Power Amp - set to full constantly ?

[voodoo1967]

Hello all - new here - 2 questions in one post

 

1.Can someone explain to me why you would do this - set your power amp to full on both channels and then control the volume mix from the mixer ? - rather than just set the power amp to half and turn up if needed.

2.Also if you used 2 x4ohm speakers (ie 2 ohm) on one channel - would that cause an issue with the amp ?

[azlan]

1. you'll get finer control of the volume level and have bags of headroom should it be needed (my understanding anyway)

 

2. depends on the model of amp, some will be happy with a 2ohm load but others may have a minimum load of 4 ohms, check on the back of the amp or the manufacturers website

[mrcog]

1. Because you've then got your control from your desk? However you then could be running your output level from the desk very low, and because the faders are logarithmic, it makes it very hard to make small adjustments near the bottom of the travel...

 

2. Depends entirely on the amp I believe. I'm guessing this is a real life situation, so what's the amp that your talking about?

 

James

[paulears]

Many people do like to leave the amps with the attenuators (note: they're not supposed to be used as volume controls) fully clockwise - however, I don't. I could, but in my case, I've got a bit more available volume than I actually need, so I have the inputs to the desk set to give me a decent range between off and the unity gain position where the faders will usually be when open and the input gain adjusted properly. Same again with mixer output - with the output faders at the normal position, around 75% of full travel. This way the in and out meters all work across their range, showing me what's going on. With this output level the volume would be extreme - so the choices would be run the mixer master faders about 10mm from the end and amp fully clockwise, or set a more useful position and knock the amps back to give the ideal level.

 

For me, convenience scores over perhaps absolute sonic integrity - and, I can't hear the difference!

[mrcog]

If they arn't meant to be used as volume controls Paul, then why are they on the amp in the first place? Is it flexibility, so that the amp can be used for different sized speakers, or for some other reason?

[paulears]

They're supposed to be viewed as attenuators - so in the gain structure you have the ability from the desk to drive the amps at anywhere between max and off. If you use them as volume controls of course it works, but you're actually limiting the available dynamic range. Many people who have 2 or 3 way systems have the balance set at the crossovers, so it makes sense then to run the amps fully on, as the balancing is done upstream.

[Dave SA]

...and stops someone coming along, winding up the amp further and either destroying your sound balance, or your speaker components.

 

Just wanted to add that Paul, due to the relative nature of the OP's post.

[ramdram]

Ref Q1. For what it's worth the youngsters I work with wanted to to run the theatre amps with no attenuation, ie wide open, then tried to bring the radio mic levels down on the desk...because that is the way they had been taught to run a desk...but for doing gigs with instruments...goodness knows how they did the mic level thing, which were not at line level. The CD player o/p is at line level so the pot was adjusted accordingly. We don't put anything else thru' the desk.

 

It took the best part of the season to get them to work with lower level settings...(did I mention two blown speakers? The bulbs blew on the limiter protection board so no real prob just a lot of effort bring the cabs down and replacing the bulbs). Eventually they twigged that having to mess around with the pre amps level on each channel could be avoided by simply not running the amps so high...and the faders could sit on the zero level. The preamp pots were not linear and the critical level was on the "knee" of the law so to speak..so flippin difficult to get the the mic level just so.

 

All this headroom stuff is all very well for music but for speech??? It all depends on what sort of "stuff" you are putting thru' the system:

 

http://www.crownaudio.com/amp_htm/amp_info..._much_power.htm

 

Ref Q.2, you would have to read the amp manual to see what the o/p stages were designed for, and if you ran the speakers in series or parallel:

 

http://www.termpro.com/articles/spkrz.html

 

there are quite a few sites via google so the above are probably as good as any.

 

HTH

[David A]

http://www.gain.pe.kr/spboard/board.cgi?id...load&gul=14

 

Very occassionally, by pure luck, having the amplifier gain at full will match the output of the previous unit but 90+% of the time it will be wrong and this will increase your noise and decrease your dynamic range.Gain structure is the most important and least understood factor in setting up sound systems.It is very common practice to set the amp gains at full and this is bad practice, read the link above and find out how to do it correctly, this will make your future life much easier.

[Dave SA]

David you are of course correct. But I am far too paranoid about some ''helpful'' person coming along and ''correcting'' the gain structure. ;) Perhaps I need to make some more friends :).

[David A]

you should set the gain structure, then mark everything with a paint pen, then put white gaffer tape over the controls and write on it, "I will kill anyone who touches these"

The "volume" controls on amplifiers are often not attenuators, they are someway into the circuit, as soon as you overdrive any part of your sound system from the input to your mixer to the input to your amp you will clip and nothing can unclip it, thus totally wasting the potential of your system, in a properly gained system every component is running just under that clip point at maximum output.Running your amp at full completely screws it up. and yes for some unknown reason it is totally common practice but no competent technical body has ever endorsed it as it is totally illogical.

[voodoo1967]

Sorry I have added the amp is a T-amp 2400 (thomann) and the mixer is a Yamaha 266c.

They had the amp on max , from the mixer - the instruments were panned left and so went to channel B , and the vocals panned right so went to channel A. The vocal speakers for channel A were rated at 4ohms each, so because they are in series - does this then make them 2ohms ? - I would have thought this may cause problems with the amp ?

 

It's a mates band and I was trying to figure out why they had it setup this way

[stevebunting]

http://www.gain.pe.kr/spboard/board.cgi?id...load&gul=14

 

Very occassionally, by pure luck, having the amplifier gain at full will match the output of the previous unit but 90+% of the time it will be wrong and this will increase your noise and decrease your dynamic range.Gain structure is the most important and least understood factor in setting up sound systems.It is very common practice to set the amp gains at full and this is bad practice, read the link above and find out how to do it correctly, this will make your future life much easier.

 

This is correct. Read the paper. This will tell you how to determine amplifier attenuation.

 

Having said that, most people run them wide open and control the gain from the DSP. Getting absolute gain structure correct is not as important as it was years ago when equipment was noisier, and you can sacrifice this last stage for the repeatability of getting all your amplifiers at the same level, which is more important. This is especially true for large systems and line arrays.

 

There may still be benefit to winding back the amplifier attenuation if you have especially noisy equipment, and if you find it helps, do it (according the the process for setting gain structure outlined in the paper).

 

The reason it's called attenuation rather than volume is that they control the input level, not the output. You can still use the whole output power of the amplifier with the attenuator turned down. Drive an amplifier to clip with the attenuation control at full, then turn the amplifier down by 6dB and the console output up by 6dB. You will still clip the amplifier and get the same output level. The difference comes when you have turned it down so much that you can no longer drive the amplifier to clip before the console clips (or other hardware in the way).

[pumphouse]

2 x 4 ohm loudspeakers in series = 8 ohms.

2 x 4 ohm loudspeakers in parallel = 2 ohms.

 

Are you sure they're actually series wired (this is a bit fiddly to do) and not just linked together using speakon connectors with an 'in' and 'out' on the first loudspeaker, as this is actually parallel wired.

 

Jason

[David A]

By running your amps at full you are forced to reduce gain somewhere else which raises your noise floor, not a problem with rock'n'roll but a major problem for theatre.Just setting everything right should not be such a big issue, but it is.