How much should be it the Zin of a Power Amplifier?

[fotios]

With regret i discovered that a lot few here deal in really with the DIY. Isn't this the aim of this web site? Or i have perceived error? This means that vast majority or has bought ready appliances what has damage and searches how him it repairs or it deals with theories without it applies something in the practice through them. I place this question for the Zin and wait for answers and opinions. With regard to me, I have already ready my answer as experimented a long time with this subject. Thus I decided to make for a time the opposite from usual. I leave to be exposed first you, and afterwards I comment your opinions in spite of search 1.500.000 posts in all site. Be careful because it is so much critical subject as the output of amplifier. She is the one from the two doors of contacting him with the outside world.

[anatech]

Hi fotios,
I like to see a high input impedance unless I have a noise issue o deal with. So a 250K input Z is okay by me.

-Chris

[EC8010]

1M.

[anatech]

Hi EC8010,
Sure, I'll buy 1 Meg.

-Chris

[unclejed613]

anywhere from 10k on up works just fine for me. it's good to have a predictable value that you can base calculations on. you will probably notice that many amps have a 10k, 20k, 47k, or 100k resistor across their inputs. many years ago, when audio engineering was influenced by telephone system design, there were many amps on the market with 600 ohm inputs, but for the most part, impedance matching has been replaced by voltage matching. most preamps and other line level devices have an output impedance of around 1k, and most devices such as amps have very high input impedance, so there is not a lot of voltage loss in the cables.

[lineup]

10.000 Ohms.

If this is too low for CD-player output (or other intended source)
then I can accept 22.000 Ohms. But not any higher


When comes to using sources with weak outputs,
that require higher input impedance
than 47 kohm
I strongely recommend:

The use of a good pre-amplifier or a good pre-buffer.


lineup
Lineup Audiko Lab

[lumanauw]

How to calculate Zout?

I wanted to know what is the output impedance of a common emitor stage without any resistors referenced to ground or rails.

What is the output impedance of collector of VAS stage?

Also current mirrors, what is the output impedance of the collector of current mirror (that is used for differential pair load)?

[DSP_Geek]

Originally posted by lineup
10.000 Ohms.

If this is too low for CD-player output (or other intended source)
then I can accept 22.000 Ohms. But not any higher

This is the advantage of building your own preamp: you can drive whatever amplifier you like. I'm setting the amp input Z between 2k and 3.3 k, which is high enough for an opamp to be able to drive it without too many problems. I like lower Z than usual because some subtle cable variations seem to go away at lower impedances.

[Piercarlo]

Originally posted by lumanauw
How to calculate Zout?

I wanted to know what is the output impedance of a common emitor stage without any resistors referenced to ground or rails.

What is the output impedance of collector of VAS stage?

Also current mirrors, what is the output impedance of the collector of current mirror (that is used for differential pair load)?

Look on web for a tutorial about H parameters and their use for calculating common emitter features (gain, impedances and so on.

Alternatively look for a tutorial about Giacoletto Equivalent Circuit, which usually introduce to the internal "machinery" of an active device (mainly bipolar devices but, with some change in parameter, this kind of circuit may be useful for any three terminal active device, bipolar, FET, vacuum triode...)

Hi
Piercarlo

[Piercarlo]

About the original question:

The optimum input impedance of an amplifier is that maximum value which still remain resistive and linear on the whole bandwidth and on whole power escursion for which the power amplifier is designed...

Hi
Piercarlo