Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Power Amp Positioning
References: <[email protected]> <[email protected]>
"Grant"  writes:

> Looking at the Pros&Cons of both situations 
> Amps by mixer [+]
>  -you can look at the signal lights to confirm that they are on and
> recieving an audio feed.

That's one pro of this setup. I have seen in many systems
this debug option used even when amplifiers are near speakers.
They can in some cases installed in place where their lights
are still visible to the people using mixer but out of the view
of the audience. 

>  -in the event of anything failing you can quickly replace a fuse 

That's true. In real case this kind of occurence is very rare.
And most often if the fuse fails, there also something else
wrong (shorted wire or broken amplifier stage) that usually
is not fixable during the show. In case of amplifier or
crossover failure those are easiler to repatch of they are
near you. But in practice the fact that this happens
is quite rare during the show and if it happens then it
might not be convient at that time to do that
(for example if the equipments are quite permanently installed
to equipment stack then replacing or repatching something on
the run might not be that easy is equipment crowded mixing
place).

> Amps by mixer [-] 
>  -the VERY long lengths of speaker cable will probably lead to interferance,

Long speaker wires will more effectively pick up interferences.
That's true. Usually the more severe problem in very long speaker
wires is thew wire resistance, which affects (if too large)
the frequnecy response of the speaker and controlling of the bass element.
If this has any noticable effect depens on the system and the needs
put to it. Long wires also loose some speaker power.
If you use thick enough speaker wire you can go up to few tens of meters.


For frequency response change here is a table of wire
lenghts which keep the frequency response error in less than
0.5 dB for different speaker impedances (impedances in ohms,
wire thickness in mm^2):

Speaker 8   6   4   3   2   1   ohm
Wire  
2x.75   10  8   5   4   2.5 1.3 m
2x1.0   14  10  7   5   3.5 1.7 m
2x1.5   21  16  10  8   5   2.6 m
2x2.5   35  26  17  13  8.5 4.4 m
2x4.0   56  42  28  21  14  7   m           

>  -and a real possibility of the amplifiers overheating in trying to drive an
> overly excessive load.

Adding the speaker wire to the chanin will increase the total
resistance of the system, so it basically makes the load to take
less power from the amplifier, so makes the load easier.

Only ting in the long cable which makes the load harder for
an amplifier is possible wire cable capacitance. On normal
wire pair type of speaker cable this is neglegtable,
but if the speaker cable is shielded wire (nor recommended)
then in long runs the large capacitance can become a problem
for an amplifier.

> Amps by Speakers [+]
>  -Much cleaner signal getting to speakers

That's true. You can avoid almost all of the effects of the speaker cable.
The standard approarch in practically all touring audio systems and
many fixed stage audio systems I know is that the amplifiers
are fitted to one or two amplifier racks near the main speakers.
Typically in many systems there is one amplifier rack on either
side of the stage and both left and right speakers are wired
to it. In very high power systems where one amplifier rack is not
enough and main spakers are far from each other (some outside
occasions) the typical approach is that the both left and right side
speaker stcs have their own amplifier stack. 

The ultimate solution for avoiding the speaker wires is to use
active speakers which have built-in amplifiers in them,
so each speaker has it's own amplifier.

For operation convience in might be best for any small to medium
side systems to install all amplifiers in one place and wire
the main speakers from there. 

>  -NO distortion on signal cable getting to amp from mixer (assuming your
> amps do take a ballanced line feed Fred)

If your mixer and amplifier have properly designed balanced
inputs and output you can transfer the signal very well from
the mixer to the amplifiers for long distances. Distances
up to few hindred meters are no problem (I have operated
systems with wire run up to more than 100 meters).

If you encounter problematic equipments, then tricks like
line isolation transformers or ground lift switches on
audio shield are sometimes needed.

> Amps by Speakers [-]
>  -You have to find somewhere to actually put them, out of the way that is.

That's one thing to considers. Fortunately even few kilowatts of
amplifier power installed to an equipment rack with crossovers
and equalizers is just somethign like 0.5 x 0.5 x 0.7 meters in
size (like typical touring racks with around 5 kW of amplifiers in each...). 

The operation of the amplifier stack is usually just by turning it
on before the show and turning it off after the show. With a reliably
built and wired speaker/amplifier system there is usually not
need to touch it during the show.
 
-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/