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[David Morison]

So you're saying that if I have 2 indentical mics on 2 channels, one that has the pre-amp set at a reasonable level and the other is turned all the way up to where it's clipping with any input; that I can get the same overall GBF from both channels?  Same actual speaker output? The clipping channel will not feedback at a lower volume?   
 I will test that out myself.

I'd qualify that by saying up to the point of clipping, you'll get the same results.
Clipping adds extra frequency content to the signal, so that difference could be enough to trigger feedback where it otherwise wouldn't occur.
If you do your test with one mic 5dB below clipping and the other mic 20dB below clipping, and then use the channel faders to bring them back to the same level at the desk output, then they'll exhibit exactly the same feedback sensitivity. The only difference you may notice is very slightly more background hiss on the low gain/high fader channel, but it certainly shouldn't be enough to push you noticeably nearer feedback.

HTH,
David.

[Tim McCulloch]

Bob Burke- one more time.  "Unity" is just a convenient place with a silk-screened legend and possibly a detent.  It is no more relevant than the "Normal" setting on a clothes dryer.  What is normal?  I'd say anything that isn't (based on my dryer) towels, heavy items, delicates, air fluff, freshen up and 'fast dry'.  IOW, not much is "normal"; it's a default setting if you don't have a better preference.

I'm an old analog guy, Bob.  I got my start on a Tapco 6200 and 'graduated' to BiAmp and Peavey, then Yamaha and Ramsa.  Many of the things we did back then weren't ideal from a technical POV, but we did them and mostly still do, even with digital mixers (another rant for another time).

Mythbusted #1 - Preamps are a big deal.  No, they really aren't, and John Roberts (who designed consoles and preamps in a former life) has patiently explained why in multiple posts.  But the perception that console mic pres are some kind of voodoo or *must* be operated at a particular level is wrong.  In the Olde Dayz, we'd PFL a channel and raise the input trim until the PFL meter would peak at -0- or even hit +3 on the meter.  The reason?  "It's for the preamps", we'd be told.  The result of this technique was frequently input faders somewhere down in the bottom 1/3 of their travel, but the output faders would be around the -0- mark.

Mythbusted #2 - Faders MUST be at 'unity' or -0- to work properly.  Uh.... really?  Then why make them moveable?  Nope.  Just more level, but that's a good thing, right?  Maybe yes, maybe no...

Mythbusted #3 - Amps should be run wide open.  Well, from the 'make it impossible to screw up' standpoint, running amps with no input attenuation makes sense, but it is not a requirement.  With the exception of a couple of designs not commonly used in pro audio, the input level control on an amp is a pad that results in a higher input level being needed to make the amp reach the desired operating level; the output level *capability* (potential output voltage swing) of the amp remains the same.  It was (and still is) common for amps to have an input sensitivity of around 1.4v (or half that, 0.775v), and that voltage would drive the amp to full gain output.  Interestingly, "0 dBVU" on the console corresponded to about 1.4v....  You get above 0dBVU are your amps are clipping and it's probably loud - Loud - LOUD!

Bob, all of these things conspire to make a whole lot of level, using a whole lot of electronic gain, and much of that gain isn't necessary for the quiet operation of most small portable sound systems used in casual entertainment.  The result of that brings us to:

Mythbusted #4 - If you don't have a graphic EQ, you'll never stop feedback.  The EQ is a frequency band-selective (well, sort of selective but that's for another discussion) volume control.  Considering Myths 1 -4 above, doing things the Ye Olde Way will pretty much require some kind of level reduction, and the humble graphic EQ can do just that.  I refer to such use as 'gain reduction through equalization' and that's pretty much what we did with them (and many folks still do).  When you see an EQ that has every fader from 150Hz to 8kHz down from -3 to even -12dB, this is exactly what's happening.  It would be more productive and sound better to bring down the level ahead of the EQ (mixer, at whatever stage or stages might be appropriate) and make less feedback to start with, but conventional bad advice from the Olde Dayz prevails.  That all said, having an EQ to help with tonal shaping and some feedback EQ is still a good idea, but not overdoing the amount of signal being sent to the system makes it much easier to use successfully.

Bob, I'm a big believer in acoustic solutions to acoustic problems, and if we don't put unneeded gain in the electrical signal path, we create fewer issues with the electro-acoustic interface.  If we don't put additional, unneeded sound producers in the same physical space, we reduce the amount of acoustic interference we create (the pair of monitors side by side, and the way they interfere with each other AND the PA).  That brings us to:

Mythbusted #5 - Every speaker is independent and works in its own little world.  Nope, uh uh, not even.  Think back to your high school physics class and the wave tank experiments (or you can do this at home with a baking pan, some tap water and a couple of BBs or pebbles).  You drop a pebble in the center of the tank and the water ripples in a circle, and they reflect off the sides of the tank, returning to center in interference to the first wave.  Let it get still, and simultaneously drop 2 pebbles, one in the center and the other a little off to the side, or ahead/behind.  Watch the way the ripples interact (interfere) with each other.  Now do it a few more times, changing where you drop the pebbles and observe.  Graphically this demonstrates what is happening in acoustic space when you have multiple sources that are not identically located.

There is much more, but these 5 things address most of what I see when I hear a bar band or observe many weekend warrior sound providers.

I get the biggest looks of astonishment when I'm brought in to "fix" something and wind up either turning something(s) down; turn some stuff off; or press 'bypass.'  And I give the client a full day-rate bill.

Have fun, good luck.

Tim Mc

[Bob Burke]

Tim,

  Thanks for the detailed explanation. It's a lot of info to assimilate, but I will try. Funny you should mention the Tapco. My first mixer was a 6000R (I still have it).

  It's painfully obvious that most of my problem comes down to simple gain staging, or improper use of it. I will try to emulate your methods. As for the two monitors side by side, I thought it would work better than it did. I will be going back to my single Yamaha CM12V, (which sounds much better than the PR-12's anyway), since I succeeded in blowing the tweeters in the brand-new Peaveys. The CM12V is also a dedicated monitor, with the wedge at the proper angle.

  Where do you stand on the set-it-and-forget-it technique of setting up the system and adjusting the input EQ only at the venue? That still sounds suspect to me, given the varied places that we play.

  I can't wait to put all of this great advice into practice!

Thanks again.

Regards,

Bob

[Mark McFarlane]

One argument for running channel faders 'near 0' is you may have a little more precision in small adjustments due to the non-linear behavior of some faders (e.g. at the bottom of the fader throw 1/4" = 10db but near unity 1/2"= 5db...

[Tim McCulloch]

One argument for running channel faders 'near 0' is you may have a little more precision in small adjustments due to the non-linear behavior of some faders (e.g. at the bottom of the fader throw 1/4" = 10db but near unity 1/2"= 5db...

Agreed, and most of the time that means NOT "PFLing to 0" unless the entire rig is gain-staged for that.

[John Roberts {JR}]

So you're saying that if I have 2 indentical mics on 2 channels, one that has the pre-amp set at a reasonable level and the other is turned all the way up to where it's clipping with any input; that I can get the same overall GBF from both channels?  Same actual speaker output? The clipping channel will not feedback at a lower volume?   
 I will test that out myself.

Dick Rees is correct. Yes you can get the same GBF from both but you will have to turn down the clipped channel in later stages to realize the same GBF which is related to total path gain, not just some single stage.

Turning up a gain stage past clipping does not somehow limit the gain, even though the sound will no longer get proportionately louder. The dirty little secret for why undersized amps can be associated with more feedback is that inexperienced operators can keep turning up the power amps after clipping trying to make the louder. Feedback is caused by the gain not the loudness. 

I used to make feedback on purpose for FLS demos years ago and I could easily make low powered systems feedback... it's the total path gain.

JR

PS: consoles are designed to be pretty tolerant of how the operators choose to use them. Don't over think this...

[DavidTurner]

Hi Bob:

Mr. Turner is my dad

I have not taken the time to read all the responses to your question so forgive me if I am repeating some other advise. No doubt you have been given some excellent pointers, but since you asked:

Looking at the picture of your rig, I would start with everything set to flat (except for a hi pass on the vocals) and adjust the mains eq  while speaking/singing into the lead vocal mic - being careful not to cut too much. Assuming that the vocal is the most important thing being amplified, I wouldn't care much about whether the rig was "flat", but whether the voice sounded good. I would then use the channel eq to make the other things being amplified sound the way I wanted.  YOur mileage may vary 


 

Mr. Turner,

  Actually, it's not giving me the results I want. Hence the question. How do you do it? (bear in mind, I don't have a driverack).

Thanks.

Bob

[Bob Burke]

Hi Bob:

Mr. Turner is my dad

I have not taken the time to read all the responses to your question so forgive me if I am repeating some other advise. No doubt you have been given some excellent pointers, but since you asked:

Looking at the picture of your rig, I would start with everything set to flat (except for a hi pass on the vocals) and adjust the mains eq  while speaking/singing into the lead vocal mic - being careful not to cut too much. Assuming that the vocal is the most important thing being amplified, I wouldn't care much about whether the rig was "flat", but whether the voice sounded good. I would then use the channel eq to make the other things being amplified sound the way I wanted.  YOur mileage may vary 

Thanks, David.

  I will govern myself accordingly.

Regards,

Bob

[Tim McCulloch]

Tim,

  Thanks for the detailed explanation. It's a lot of info to assimilate, but I will try. Funny you should mention the Tapco. My first mixer was a 6000R (I still have it).

  It's painfully obvious that most of my problem comes down to simple gain staging, or improper use of it. I will try to emulate your methods. As for the two monitors side by side, I thought it would work better than it did. I will be going back to my single Yamaha CM12V, (which sounds much better than the PR-12's anyway), since I succeeded in blowing the tweeters in the brand-new Peaveys. The CM12V is also a dedicated monitor, with the wedge at the proper angle.

  Where do you stand on the set-it-and-forget-it technique of setting up the system and adjusting the input EQ only at the venue? That still sounds suspect to me, given the varied places that we play.

  I can't wait to put all of this great advice into practice!

Thanks again.

Regards,

Bob

Hi Bob-

Well, we emulate the big boys - system EQ makes the *system* work and play nice together, and "house" EQ is used to make whatever changes might be needed for that show, that day, in that room.

In the big boy world those are usually physically different devices although DSP technology has allowed us to blur those lines.  In weekend warrior/bands mixing themselves world, there is usually one EQ that does all the jobs.  Not ideal but it's pretty much how we all started and many continue to work.  Nothing wrong, inherently, but it can make things a bit confusing when you have to remember "was that -6db cut at 3k for the system or the glass atrium we played in last week?"

Here's what I'd consider:  Set your rig up in your back yard, using the spacing and relative positions you use most of the time.  Since you don't have sophisticated system control, voicing (EQing) the system to sound "right" is the starting point.  For a reality check, have your wife and a couple of friends/neighbors offer an opinion as to how the playback sounds with your material (you'll find a lot of variance, probably).  You're looking for "pretty good, on most of the songs."  You've done your system EQ, for the most part.  Note that we didn't really get into "flat" because "flat" systems sound boring; they don't have the amount of LF that we association with terms like warm or full, so I suggest voicing the rig to be compatible with your music.  If you have something odd, like a big boost at 8kHz, it's probably related to the music you're using to tune the rig; if it's uniformly this way consider "splitting the difference" between "flat" and whatever boost you think sounds good.  But in the end, remember this:  No building, wall, or open space can alter the physical and electronic composition of your *system*.  That's an important distinction.

Also note that we didn't use the monitors yet...

Now bring up your live mics and guitars... slowly.  If you get feedback before you reach a useable SPL, your next step is to figure out why.  Start by looking at channel strip EQ for a boost around the frequency that is feeding back.  None?  Then either try to raise the PA speakers, move them further downstage of your mics, etc.  If you have the channel strip EQ fairly flat to start with and very little fader movement introduces feedback, turn down the input trim until you can raise the fader to a more desired physical position.  This is where "unity" when used as a physical reference point, can be useful.  Most often, though, setting the mic pre to the unity detent and the fader to unity detent will result it the wrong amount of level (see previous post), but I digress.  If every input seems to 'take off' with ringing or feedback, you can also simply turn down the main output fader if the input trims seem rational (no red lights, PFLing shows inputs peaking at -10, etc); there is too much down stream gain.  You can turn it down at the mixer or you can turn down the amp.  When you're done with this and are happy with tonality, feedback control and over all SPL, use your phone to take close up pics of the EQ settings.  They are your "system EQ" settings and will be the baseline upon which you make other, venue-dependent changes.

At any rate the purpose of this exercise is to establish "normal" (so you can dry your clothes ).  Remember that I said no building or walls or open space changes your system.  When you move this indoors you'll find 2 things:  the first is that you'll be turning the over all level down and that all the HF you didn't hear outdoors is suddenly prominent; you may also find you have either more or less very low end.  These are the results of the various boundary surface affecting the audio you hear because of reflections (back to Myth 5, both relating to multiple speakers and, in this case, to the reflections that are almost as loud as the original waveform).  You can't EQ reflections away.  You can do a couple of things, the first of which is to not "spray" the PA onto reflective surfaces by altering speaker positioning.  Flat, parallel orientations are worst, and using the speaker's nominal coverage angles to help you point the sound at the people and (to the extent possible) away from things that are reflective.  The other thing you can do is not "excite" reflections and room modes by removing (with EQ) some of the frequency content that is being reflected (see glass atrium comment above).  Note that listeners who do not hear the contribution of the reflection will still be hearing the cuts you make with the EQ.  If you need to make changes to the EQ, do it.  You have the pictures, right?

If you find yourself consistently making the same EQ changes regardless of room, consider making them permanent "indoor" settings and take pics of them.  No harm, no foul.  You have to make it work for you as consistently as possible.

You can often tell what a room is going to sound like by simply standing where you'll perform and speaking in a loud voice; you can tell about the audience area doing the same thing.  Chances are you can't do this in a restaurant, and a bar will likely be too noisy, but you can use your ears to great advantage once you get used to it with some practice.

It's time to head out to build some PA and assist the BE do his thing.  More later, after you've had a chance to get some questions based on experimenting with your setup.

Have fun, good luck.

Tim Mc

[Bob Burke]

Hi Bob-

Well, we emulate the big boys - system EQ makes the *system* work and play nice together, and "house" EQ is used to make whatever changes might be needed for that show, that day, in that room.

In the big boy world those are usually physically different devices although DSP technology has allowed us to blur those lines.  In weekend warrior/bands mixing themselves world, there is usually one EQ that does all the jobs.  Not ideal but it's pretty much how we all started and many continue to work.  Nothing wrong, inherently, but it can make things a bit confusing when you have to remember "was that -6db cut at 3k for the system or the glass atrium we played in last week?"

Here's what I'd consider:  Set your rig up in your back yard, using the spacing and relative positions you use most of the time.  Since you don't have sophisticated system control, voicing (EQing) the system to sound "right" is the starting point.  For a reality check, have your wife and a couple of friends/neighbors offer an opinion as to how the playback sounds with your material (you'll find a lot of variance, probably).  You're looking for "pretty good, on most of the songs."  You've done your system EQ, for the most part.  Note that we didn't really get into "flat" because "flat" systems sound boring; they don't have the amount of LF that we association with terms like warm or full, so I suggest voicing the rig to be compatible with your music.  If you have something odd, like a big boost at 8kHz, it's probably related to the music you're using to tune the rig; if it's uniformly this way consider "splitting the difference" between "flat" and whatever boost you think sounds good.  But in the end, remember this:  No building, wall, or open space can alter the physical and electronic composition of your *system*.  That's an important distinction.

Also note that we didn't use the monitors yet...

Now bring up your live mics and guitars... slowly.  If you get feedback before you reach a useable SPL, your next step is to figure out why.  Start by looking at channel strip EQ for a boost around the frequency that is feeding back.  None?  Then either try to raise the PA speakers, move them further downstage of your mics, etc.  If you have the channel strip EQ fairly flat to start with and very little fader movement introduces feedback, turn down the input trim until you can raise the fader to a more desired physical position.  This is where "unity" when used as a physical reference point, can be useful.  Most often, though, setting the mic pre to the unity detent and the fader to unity detent will result it the wrong amount of level (see previous post), but I digress.  If every input seems to 'take off' with ringing or feedback, you can also simply turn down the main output fader if the input trims seem rational (no red lights, PFLing shows inputs peaking at -10, etc); there is too much down stream gain.  You can turn it down at the mixer or you can turn down the amp.  When you're done with this and are happy with tonality, feedback control and over all SPL, use your phone to take close up pics of the EQ settings.  They are your "system EQ" settings and will be the baseline upon which you make other, venue-dependent changes.

At any rate the purpose of this exercise is to establish "normal" (so you can dry your clothes ).  Remember that I said no building or walls or open space changes your system.  When you move this indoors you'll find 2 things:  the first is that you'll be turning the over all level down and that all the HF you didn't hear outdoors is suddenly prominent; you may also find you have either more or less very low end.  These are the results of the various boundary surface affecting the audio you hear because of reflections (back to Myth 5, both relating to multiple speakers and, in this case, to the reflections that are almost as loud as the original waveform).  You can't EQ reflections away.  You can do a couple of things, the first of which is to not "spray" the PA onto reflective surfaces by altering speaker positioning.  Flat, parallel orientations are worst, and using the speaker's nominal coverage angles to help you point the sound at the people and (to the extent possible) away from things that are reflective.  The other thing you can do is not "excite" reflections and room modes by removing (with EQ) some of the frequency content that is being reflected (see glass atrium comment above).  Note that listeners who do not hear the contribution of the reflection will still be hearing the cuts you make with the EQ.  If you need to make changes to the EQ, do it.  You have the pictures, right?

If you find yourself consistently making the same EQ changes regardless of room, consider making them permanent "indoor" settings and take pics of them.  No harm, no foul.  You have to make it work for you as consistently as possible.

You can often tell what a room is going to sound like by simply standing where you'll perform and speaking in a loud voice; you can tell about the audience area doing the same thing.  Chances are you can't do this in a restaurant, and a bar will likely be too noisy, but you can use your ears to great advantage once you get used to it with some practice.

It's time to head out to build some PA and assist the BE do his thing.  More later, after you've had a chance to get some questions based on experimenting with your setup.

Have fun, good luck.

Tim Mc

Wow.

  That is perhaps the best advice I've ever gotten. I had never even heard of the concept of "system EQ".

  Can't wait top try this.


Thanks a million!


Refards,

Bob

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