AMPLIFIER GAIN STRUCTURES
This is a subject that, on the face of it,
seems very simple but it is often mis-interpreted so it demands
further clarification.
In these days of small, high power loudspeaker
systems almost all designs use some form of "System Controller"
to get the most out of the driver/cabinet configuration.
In these systems the controller knows all about
the behaviour of the loudspeakers in the system and tells the
amplifiers how to "behave" to get the best results from
the system. (For more information, see the following section on
"Controller Myths").
However, optimum controller operation is achieved
only when amplifiers in the system are "gain matched"
together.
Most modern amplifiers now have a 26 or
32dB
voltage gain setting together with a 0.775V and 1.4V sensitivity
setting.
Do not confuse voltage gain with sensitivity
! This is very important !
The 26dB setting sets the voltage gain of the
amplifier. For example ;-
* Put a 0dB signal in at the input and you
will get 26dB (of voltage gain) at the output
* Put a –10dB signal in at the input and
you will get 16dB at the output (This is still 26dB of voltage
gain)
* Put a +3dB signal in at the input and you
will get 29dB at the output (Still a gain of 26dB).
So if you have 6 amplifiers all set to 26dB
of gain, an input of 0dB will produce the same 26dB of gain in
ALL of them. This is the sort of "synchronisation" that
produces the best results when using controllers.
If all your amplifiers have the same voltage
gain then it is possible to reduce the number of controllers in
the system by using one controller to feed multiple amplifiers.
Any increase or decrease in one amplifier is accurately repeated
by any other amplifier in the system as they will all track the
same.
Some manufacturers state 26dB gain is optimum
– some will state a different figure - say 32dB. The important point
is that all the amplifiers must be the same.
Consult your supplier to find out what is the
optimum setting for your controller.
QUESTION?
What happens if you use the 1.4V
setting on all the amplifiers? They all produce the same output
then – don’t they?
ANSWER
No, they don’t !
Here’s why....
The 26dB voltage gain setting sets the amount
of gain your amplifier has i.e. for the 26dB setting a measurement
from input to output will ALWAYS be 26dB no matter what the input
is and no matter what the rated output power of the amplifier
is. So, it follows that this setting is totally independent of
amplifier model or type (or manufacturer if the other manufacturer
offers a 26dB option).
Both the 0.775V and 1.4V setting do not set
gain. These settings are sensitivity settings.
These settings set the input level required
to get maximum output from the amplifier. Thus, if your amplifier
has a sensitivity of 0.775Volts and has an output power of 500
Watts, that’s what you’ll get out of it if you put 0.775
Volts in.
If you have 1000 Watt amp and its sensitivity
is 0.775 Volts, you put the same signal into it, you'll get 1000
Watts out – not 500.
This is what fools the controllers. If amplifiers
are all receiving the same inputs and the amplifiers are all putting
out different outputs, what’s the controller supposed to
do ? Which amplifier is set correctly ?
Using the voltage gain matching setting sorts
the problem. They will now all track together.
Note: Don’t forget that if you are using
an amp in bridge mode this adds an additional
6dB !
Most customers will not have access to the battery of test
equipment we have and we acknowledge that setting proper gain
structures can be difficult to achieve if your amplifier does
not have a voltage gain setting.
If you have an audio oscillator and a digital meter that has a
mVAC range, it is possible to calculate the gain by applying the
following formula :-
20*Log (Voltage OUT/Voltage IN)
Connect the audio oscillator to the input (pins
1 + 2) of the amplifier (short pin3 to ground). Using the meter,
measure across pins 1 and 2.
Adjust the oscillator so that you read 0.775VAC
across pins 1 and 2. This corresponds to a 0dBu input. Turn the
channel volume up all the way to maximum and using the meter measure
the (AC) voltage at the output terminals.
So, let’s say you measure 0.775V at the
input and 15.45V at the output.
Substituting the values in the equation:
20 log (15.45/0.775)=20 log (19.9350).
Gain therefore = 26dB.
Quite easy really isn’t it!
If you want to give your amplifier a specific
gain structure it’s possible to do this too :
Refer to the table below. This shows what voltage
you will need to measure off the output terminals of the amplifier
to give you the required input/output gain.
In all cases input voltage is assumed to be
0.775 VAC. The indicated voltages are near enough for you to set
your gain accordingly.
| Gain
DB |
Output
Voltage |
| 20 |
7.75 |
| 21 |
8.70 |
| 22 |
9.76 |
| 23 |
10.95 |
| 24 |
12.29 |
| 25 |
13.79 |
| 26 |
15.45 |
| 27 |
17.35 |
| 28 |
19.48 |
| 29 |
21.85 |
| 30 |
24.52 |
| 31 |
27.51 |
| 32 |
30.87 |
| 33 |
34.65 |
| 34 |
38.85 |
| 35 |
43.60 |
Put
your 0dB signal in (measured as above) and set the input attenuator
control (that’s the front panel level control) for the channel
to give you the required output voltage corresponding to the required
gain and that’s it!
If you have a method
of locking off the controls to prevent them being moved so much
the better, but using amplifiers adjusted in this way will give
good calibration against each other in "controller based" situations.
It
may be a little messy but it works !
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