RMS, Program and power amps

[Olliedem-c]

Hi again,

 

Just got a t.amp 2400, which is 2x1200wrms@4Ohm, and I have been using a 650w@4ohm power amp on an old set of PEAVEY XT Series speakers.

 

The Xt have bins and tops with passive crossover, each speaker says 350 rms, and 700 program @4 ohm.

 

I'd like to use the new t.amp and I dont want to burn out the peaveys, so if I use the power amps at just above half, is this a safe practice?, I know under powering is dodgy, but its a full on 2 hour show and I dont want to kill any equipment.

 

Thanks

 

 

<_< - All upper case title edited - no need to shout, we can all hear you

[mal421]

By running your amp at about half ,do you mean having the gains set to about half ? Most amps work best with the gains set to maximum . What you need to know is the output voltage from your desk and the input voltage for you amp . They should be between .775v and about 1.3v . E.G. If your desk only puts out .775v when set to maximum and your amp requires 1.3v to give maximum power you will never get full power out of your amp . If the reverse is true you could be getting full power out of your amp long before you reach maximum on your desk resulting in blown speakers when you think you are safely within their limits . I think you should use your ears . Set up your system with the amp set to max and see how far you can go on you desk before the speakers start to complain . Knock a few db's off and make an allowance for peaks and you will have some idea of how far you can push your system for real .

[Bobbsy]

Er, I'm going to take issue with some of this.

 

First off, the level controls on amps are pretty well never "gain" controls. They're input attenuators. The amplifier section remains unaffected and is still capable of putting out its maximum if the input stages are hit by a hot enough signal.

 

Similarly, mention of .775v RMS equates to the 0dBu output from the mixer. However, any decent mixer is capable of outputting considerable more than that. For example, if you can hit +8.2dBu without clipping, this equates to 2 volts RMS.

 

That said, there's no one right way to set up in this situation. Many prefer to run their amps at full and reduce level on the mixer since a clipped signal is far more likely to do damage and a clean one at high level. However, there's just as compelling an argument to keep the desk in the more linear part of its range and reduce the input attenuation as you propose to do.

 

There are as many other "flavours" of set up in between as there are different compromise settings.

 

Bob

[Tomo]

I've always gone by the tenet of "Everything should clip simultaneously"

That way, you're using the full range available to every component in the system.

 

I also know that if I keep the desk graphs below clip, all the rest is also below clip.

 

That said - I'm not a sound engineer, and my main strength is digital electronics.

[Silver_Rayne]

I don't think you are at risk of blowing out if you are careful. With the amp you have running on stereo you should be fine. I think if you read the manual that your amp will be outputting around 300rm in stereo if you are bridged it will go up considerably. XT's are sturdy speakers though I would start heavy on the base and work my way up as the tops are a bit more fragile and the first thing to go.

Before I started at my venue (two weeks before) A new amp was ordered it is likewise a 2400 but on but the output in stereo is closer to 275w per ch. In bridged or mono it goes up to 460 I think.

That probably makes little sense but the guist of it is as long as you are gentle and don't over stretch things your system will be fine. If it is encountering problems you will hear them comming and then it is time to clip.

[Uhdinator]

under powering is worse than over power. If you have say a 200w amp going into a speaker that is rated at 400w. The volume

or SPL will not be as loud as if you gave it the rated 400w that it needs. what this translates into is that you run the board output

too hot to get volume and if you clip the amp because you are maxing out the board and the input into the amp then you get clipping in the amp and or board and you cook the voice coils in the speakers. if you over power the speakers you will have alot of headroom as you will not have to run the board main volume as high to get the rated Volume/SPL out of the speakers.

 

IF you turn down the input sensitivity on the amp you will not reach full power but you will also loose headroom by doing this.

Many do not know how to properly gain stage their board inputs. I have seen many people turn up each input preamp on the mixer until they get a peak light and back it off until the light goes out...............wrong!

 

push PFL switch on the input channels one at a time to read the signal on the master peak indicators. Turn up the pre amp gain until you get 0 db on the meter. This is for one channel only. IF you are using 16 inputs for the mix you do not peak each channel at 0 db. Its a ratio thing. IF I am using say 10 inputs for a mix I set each channel to peak at say -12 to -15 db.

That way when all 10 channels faders are set at 0 or unity gain the peak meter should be hitting 0 or slightly above when the master is at 0 also.

 

If you don't have PFL or solo function just turn down the power amp inputs and turn the amp off and set the master vol fader at 0 and set the channel fader at 0 also then adjust peak level using preamp gain. Then turn down ch 1 and check ch 2, and check all channels independently. When done checking all channels then turn all the faders back up to 0 and see if you are reading 0 on the master peak (with master volume at 0 too of course) If using 16 channels I peak each around-20. Then when done power you amp back up. You will find if you do this right you will prob never be able to run the master up very high as you will have plenty of volume without cranking the mixer. Keep in mind that a pro mixer is +4 db when set at 0 or unity gain and if the specs

on the mixer says max level before clipping is +22 then you have 18db above unity gain or "0" before clipping in the board.

IF your amp says its nominal level or input sensitivity is +4 db and it clips at +18 db, that it is possible to not hit clipping level in the mixer but still clip the amp input...........make any sense? you are prob really scratching your head now.

 

Any way if you underpower you can cook speakers, if you overpower you will hear distortion like in an overdriven guitar amp.

[Simon Lewis]

IF your amp says its nominal level or input sensitivity is +4 db and it clips at +18 db, that it is possible to not hit clipping level in the mixer but still clip the amp input...........make any sense? you are prob really scratching your head now.

 

Yep, I'm scratching my head!

 

The amplifier input sensitivity denotes the input voltage required to achieve the full rated power output. Exceeding this voltage will cause the amplifier to clip. You do not have 14dB of headrom in your example.

 

IF you turn down the input sensitivity on the amp you will not reach full power but you will also loose headroom by doing this.

 

Changing the sensitivity simply alters the voltage needed to achieve full output power. Given that the desk manufacturer has given you considerable headroom, it is better to attenuate the amplifier input and run the desk outputs quite hot. This will actually improve the overall system dynamic range and lower the effective noise floor.

[Light Console]

Changing the sensitivity simply alters the voltage needed to achieve full output power.

That is a great sentence to use when someone asks why they are not gain controls.

 

Given that the desk manufacturer has given you considerable headroom, it is better to attenuate the amplifier input and run the desk outputs quite hot. This will actually improve the overall system dynamic range and lower the effective noise floor.

 

I have always been taught to run everything as near to unity gain as possible. In a situation where idots are in control of a system, I will use the master level on the EQ to set a maximum level (as it is a DSP8024, and the controls can be locked out). Is this the best practice over time for an installed venue?

 

Edit: Oh my 300th post outside of the Tea Break threads, I shall go and have a cup of tea to celebrate!

[Simon Lewis]

That is a great sentence to use when someone asks why they are not gain controls.

 

thank you!

 

The subject of gain structure can get much more complex, but rather than reading me ramble on, why not have a look at Chuck McGregor's excellent tutorial on the subject?

 

Hope you enjoy it!

 

Simon

[Light Console]

Thanks again, I did enjoy it, but now my head hurts - I'll stick to lights!

It reminds me of the time Balanced signals were explained to me. (It's the difference between the + and the -)

 

Another question then;

The "line in" control on my EV SXA100+'s, are they gains or attenuators?

[Bobbsy]

You can understand 3 phase power but not a balanced signal? We sound guys only have 2 phases on a balanced line! :(

 

The pot on your SXA100 is an attenuator...it's the input to the amplifier section just like the example above.

 

Bob

[paulears]

well, Electrovoice label them 'gain' so that seems to answer that one.

[Bobbsy]

Lots of amps have controls labelled "gain" but I've yet to see one where this wasn't attenuating the input voltage rather that actually adjusting the gain of the amp. I'm 99.9% recurring sure this is also the case on the EV active speakers.

 

Bob

[cineko]

I think if you read the manual that your amp will be outputting around 300rm in stereo..

Am I missing something here (apart from sleep!) on the Thomann site the t.amp 2400 is rated at "2x650w into 8 ohms", why do you say 300? (just curious as I'm thinking of getting a couple of these)

[lonfire]

hi guys,

just as a way of an example... I've got two HZ DPX1500 amps which can do 750w RMS into 4ohms.. one amp runs my peavey impulse subs (rated at around 300w RMS 4ohms) and the other amp runs by peavey impulse tops (rated at around 300w RMS 8ohms).. never had a problem with either, and I've tried running them quite hard and they seem to take it ok..

 

just a quick thought, what are peoples thoughts on setting a limiter just before the point of clipping on the amp? is that a good idea? I guess you'd have to lock off the input attenuators on the amp also to stop people fiddling.. I guess you could still clip on the desk which would be just as bad..

 

rgds

chris