Trying to increase GBF with channel EQ

[WynnD]

Sounds like a bad idea to me, but if you did, would you try this before ringing out the system? That sounds like a second mistake to me. I would rather keep the channel EQ for adjusting tonal quality of the mic.

[agedhorse]

Since feedback occurs based on peak deviations in closed loop frequency response (and at points where the phase is close to zero), decreasing the amplitude of these peaks will increase the gbf.

 

The rule of thumb is to improve acoustics that may be influencing gbf first, then narrow band eq, then lastly wider band eq (channel eq).

[Audiopile]

Your question brings up possibly an interesting topic for conversation, being the underlying problem exhibited by the symptom.

Increasing GBF?

Why the need?

I'll suggest one or more likely reasons:

1)  Low input level at the source/mics.

2)  Incompatible positioning of the speakers in relationship to the open mics.

3)  Spikes (unevenness) in microphone or speaker response.

4)  Venue acoustics.

5)  High SPL requirements (generally due to high stage volume).

6)  Quantity of open mics.

7)  FX and/or processing that compromise the GBF.

8)  Phase issues.

This past weekend I dealt with a serious GBF problem that was a self fulfilling prophesy, being that the GBF symptom existed and quickly escalated due to the GBF problem that existed.  I resolved the problem, to the order of gaining about 40dB addition headroom, with a few 9v batteries... and no twisting of knobs.

[Gregidon]

Mark,

 

Tell us more, how did you use 9V batteries to eliminate the problem?

 

Greg

[dboomer]

 

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Audiopile wrote:

 

 

 

 

This past weekend I dealt with a serious GBF problem that was a self fulfilling prophesy, being that the GBF symptom existed and quickly escalated due to the GBF problem that existed.  I resolved the problem, to the order of gaining about 40dB addition headroom, with a few 9v batteries... and no twisting of knobs.

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OK  ... I'm all ears.  Since the threshold for feedback is fixed (at least until you move something) that would imply that you were operating at least 40 dB below that threshold to begin with.

[agedhorse]

Batteries for in-ear monitoring receivers?

[RoadRanger]

I actually have a cheapie Nady wireless mic where it's gain goes WAY up just before the 9V battery dies entirely .

[Bobby1Note]

 

 

Trying to increase GBF with channel EQ

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WynnD wrote:

 

Sounds like a bad idea to me

, but if you did, would you try this before ringing out the system? That sounds like a second mistake to me. I would rather keep the channel EQ for adjusting tonal quality of the mic.

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Depends on the mixer;,,,, a good multi-band EQ section, and you can take care of the problem right at the source, rather than apply a global cut, and still have EQ left over for tonal changes. I'm thinking mostly along the lines of acoustic instruments (guitars, etc) that are predisposed to..standing waves within the intrument cavity. Not all of them can use sound-plugs.

[Reson8tor]

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Bobby1Note wrote:

 

Depends on the mixer;,,,, a good multi-band EQ section, and you can take care of the problem right at the source, rather than apply a global cut, and still have EQ left over for tonal changes. I'm thinking mostly along the lines of acoustic instruments (guitars, etc) that are predisposed to..standing waves within the intrument cavity. Not all of them can use sound-plugs.

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Exactly this. Even the little compact A&H and Soundcraft mixers I use have a sweep midrange EQ (semi-parametric), and I'll use that on things like acoustic guitar. Most guitars will have a body/airmass resonance somewhere around 100 -110 Hz, so I'll dial in a cut there. I still have low and high EQ, if needed, for a little tone-shaping. With good quality instruments and mics, I never need to do much tone-shaping at the board anyway.

The usual guidelines for setting up mic/speaker distance and angles still apply for avoiding feedback, but "pre notching" trouble spots with channel EQ gives you a bit of a headstart with maximiizng your GBF headroom. I can't think of any reason not to do it, as long as that particular EQ band isn't needed for drastic tone-shaping.

 

[ChiroVette]

 

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Bobby1Note wrote:

 

Trying to increase GBF with channel EQ

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WynnD wrote:

 

Sounds like a bad idea to me

, but if you did, would you try this before ringing out the system? That sounds like a second mistake to me. I would rather keep the channel EQ for adjusting tonal quality of the mic.

---

 

 

Depends on the mixer;,,,, a good multi-band EQ section, and you can take care of the problem right at the source, rather than apply a global cut, and still have EQ left over for tonal changes. I'm thinking mostly along the lines of acoustic instruments (guitars, etc) that are predisposed to..standing waves within the intrument cavity. Not all of them can use sound-plugs.

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Yeah, I have been finding in my own limited experience that there is a point of diminishing returns when you start ringing out the front end, and as you said, it is better to address problem frequencies (feedback) on the channel strip than globally. I used to try to get as much headroom as humanly possible from my front end, and then found that at some point, yeah you can make your system (even my old MR835 + Crown Powerbase 2 amp) loud as f***, but it came at the expense of cutting a ton of frequencies out of the mix, many of which are important frequencies for the overall sound and mix.

 

I am finding, for some reason, that my new PA likes to feed back a lot more than my old one, so I have to get used to that. I feel like I have far less GBF now than I did, say six months ago, but I am sure that the reason is that the new system is, in actuality, much louder than my old one.

But what I love about my Mackie DL1608, so far, is that it has four Parametric EQ bands and all of the bands are completely movable and all have very variable Q settings, with the top and bottom having both shelf and bell.

 

One of the things I found, experientially, is that my own vocal mic (an EV N/D 767) is usually the worst culprit, both in the monitors and the mains. I have also found that the frequencies this mic just LOVES to feedback on, even at moderate and not high volumes, are about 1.8K and 300 Hz. So I can cut these frequencies on my vocal mic channel without the need to carve up a global monitor mix or the FOH mix.

 

This is, by the way, irrespective of the room, speaker placement, or even outdoors.