Help With Setting up a Limiter

[zbad55]

Hi all,

 

I have a DBX 166A comp/limiter that I would like to run as a limiter after the EQ for our FOH for speaker protection. I have been trying to read and learn all I can and not sure what some of the settings should be. The manual has good info about the peakstop and adding mild compression, but I would like someone with firsthand experience about this.

 

From what I have read the treshold should be set a couple of db below the point of clipping the amp. Where do I find out at what point the amp will start clipping. I'm still trying to learn the relationships between dbu, dbv and dbVU and I think this is where the answer lies.

 

The amps that I use are a Crown XLS602D and a Samson SX1800, and with turning the amps sensitivity controls, they are not marked so how do you know, just how much you have either increased or decreased the sensitivity. Any help would be appreaciated or any links that may help. Thanks

 

Carl

[dboomer]

Setting the limiter a couple of dB below amp clipping doesn't necessarly protect your speakers at all.

 

What you want to do is limit the output from your amps to just below the average power that your speaker can handle. That will protect the speaker from thermal burnout. You'll also need to set hP filters to protect against excursion failures.

 

The formula to set the threshold is ...

 

Limiter Treshold(dBu)=Speaker voltage limit(dBu)-Amplifier Gain(dB)

[zbad55]

dboomer, thanks for the reply, where can I find those numbers, the speakers are SP-5's and handle 400w Continous, and the spec says for 99db SPL (2.83V Input). How do I calculate the voltage gain for the amplifier gain. This is where I get confused is how to calculate these numbers. Maybe its simple and I am just missing something. Thanks

 

Carl

[agedhorse]

Here's the easier practical way to do this, the amps are reasonably sized in stereo mode, (I'm assuming you are using one amp or the other and not both) so with the speakers not plugged into the amp, the amp powered up, drive a "typical" signal (like music) from a CD through the eq (set flat) through the comp, and with the comp portion set in bypass position (ratio to 1:1 or the threshold set all the way up) until the clip lights start flashing. Now reduce the threshold on the peak stop limiter until the clip lights just stop flashing. That's it. You do not want to use the compressor portion of the unit unless you intentionally want to compress the program also. I suggest waiting until you get this right first.

 

An alternative to this is to buy amps that have limiters built in, and size the amps correctly. The results will be identical when calibrated as above. I do not believe either of these amps have limiters but it's worth a double-check.

 

I checked the XLS-602, so take the 166 out of the signal path and just be sure the limiters are engaged on the amp. It's more foolproof. The clip LED now indicates limiter action. Do not run the signal hard into limit in any event because now you run into thermal issues long term as Don mentioned.

[PlatinumProd]

I may get dinged for not doing this the technical volt-reading way, but I have never blown an amp nor a speaker component. I thinkyou should follow the way the experts here are telling you, but I will tell you what I do for "educational" purposes.

 

I approach limiting for an amp the way I approach compression/limiting in the studio.

 

I set my Main's out to about -6 db below unity.

 

THEN, I start setting my level's at the source.

 

I do a sound check at the loudest levels all instruments can play. I make them turn their instruments/amp per channel the loudest they will play on the loudest, most frenzied song. I make sure no signal on individual channels of the mixer peg red, EVER ... and I try to get them into the median zone about 50% of the time (usually designated by yellow).

 

These channel mixes are summed in the output buss. With all of them playing, you will find almost all musicians play louder together than they do invidually (even though you previously asked them to play their loudest ... go figure ). Reset their channel levels again as before, and place light compression on any channels that may be problematic ... particularly percussion and bass, in most of my cases. Bring the main fader up to the volume you need ... hopefully your amplification gets you there without having to go higher than unity on the mixer and you have plenty of headroom.

 

NOW, with all your individual channels never distorting, you mixer never distorting during normal program, and some headroom on the main mix out for unexpected transients, set your compressor/limiter to show a compression level about -3 db. This ensures your compressor kicks in at the level you know all sources are maxed, you are well below distortion on all channels and the main out, and your amps show no clipping.

 

Finally, cut your main slider back, where the -3b of compression goes away.

 

VOILA!

 

I have recorded COUNTLESS multitrack sessions being able to walk away from my mixer and go on stage. If anyone has recorded live here, they know how critical it is to keep distortion out digitally recorded tracks. If you listen to the way my lead singer squalls his vocals and how the background may suddenly soar an octave from phrase to phrase, you would know it's hard to accomodate that dynamic range without being at the board constantly to monitor. If I can control the levels like that going to digital tracks, I think the output to an amp will be ok.

 

Maybe with the finesse advice these electronic gurus give you, you can consider some of my old-school tactics as food for thought while you engineer your sound.

 

Success to you!

[agedhorse]

So what ratio would you set your compressor at?

 

What happens when one of the boys knock a mic over, what about an accident with monitors?

[zbad55]

Thanks for the reply's that have been given, I'm starting to understand this a little more. Maybe I'm making this more complicated that it should be, but what is still confusing to me is the amplifier gain. With the dials on the amps that change input sensitivity not any having any numeric values on them, how do you how many db this equates to, in reference to the formula provided by dboomer. And is the reference for the speaker "2.83v" the maximum that this speaker should be run at

 

For example, the DBX 166A has a threshold of up to 20db for the peak/stop limiter. If the VU meter on the board is running at "0" Unity and by mistake a fader is pushed up to far and starts clipping the VU meter on the board at what point would the amp start clipping (How many db). If I were to set this dial at 15db (as an example) is this sufficient and the limiter will kick in and the amp will never see this hot of a signal. That is what I'm trying to understand, so what ever the VU meter on the board (not that I would run it that way) is doing I know it will not clip the amp because of the limiter setting.

 

AH, you said something that I have heard and it might be one of those myth's about never running an amp without any load on it. Just wanted to clarify.

 

Sorry to ask some questions that may seem easy, but I'm trying to learn and understand how all this works, with all the references to db's, dbu, dbv and the VU meter and how they all relate. As well as how other people run their setups. Thanks for all the help,

 

Carl

[PlatinumProd]

So what ratio would you set your compressor at? What happens when one of the boys knock a mic over, what about an accident with monitors?

 

If you set a compression ratio of 3:1, an input signal exceeding the threshold by 3 dB will only allow a 1 dB increase in level to the amp.

 

I've had mics dropped, idiots pull leads out of their amps or direct boxes and NEVER killed an amp yet. (Knock on wood)

 

What kind of accident with monitors?

[agedhorse]

 

 

 

Feedback???? the most common accident by far.

 

Killing the amp is not what we are talkingabout, nor will the amp be killed by over-driving, but the speaker may.

 

A ratio of 3:1 is nowhere near enough to be viable as much in the way of protection, if you set the threshold as you suggested in your post, first of all you never reference the clip point of the amp, and secondly, if somebody yells into a mic that's loud already, 3:1 compression isn't going to help you out all that much. A little maybe, but not all that much.

 

Just because you haven't experienced damage doesn't mean you have a good grasp of what the average user here deals with. You seem to have it all figured out, styled and dialed to your speific situation and quality/performance standards, but there's more to live audio than a recording perspective might imagine.

[Dogoth]

 

Maybe I'm making this more complicated that it should be

 

 

Definitely.

 

Most modern equipment will interface pretty easily so relax, set the amp to 3/4 and see if you can hit the limiters before your board's output runs out of steam (do this with out speakers connected (as stated) this is perfectly safe). If your mixers output meters are barely tickling the bottom leds and your already flashing the amps limit lites then turn down the amp, but odds are you'll be in the ballpark.

[dboomer]

If you use an amp thet is properly sized and it has built-in clip limiting then the whole thing is automatic. This becomes a bit more difficult to do with bi/tri amped systems because it's difficult to get small enough amps for the HF. The other thing is that it restricts you to using small amps (I know Andy ... you love this) but using oversized amps allows for more headroom and dynamic range in the system and that just sounds better.

 

Platinum ... your method doesn't take into consideration the size of the amplifiers at all. The reason this is working for you is you simply are not exceeding thecapabilities of your equipment ... thats a good thing ... but you're not really getting any protection.

[dboomer]

 

dboomer, thanks for the reply, where can I find those numbers, the speakers are SP-5's and handle 400w Continous, and the spec says for 99db SPL (2.83V Input). How do I calculate the voltage gain for the amplifier gain. This is where I get confused is how to calculate these numbers. Maybe its simple and I am just missing something. Thanks Carl

 

 

The gain of the amp should be on the spec sheet. That gain will be if the knobs are wide open. As you turn them down (probably a good thing) just subtract by the -dB numbers.

[zbad55]

Thanks all for the help on this subject, I have a better understanding of it now. I did some more research and reading in addition to your help as well. And today I started to do some adjusting with the levels to see what different settings result in. Thanks again

 

Carl