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June 17, 2006 - 1:49pm
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Amplifier Input Sensitivity/Impedence Specifications
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Well, I have read on audiokarma that you can calculate maximum power of your amplifier based on the input sensitivity of the amp and output of the component with the following formula:
20*log(V1/V2)=dB
V1=component output voltage
V2=amp input sensitivity
So, my CD player output is 2000mV and the amp's input sensitiviy is 200mV.
20*log(2000/200)=20dB
So, -20dB on the volume should be maximum output of the amplifier with this component. In other words, maximum volume should be obtained at -20dB on the volume scale.
I tried this and it was a complete failure. dB continued to increase all the way up to near 0dB. So, I don't know what this guy is actually getting at:
http://audiokarma.org/forums/showthread.php?t=58829
As you have mentioned, input sensitivity and input impedence are 2 totally different, and actually unrelated. Lets take a closer look at each:
1. Input Sensitivity:
It is the minimum input VOLTAGE required at that point ( eg the Aux input of your integrated amplifier ) to produce the Full ( Rated ) output.
The ratio of the Output Voltage to the Input voltage is the GAIN of that amplifier.
The formula that you have quated provides the GAIN in dB of the device.
The Gain in itself is not a critical parameter, as long as there is enough of Gain to ensure that the device can be driven to its full volume.
Let me illustrate with an example :
a. Your CD palyer with an output of 2000 mV ( max ) will easily drive an amp which has an input sensitivity of 200mV. In fact the volume control will be way down under normal listning conditions, as only a fraction of the full 2000mV output will be required by the amplifier.
b. However, if you directly connect a Moving Magnet Cartridge from your turntable, to the same amplifier input, it will hardly be audible, even with the volume control all the way up.
That is because the Cartridge provides only 5mV ( typically ) max, and the amplifier needs 200mV for full output.
So you will need a more sensitive amplifier.... the Phono pre amp between your cartridge and the input of yr amplifier, plays that role.
IMPEDENCE:
This is loosely speaking, the resistance.
This indicates the Input CURRENT that the amp will draw from the source, like the CD player or cartridge.
As long as the source can deliver what the amp input demands, you are OK. The 2 components are then considered "MATCHED" for each other.
The ideal source component ( eg a CD player ) should have a Huge output voltage, and zero source impedence, so that it can deliver an infinite current if so demanded.
An ideal amplifier input should be VERY sensitive, and have an Infinitely large input impedence, so that it draws almost zero current from the source, and stresses / loads the source minimally.
As in ALL engineering matters, there are compromises and limitations, and there in turn lead to consequences in the sound quality.
There are ofcourse several schools of thought on the "Best" specification.