Amplifier Input Impedance

02-10-14: Cobra2
Low input impedance is an advantage regarding noise...

I might be missing something here but in an audio power amp we are considering only voltage gain & not power gain. So, how does input impedance factor into noise when we are considering only voltage gain (in an audio power amp)?

02-10-14: Arnettpartners
I should keep my mouth shut because I'm ignorant, but I have read that it's hard to design a SS amp with high input impedence and still get enough power or ...

again, I could be missing something - how does input impedance figure into the voltage gain of an audio power amp? If we were talking power gain then I could see input impedance figuring into the overall (power) gain calculation.

02-10-14: Atmasphere
Stickman451, you are correct as is Jmcgrogan2. Its easy enough to design a solid state amplifier with a high impedance input (100K);...

so, what sets the input impedance of an audio power amp?
My understanding was that an audio power amp's input impedance could be increased by using global negative feedback (which is a no-no 'coz of all the other issues this brings with it) & by selecting the type of semiconductor device used for audio power amplification. If the designer uses BJTs the input impedance is going to be lower than if the designer used a MOSFET or even a JFET. What are the ways to make it arbitrarily high in a s.s. amp?
In tube amps this might be different as the designer can put a resistor of arbitrary large value in series with the tube input. Is that why you say that the amp input impedance could be arbitrarily high, Ralph/Atma-sphere??

How come Almarg hasn't spotted this thread as yet?? ;-)

02-11-14: Bombaywalla
How come Almarg hasn't spotted this thread as yet?? ;-)

Even the great and powerful Al needs a little vacation time every now and then. ;)

02-11-14: Bombaywalla
How come Almarg hasn't spotted this thread as yet?? ;-)

02-11-14: Jmcgrogan2
Even the great and powerful Al needs a little vacation time every now and then. ;)

:-). Thanks, guys. No vacation time needed, though, as I'm always on vacation (i.e., happily retired).

Actually, I had spotted the thread. But having never designed an audio power amplifier, I can't speak knowledgeably about what the tradeoffs would be if a solid state one were designed with a high input impedance. Certainly it's readily doable, but I don't have a good feel for what the inevitable tradeoffs would be.

In addition to those tradeoffs, of course, as John and others have noted I don't doubt that in many cases a significant factor is a lack of motivation to provide compatibility with tube preamps.

Best regards,
-- Al

Input impedance has nothing to do with gain.

You can have a high input impedance on a solid state amp by using FET devices at the input.

Large solid state amps with a low input impedance often do have a lot of gain as they are designed for use with loudspeakers that need a lot of power. You need gain for that- 30 db might barely be enough. Conversely if you are driving a horn speaker you don't need nearly so much gain so many SETs don't have all that much- sometimes 15 db is plenty.

So there is a rough correlation but it is not based on input impedance => gain.

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?

I'm not arguing with an audio engineer--just asking?