power amp settings

[fireman]

is it better to run power amps on "full" or set the dial to 3/4 to get the best headroom?

[norty303]

Depends on your environment. If you want to be absolutely perfect in maximising your dynamic range then it will almost invariably end up with your amp pots at less than max (depending on what the input sensitivity of your amps are and the output gain of the last piece of kit - usually crossover)

 

However, if you're in an environment where people are likely to meddle with things they don't undertand, it's best to run the amps at full and control the signal going to them with the crossover outputs. What you lose in dynamic range is made up for by kit that generally survives for longer.

[Shez]

What Norty says about meddling people is very true - they will assume that they can just turn it up and make it louder without any problems.

What you need to realise is that the amp can work flat out and produce maximum power at ANY setting of the control on the front. The only difference this control makes is what input level is required in order to achieve full power.

Most pro amps, when the input attenuator is turned all the way clockwise, will produce max power for an input not much more than +4dBu. If it's connected directly to a desk, that gives you no headroom. Turn the attenuator down by e.g. 12dB, and you can then feed it up to +16dBu which is a slightly more sensible amount of headroom.

Of course it also depends on the speaker's power handling. You need to work backwards along the signal chain from there in order to work out appropriate level settings.

[fireman]

thanks , you've both helped me out there.

 

I usualy run the amps at about 3/4 and they are fed from a crossover, just couldn't remember how the "volume" knob worked, I.e cutting the input or cutting the output if you know what I mean.

 

good point about messers I always worry about someone going daft with the levels and something going pop.

[Simon Lewis]

However, if you're in an environment where people are likely to meddle with things they don't undertand, it's best to run the amps at full and control the signal going to them with the crossover outputs. What you lose in dynamic range is made up for by kit that generally survives for longer.

 

I can see the argument here, but if tampering is an issue (and I presume that even the crossover could be altered?) why not put a suitable balanced pad ahead of the power amplifier to give the required degree of attenuation and ensure correct gain structure. The resistors can even be soldered into the XLR plug.

 

Furthermore, there's just as much potential for damage to speakers when an amp is run wide open and the desk goes above 0dB ......

[Rob_Beech]

I would assume we'd be talking about digital ones and the ability to lock them so that you cannot alter anything without a password, but it wasn't stated one way or another.

 

Rob

[norty303]

Furthermore, there's just as much potential for damage to speakers when an amp is run wide open and the desk goes above 0dB

 

Yes, but less than when the desk goes above 0db AND the pots get cranked another 1/2 or 1/4 turn.

[paulears]

I Have this problem frequently. I have a reasonable PA installed in the theatre I work in for the summer - the venue has no permanent installed equipment, and we run a series of 67 day a week one-night shows. The access to the venue is a pain, and many of the visitors leave their PA on the truck and use mine - for sheer convenience. They bring in monitors and a desk (or two) if required and most go into a couple of our desk channels, with just one being given direct access to the amp inputs (we trust him!)

 

The system is set up with a standard 'main' output to left and right, with another pair of outputs feeds aother left right system designed to cover the edges - the auditorium is quite wide. When a show sells less than 50% (this is common as full, we hold 1200 or so, but most houses that have 700 are considered pretty good for us. The 'wide' system can be brought in, if required. In addition, an extra flown centre cluster in mono) is available to fill in a fee weak areas near the stage in the centre.

 

Now the snag is that some of the people who use the system, don't even ask about the system, and simply use the main L/R - which is fine - it's perfectly capable of proving enough SPL at the mix position to make speech impossible. The gain structure has been tweaked away from what I usually use, and one of the graphics in the rack has a nice led VU meter very evident from the visitors mix position. When the red leds start to light, there is still headroom left on the amp racks (quite a bit, actually) - so all they have to do is keep to an occasional red on peaks and all is well. The trouble is, many still can't set the channel gains properly on our desk, and then when they push it distortion is introduced at the desk end. I suppose if there has to be distortion, I'd rather it be that end, but it is still unkind on the loudspeakers.

 

Last night, the sound check on the drums alone was shaking the crew room walls! As soon as I'd gone, up went te levels. I did think about apermanent limiter in the link, but I'm sure when it kicked in, they'd just push and push till it sounded a real mess.

[norty303]

Although not strictly designed for the job, my favourite piece of kit is my Formula Sound AVC2. It's a dynamic level control rather than compressor/limiter, and it has a slow response, so your LMS limiters catch the fast peaks, and the AVC deals with the ever increasing overall level.

 

Once it starts to work, the level of attenuation becomes slightly greater than the input level as the input level rises, so that by the time there's real distortion going to the amps, it's pulled the level back a way to protect things. It's meant that I can now feel happy leaving a rig with DJ's for the night, amps up full, LMS locked off, AVC watching and waiting, knowing I'll comeback to collect and everything is going to be intact and working. Can be had for £60-£180 on ebay, usually in mint condition as they've been taken out of installs. Also has a 10v output that can be used for all sorts of things, that activates once the first limit threshold is reached (strobe? bright light? 'Turn Down' sign? Electrified master level controls?) I'm sure you could go all 'Bond Villain' if the desire took you

[SteveAATW]

Another thing to bear in mind is that most digital crossovers need to be worked hard to fully use the resolution of the A-D and get the best sound out of them, invariably this means turning the crossover outputs up and the amps down. In a hire environment this can be somewhat difficult granted but you can get gain control cover plates for the QSC powerlight range and the MC2 E series have attenuator switches on the back which can reduce the range of the gain control on the front panel so you can have it up full but still have it turned down. The improvement in sound quality is noticeable on decent systems :P

 

In the DJ situation volume limiters are all well and good, but if the signals distorted then its distorted be it quietly or loudly and square waves are bad news. I've never fathomed why mixer manufacturers leave the master gain controls on the front panel to be honest. Mine end up taped up or removed normally. Also if you get a mixer which has trim pots on the back panel you can reduce the input gains so that even if the DJ turns the line gains up full it can't distort - the FSM 600 and FF-6000 are excellent for this and you can make the system virtually DJ proof with no need for an automatic gain control.