase (for a fuller sound) with the signal generated by the guitar top's reaction to speaker sound. A phase inversion (of one of the two source signals) needs to be pre-blend when testing for which phase position will yield the best possible blend.

[guitaniac]

Perhaps I should have elaborated more with the Aura Spectrum preamp example. In addition to the phase inversion function for the processed signal by itself (sometimes needed for a better blend of processed and dry signals), there's also a "global" phase inversion switch which affects the full output signal.

This global inversion switch can be used to either a) ensure that the guitar signal generated by playing is more in phase with the guitar signal generated by speaker sound interacting with the guitar top (to get the fullest sound) or b) to ensure that the guitar signal generated by playing is more out-of-phase with the guitar signal generated by the interaction of speaker sound with the guitar top (to inhibit feedback).

Sometimes the phase switch position won't have much effect in a live sound situation. Its always a good idea, however, to try both positions if you're having feedback issues or your amplified guitar sound seems unusually thin.

It should also be mentioned that changing your position on stage may change the phase relationship between the signal generated by playing and the signal generated by the guitar top's interaction with speaker sound.

[sdelsolray]

All EQs (except phase linear EQs) use phase shift to do what they do...increase or decrease certain frequencies. It is debatable whether you or I can hear phase shifted artifacts generated solely from EQ filters.

Blending two sources can create its own set of phase-shifted artifacts, the most common being the comb filtering effect. If there are these artifacts when blending, use of EQ might enhance them, minimize them or have no effect at all.

[perttime]

There's players who successfully put two or three signals through separate EQs, and then combine. So, clearly it can work.

[ljguitar]

Originally Posted by AndyC

In a dual source system, it would seem to make sense that if you had seperate EQ control over each source that should be a good thing. (using something like a Grace Design Felix for example).

But what about when you blend the sources into one? Is there then a possible issue with phase cancellation affecting the sound? Or would the phase invert ability on most preamps resolve this problem?

I've heard an argument that says it is better to blend the two signals and then apply EQ to the single blended output.

Comments appreciated.

Hi AndyC

Never had that issue, and I've played 2 fairly high quality dual-source rigs for the past 18 years (both with internal mic and pickup). The pickup, and the internal microphone on my rigs have different specialties and limitations. My preamp has 2 channels - each with volume and EQ controls - for a reason.

I set the EQ on the pickup to handle low end and cut back the all-too-prone mids. I set the internal mic EQ to produce clean low-mids through the highs. That way I can fill-in the weakness of the pickup with the mic. And I can correct the tendency for bass on an internal mic to go boomy by cutting the bass end of the mic. Then the combined output is tonally balanced again for the amp/PA for the room/stage (and my pleasure).

I have fired up just the pickup for people - which sounds very nice on it's own. And then I dial in the mic…and players go "OOoooo…that's better." It's actually more natural and more guitar-like (which is the aim).

I want my pickup/preamp rig to make my guitar sound more like a really nice guitar than like a really nice guitar played through an amp/PA.

With my rigs the only time I reverse phase is when I put in a big-black-soundhole-plug. It evidently physically throws the mic out of phase (because the mic is ¾" below the level of the soundhole pointing out the soundhole) with the mic. Reversing phase on the mic restores it to the original tone/sound.

[tadmania]

Ah, I was hoping you would chime in, sir. That is my experience, as well.

[guitaniac]

Yes, blending can be more satisfying than using just one pickup. Its pretty much "do the best you can" with respect to phase issues, however. I occasionally see someone complain about phase issues with their dual source, and I suspect that my own blended source results would be a little better with perfect phase compatibility.

I've gotten my most satisfying steel string guitar results with a PUTW I/O UST & Baggs Lyric combo (no EQ on UST, considerable EQ on the Lyric, no phase inversion). A Baggs Hex & MiniFlex2Mic combo (minimal EQ, no phase inversion) works very well in a classical. On the downside, I've never managed a very satisfying blend with the Headway UST & Baggs iBeam in my old D28.

Most recently, I've been using a Zoom A3 to do the blending.

[Andy Howell]

Blessing an outboard mic and pickup with the Headway this has never been an issue for me. Blending two on board pickups may well need dual eq. Depends what you are up to!


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[AndyC]

Guys, thank you so much for all of the extremely helpful comments

[guitarwebguy]

This is why I have and use a Grace Felix, the ability to control the blend and tweak the EQ on each of the the inputs as needed .....