Amplifier Input Impedance

Stickman451, you are correct as is Jmcgrogan2. Its easy enough to design a solid state amplifier with a high impedance input (100K); they could even make it switchable if they wanted.

Although our preamps are vacuum tube, they can drive even 5K with ease (they are designed to drive 600 ohms). But this is unusual with tube preamps so for the most part its likely that this will continue to be part of the equipment matching conversation for the foreseeable future.

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?

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.

But the real question is, what kind of source impedance should an amplifier designer reasonably expect to see from the driving source? And in a world where only the wimpiest of preamplifiers have an issue with a 10K-50K load, how much extra cost and design effort is one willing to spend in order to satisfy a few oddball cases?

^^ This.

Audiolabyrinth, if you have an amplifier, tube or solid state, with a 100KOhm input impedance, any preamp tube or solid state will work with it.

The general rule of thumb is that the source have an output impedance that is no more than 1/10th of the value of the load.

But there is a caveat- if there is a coupling capacitor at the output of the preamp, the value of the cap will interact with the output impedance of the preamp- meaning that it may be considerably higher at 20Hz than it is at 1000Hz. For this reason I feel that the output impedance at 20Hz is an important value, as it can effect apparent bass response (FWIW our preamps have the same output impedance at 2Hz, 1KHz, 10KHz and 100KHz).

If the output impedance is higher at 20Hz than 1KHz, you may experience a lack of bass with amplifiers that have a low input impedance.