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[Jon McElvain]

I am the head of the AV Crew at my church and we're looking to add a feedback eliminator to our modest gear.  We're currently looking at the dbx DriveRack PA+ that we can get for under $500 that also includes an auto EQ feature.  Anybody have any experience with this unit or other suggestions?

For background, we have a pretty simple setup.  We recently rewired the main speakers, added an amplifier, and adjusted the gain structure from the microphones to the amplifiers.  The results were even better than we could have anticipated.  Now the speakers actually have some bass response to the point that I had to dial it back on the board.  A big service for us would be six microphones for singers and a couple of guitars, the most common setup is just four singers.

Any advice would be appreciated.  The AV Crew is mostly made up of engineers, but none of us have any previous AV experience so are learning as we go.  I think we've made great strides in the past few months, but there's always more to learn.

[Corey Scogin]

Don't use a feedback eliminator or the auto EQ feature on the Driverack.  They tend to cause more problems.

I assume you are having feedback issues with the vocal mics.

A 31-band EQ inserted on the mains or monitor bus (1 eq channel for each bus channel of course) will allow you cut the offending frequencies and increase the gain-before-feedback of your vocal mics. That same EQ can also be used to adjust the overall EQ of the system if needed to dial back the bass you mentioned.  A Driverack will work just as well or better for that but manually adjust it, don't use the Auto-EQ feature.  If using the DriveRack, you may want to use the PEQ's instead of the GEQ's.

[Ivan Beaver]

feedback eliminators work best when inserted on a troublesome channel-NOT on the whole mix.

When they are on the whole mix-they will affect the freq response of everything-NOT just the mic causing the issue.

Different mics in different places will feedback at different freq-so they will all get affected in a neg way when on the mains.

Now if there is only ONE source of sound (say the pulpit mic) and no other inputs into the system (mics or line signals) then you can use it on the mains.

But otherwise it is advisable not to.

[Steve M Smith]

I agree with Corey.  For a fixed installation, use a 31 band equaliser on each output and tune it by ear (do a search for ringing out).

The resonance of the room isn't going to change unless you do some drastic rennovations.


Steve.

[g'bye, Dick Rees]

I agree with Corey.  For a fixed installation, use a 31 band equaliser on each output and tune it by ear (do a search for ringing out).

The resonance of the room isn't going to change unless you do some drastic rennovations.


Steve.

Agree as well.

The problem lies in the expectations raised by the term "feedback exterminator".  The proper way to approach the "problem" is to  properly set up the system in the first place such that you have the maximum headroom from the system within the room/environment. 

A more proper way to think of the use for such devices should be "automatic parametric EQ".  By considering it as an EQ rather than an exterminator, it should be much clearer that any issues of headroom, GBF and/or system tuning and deployment should be done BEFORE feedback becomes a problem.

FBX units do have their uses, but in order for them to be of practical use one should understand why and how feedback occurs and how to avoid it with the "ounce of prevention" rather than trying to address it with a "pound of cure".       

[Ivan Beaver]

Agree as well.

The problem lies in the expectations raised by the term "feedback exterminator".  The proper way to approach the "problem" is to  properly set up the system in the first place such that you have the maximum headroom from the system within the room/environment. 

A more proper way to think of the use for such devices should be "automatic parametric EQ".  By considering it as an EQ rather than an exterminator, it should be much clearer that any issues of headroom, GBF and/or system tuning and deployment should be done BEFORE feedback becomes a problem.

FBX units do have their uses, but in order for them to be of practical use one should understand why and how feedback occurs and how to avoid it with the "ounce of prevention" rather than trying to address it with a "pound of cure".     

YES

Manufacturers LOVE people who want a nice simple solution-no matter if it is the wrong one or does not work as well as a the RIGHT solution.

Proper design/setup of the system is KEY.  And when that is done-something like a feedback eliminator can be helpful in some cases for the "quick got out of hand" type thing.

But as we all know- it is real easy for those type devices to totally wreck the sound quality.  But as long as the mic is not feeding back-then they have "done their job".

Why bother singing on key when we have autotune?  BECAUSE it SOUNDS better----------

The right solution costs more-but in the end is better.

[Jon McElvain]

Thank you for the information.  We very rarely have a significant issue with feedback and we are looking at the unit as more of a backup than a solution.  The past week was only the second week after we completely changed the system setup and the first week where we got more than one praise song and the pastor spoke (the first week was a video presentation).  We had some low level feedback that several of us were trying to isolate.  I wasn't running the board but wandered that direction to see if we could figure anything out.  Another one of the crew noticed that several of the singers were absent mindedly pointing their microphones at the mains above them and even worse dropping the mics and pointing them at the monitors.

Just this week I have started a "campaign" get encourage proper use of the microphones so this may solve 99% of the problems we have IF people will follow through.  I just thought the feeback eliminator and auto EQ might prevent the rare unexpected feedback event.  I assumed they functioned in an active live mode, but it appears that once it sets a filter that frequency is locked until it is reset.  This isn't really what I had in mind.

Several months ago we had one major event that hasn't happened again and we couldn't figure out what happened.  Two wireless microphones were involved and when the two people got close to each other the feedback went through the roof.  We hadn't changed anything from previous weeks and didn't change anything after so it must have been a combination of unlikely factors.

[Jonathan Johnson]

I'll have to agree with Dick & Ivan. Dick's description of a feedback eliminator as an "automatic parametric equalizer" is spot-on -- that's exactly what it is. The most common feedback frequencies will be due to room resonances, which will be present even without feedback. The proper (but debatable) way to address this is with FIXED parametric EQ on the mains tuned to those resonant frequencies.

Once you've tuned the room, any other offending feedback will usually be due to the microphone's position relative to the speakers. Change the location of the microphone and the frequency will change. You may want to notch out that frequency, but you really don't want to apply that to other channels. (Especially if the feedback frequency is 440Hz.  ) As the microphone moves, the chance of feedback at that frequency disappears, so that filter can be discarded. (The problem with feedback eliminators is that filters may be retained long after they are no longer needed.)

When applied to the mains, a feedback eliminator can lock onto desired frequencies that are most definitely not feedback, such as sustained musical notes from instruments. That is a bad situation and makes the mix sound awful.

A feedback eliminator can be a useful tool, but it should be considered a tool of last resort and be applied only as a per-channel insert. (That means you'd have to install a separate feedback eliminator for each vocal channel! And four units at $500 isn't a cheap solution anymore.) Your first tools should be proper selection and placement of loudspeakers to keep the stage out of the coverage pattern and educating your talent on the proper use and handling of microphones INCLUDING how to position them relative to stage monitors, based on the microphone's pickup pattern.

[Jon McElvain]

I do appreciate the information. Along with this forum I emailed someone at the conference level in my church and they put me in touch with their "expert" and he emailed me today. He offered to come out and see/hear the system so that may be turn into a great resource.

[Tom Bourke]

Thank you for the information.  We very rarely have a significant issue with feedback and we are looking at the unit as more of a backup than a solution.  The past week was only the second week after we completely changed the system setup and the first week where we got more than one praise song and the pastor spoke (the first week was a video presentation).  We had some low level feedback that several of us were trying to isolate.  I wasn't running the board but wandered that direction to see if we could figure anything out.  Another one of the crew noticed that several of the singers were absent mindedly pointing their microphones at the mains above them and even worse dropping the mics and pointing them at the monitors.

Just this week I have started a "campaign" get encourage proper use of the microphones so this may solve 99% of the problems we have IF people will follow through.  I just thought the feeback eliminator and auto EQ might prevent the rare unexpected feedback event.  I assumed they functioned in an active live mode, but it appears that once it sets a filter that frequency is locked until it is reset.  This isn't really what I had in mind.

Several months ago we had one major event that hasn't happened again and we couldn't figure out what happened.  Two wireless microphones were involved and when the two people got close to each other the feedback went through the roof.  We hadn't changed anything from previous weeks and didn't change anything after so it must have been a combination of unlikely factors.

When you don't know the cause is exactly when they can do the most damage.  Often they are used as an electronic solution to a physical problem.  An auto EQ can not see the stage or know what is going on.  That also raises another issue, a feed back destroyer and auto EQ as implemented by DBX are different processes.  If you don't understand the underlying process you will not know when or how to temper them.

Almost every time I have encountered feedback processors in an installed system I have improved things just by eliminating them.  The very few notable examples have been where they were used as very precise parametric notch filters and the auto functions turned off.

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