Amplifier Input Impedance

http://www.audioholics.com/audio-amplif

The above site agrees with some above comments from Bombaywalla and others. To summarize, according to the source, high input impedence in SS causes high voltage gain which in turn causes noise. It can also cause bandwidth to decrease. And finally it can introduce DC offset.

My head is still spinning from trying to absorb what I read in 15 websites which destroyed my misconceptions.
But apparently high input impedences can cause more harm than good.

OK. So are not some loudspeakers designed with very low impedences so that they will audition better because they are louder? (and I understand that this is not the same as sensitivity.) Does that not relate to input impedence in amplifiers?

Not really. The fact of the matter is that all amplifiers have less distortion driving higher impedance speakers. Some speakers are lower impedance as some solid state amps will make more power (sound quality is being traded off for sound pressure in this case). Many speaker designers don't realize that though. And this **is** in fact what we are talking about with the term 'sensitivity'. But none of that has anything to do with the input impedance of an amplifier.

www.audioholics

The above site agrees with some above comments from Bombaywalla and others. To summarize, according to the source, high input impedence in SS causes high voltage gain which in turn causes noise. It can also cause bandwidth to decrease. And finally it can introduce DC offset.

This is mostly bogus! Input impedance has 100% nothing to do with gain. A high input impedance does not have to have anything to do with noise either unless the design and execution is shoddy. We get 300KHz with 100Kohm input impedance, clearly the bandwidth thing is problematic. Think about a 6AU6 vacuum tube which is very high impedance but can operate easily at 10MHz. Finally, input impedance has nothing to do at all with DC offsets, and cannot introduce it.

I would stay off that website as it is a source of misinformation, and that is being kind.

^Once again I disagree with some of the again mentioned assertions.

Thank you, Atmasphere. A crash course in amplfier design is clearly not going to work for me.

The input loading resistor to ground cannot effect gain.
But in solid state bi-polar input dc coupled amplifiers, if it's raised too much with an open circuit input will effect the amount of DC offset seen at the speaker terminals, which could be out of reach for any dc servo's to correct. But once something is plugged into the input, (eg preamp dac etc) the output impedance of that device becomes the input impedance of the amp, and all goes back to how it was with a smaller input loading resistor.

(as for what varies gain in an amp, it's the feedback loop and values of resistors used that raise and lower the feedback.)

So in a tube or solid state (with fet input) you can raise the input loading resistor.

But in a solid state with bi-polar input transistors you can also raise it within reason 68k or even 100k, but you should never turn it on without anything pluged into the input. Otherwise you run the risk of lots of dc going to the speakers.

Cheers George