Quote:
Originally Posted by Josh
This is probably more a question for Big Daddy then anyone, but when it says that Input Sensitivity is 1.4V, and there is gain control (which affects sensitivity), what "level" is that input sensitivity rated? Zero gain? Full gain? And if you increase the gain, does that increase the Sensitivity (to say, 3V) or decrease it?
This all links back to my Passive Preamp thread  |
The simple answer to your question is yes with a big caveat. When you make the mistake of asking Big Daddy a simple question, he will give you a full page answer.
Input sensitivity is the voltage required by the amplifier to amplify a signal without clipping at its rated capacity. An amplifier takes input signals and makes them larger. How much larger depends on the input sensitivity and the maximum power output of the amplifier. You can turn the input sensitivity level all the way up, but that does not make the amplifier generate more power than its maximum. It just gets to that maximum level with a smaller input voltage.
Amplifier level/gain controls are input sensitivity controls. They are not power controls. They have absolutely nothing to do with output power. They do not change the available output power. They are sensitivity controls. They determine what input level will cause the amplifier to produce full power.
In pro audio the 1.2V (+4dBu) figure has a significance. Most professional audio equipment is designed around a nominal operating level of +4dBu. For example, most consoles will output about 1.2 volts when the meter reads 0dB.
Think of an amplifier's sensitivity the same way you think of a speaker's sensitivity. A speaker with high sensitivity doesn't need many watts to play loudly, whereas a speaker with low sensitivty needs more watts to play at the same level. Using the same analogy, a high-sensitivity amplifier does not need much of a signal from the preamp to play at full power, whereas a low-sensitivity amplifier needs more preamp drive to reach the same power.
Let's assume you want to play a song at a reasonable level. With a high-sensitivity amplifier, you might only need to set your preamp's volume control to 9 o'clock. With a low-sensitivity amplifier, you will need to set the volume control on the preamp to maybe 12 o'clock to get the same level of loudness.
Basically, you want to set the amplifier's gain so the preamp/processor is just below its limit when you are producing the maximum power you want to achieve. If the amplifier level is set higher, you need to turn the preamp/processor down, which increases the noise level. If the amplifier level is set too low, you will not be able to achieve the desired volume from the speakers without the preamp/processor running into distortion.
How to Set an Amplifier's Sensitivity Level
You want the maximum input signal to cause full power at the amplifier.
To set the amplifier controls to achieve full power with your maximum input signal use the following procedure. If your system uses active crossovers, for the moment, set all the crossover output level controls to maximum.
- Disconnect the speakers. You do not want to accidentally damage them.
- Set preamp output level to maximum. For practical reasons, it may be better to increase the volume to around 75%.
- Turn the pro amplifier's input sensitivity controls all the way down to minimum level (least sensitivity or off).
Note: Even at minimum sensitivity, it is possible that some amplifiers may produce some level of sound.
- Slowly turn the sensitivity control on the amplifier up until clipping just begins. Stop.
- This is the maximum possible power output with the maximum possible input signal. Because there is never a bigger input signal, this setting guarantees the amplifier cannot clip. You have indirectly set the dynamic range of your system. However, it is still possible that at this level, you may damage your speakers.
- Do not adjust the sensitivity level on the amplifier anymore.
- On the preamp, turn volume level control all the way down.
- Connect all the speakers.
- Play some music and slowly turn the volume up on the preamp.
Moral of the Story: Amplifier input sensitivity controls do not change the available output power. They only change the input level required to produce full output power.
It may be helpful to review the following section from
A Guide to Amplifiers.
Quote:
Input Sensitivity of an Amplifier:
A pre-amplifier has two basic functions:
1. Allow input switching.
2. To achieve synergy between the source(s) and the amplifier(s).
Input switching is well understood by most users and is rather elementary. The second function is not quite as easy for most users to understand. To select an appropriate preamp, you must know two things:
1. The output voltage of the source. Most CD players, for example, have an output voltage of 2 volts.
2. The input sensitivity of the amplifier. Most amplifiers have an input sensitivity of around 1 volt.
Output Voltage: The output voltage of the source is normally a constant level unless the source has a variable output knob. This output voltage (e.g., 2 volts of CD music) drives the input stage of the preamp which in turn drives the input stage of the amplifier.
Input Sensitivity: The input sensitivity of an amplifier is defined as how many volts are required to bring the amplifier to full power. Any amount of voltage beyond that number will force the amplifier to try to output more power than it is capable of and therefore results in clipping.
So we can conclude that an important job of the preamp is to control the voltage from the source to the amplifier. This is done by adjusting the volume control. When the preamp volume is all the way down, the output signal is attenuated completely to zero volts and no sound. As the volume knob is turned up, the voltage increases as does the sound generated from the amplifier. Assuming the output voltage of the source is more than the input sensitivity of the amplifier, the ideal working range on a volume control should be where the preamp would not add any voltage gain to its input signal beyond the amplifier’s input sensitivity.
When do we need gain in a preamp?
When the input sensitivity of the amplifier is above the output voltage of the source. It is possible for some amplifiers to need up to 5 volts to bring them to full power. On the other hand, it is possible for some to only need 1/2 volt to come to full power. It is also possible for some modified CD players or Digital/Analog Converters to have less then 2 volts.
As an example, assume an amplifier with an input sensitivity of 2 volts is connected to a source with an output voltage of 1 volts. We will definitely need a preamp with some gain. Otherwise, it will not be possible to play the amplifier as loud as it it is capable of. Even if the speakers are very efficient, the music will lack dynamics and weight at low listening levels. Turntables have very low output sensitivity. That is the main reason why you need a built-in or external phono stage with turntables,
Example: Let’s say we have an 8-ohm loudspeaker that has a sensitivity of 87dB for one watt at one meter. Assume we are trying to achieve a desired maximum peak sound pressure level of 105dB. If the signal source has an output voltage of 1 volt, how much amplifier gain is required?
Remember that in order to increase the level of sound by 3dB, we need to double the amplifier’s power.
SPL, Watts
87dB, 1
90dB, 2
93dB, 4
96dB, 8
99dB, 16
102dB, 32
105dB, 64
Therefore, we need 64 watts to achieve a peak SPL of 105dB. From Ohm’s law,
Power = V^2 / R
64 = V^2 / 8, and V = 23 volts
So the amplifier needs to provide a voltage gain of 23 times which is approximately 27dBv. Please note that 0 dBv is defined as Vo = 1 volt, and
dBv = 20 . LOG(V/Vo) = 20 x LOG(23/1) = 27 dBv.
Some amplifiers have knobs that control the amplifier’s input sensitivity. When the knob is turned clockwise, the sensitivity will increase. Turning the control counter-clockwise will decrease sensitivity. This control is not a volume control for the amplifier. The amplifier can be driven to full power with a wide range of signal levels. A low level signal will require increased sensitivity for full power. A high level signal will require decreased sensitivity.
|