Hi Terry,
 TS> Giving summary impressions first, it
 TS> sounds like you read some silly marketing
 TS> BS, with some slightly confused
 TS> suggestions that aren't quite right outside
 TS> being helpful simplified procedures
 TS> within some possible context you didn't
 TS> describe as being detailed in what you
 TS> read.  Overall, Bonnie's comments on
 TS> balancing sound output are more realistic.
Actually what I described was from a booklet from Apogee called sound
reinforcement for worship, written by the Director of education and
senior systems engineer of Apogee, Patrick Price.
The section on matching all amplifier gains in a system was something I
had never seen before, and that prompted wanting a second opinion from
someone not wanting to sell the church their sound system. :) I'm not
in charge of the church sound system, but I am learning about it.
The exact text is as follows:
Amplifier Gain - Amplifier spec sheets usually list a "gain" or
"voltage gain" specification. Gain is usually expressed in decibles
(such as +32db) or as a multiplier (such as 40 times). Given an
amplifier with a gain of 32db or 40X, this means that the amplifier
signal is amplified by 32db or multiplied by a factor of 40.
It is important for you to know the gain of all your power amplifiers
in order to properly "balance" your sytem at the offset.
If you intermix amps with different power ratings or from different
manufactures within your system you are advised to check to make sure
they all provide the same amount of gain. I should Explain that all
power amps run at some fixed gain, regardless of where you set the
input level control. It all sounds confusing but the explanation is
quit simple. All the amplifier level control does is shunt a portion of
the input signal to ground. For example, if you connect a 1 volt signal
to an amplifier with a gain of 40X you will get 40 volts out (assuming
the amp can deliver that much power). However if you leave the same 1
volt signal connected and turn the amplifier's gain control half way
down, you will have reduced the 1 volt signal down to 1/2 volt. But the
half a volt will still be amplified by a factor of 40, and you will get
20 volts out. The amps's gaian is still 40, but you have shunted half
the input signal to ground.
The point you should be concerned with is that if you use an assortment
of power amps in your system, and your high frequencies are amplified
by a factor of 40X, your mids are amplified by a factor of 45X, and
your lows are amplified by a factor of 35X, The high/mid/low balance
through the speakers will be severely messed up.
To properly calibrate a sytem using amps with different gains you must
use the gain controls on the amps to match their output levels (given
the same input) to that of the amp(s) with the lowest specified gain.
Start by putting a 0.5 volt 1kHz sine wave signal into each amp (no
speakers connected), and use an AC volt meter to measure the output
voltage with the gain controls all the way up (write each output
voltage down as you go). When your done, mark the amp(s) with the
lowest output voltage (meaning lowest gain) as your "reference" amp.
Now go through the remaining amps once again using the same 0.5 volt
input signal and adjust their gain controls so that the output voltage
matches that of the reference amp.
 ----
I attempted to say the same thing in less words in my original post, so
the question remains is the above valid or hype? I can't see how not
including the speaker efficiencies in the overall balance makes sense.
... dschmoll@nyx.net (David Schmoll)
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