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4th October 2012, 07:22 PM  #11 
diyAudio Member
Join Date: Jun 2005
Location: VA

Measuring preamp output impedance
If you want to drive unity buffer amps as you indicate, consider frequent contributor SY's ImPasse preamp, which was designed for exactly this kind of thing. SYclotron.com for more details. Also boards available from Company Store.

4th October 2012, 07:27 PM  #12  
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Join Date: Jan 2008

Quote:
Last edited by kacernator; 4th October 2012 at 07:30 PM. 

4th October 2012, 10:03 PM  #13 
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Join Date: Jun 2007
Location: Holt, Norfolk

The basic problem here is that the quoted 500 ohm output impedance is the small signal output impedance which is derived assuming the signal level is very small. As soon as you get to 2V or 5V rms you are no longer in the small signal regime and the 500 ohms therefore no longer applies. As you have seen, at higher signal levels the output impedance rises a lot.
Cheers Ian 
5th October 2012, 07:57 AM  #14 
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Join Date: Apr 2006
Location: Holland

So, output impedance is varying with output voltage? Hmmm.
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jaap 
5th October 2012, 08:38 AM  #15 
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Join Date: Feb 2010

Kacernator, can you show your schematics ?
I would like to simulate a little. 
5th October 2012, 11:59 AM  #16 
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Join Date: Jan 2008

Why should output impedance very with output voltage? I donīt get it.
Here is the schematics, it is just plain aikido preamplifier. 
5th October 2012, 12:16 PM  #17 
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Join Date: May 2007

To a first approximation output impedance will be 1/gm. gm varies with current, and therefore will vary with signal. All circuits have this problem to some extent. Good design minimises it, but cannot eliminate it, therefore output impedance will vary with signal.
An alternative point of view is that AC impedance is a smallsignal phenomenon (strictly, for infinitesimally small signals only) and is undefined for large signals where nonlinearity is an issue. 
5th October 2012, 12:46 PM  #18 
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Join Date: Aug 2009
Location: Johnson City, TN

The variation in gm with current is also the cause of distortion.

5th October 2012, 01:09 PM  #19 
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Join Date: Apr 2006
Location: Holland

My reasoning was quite simple: if two values exist for Rout (else than the sign), measured at differing operating points, the only explication can be you're measuring a variable and not a constant.
What I understand from Wikipedia the small signal model is an analysis technique while we (ought to) look at real world voltages. No 500 ohm Rout then. Dave, is bandwith a complication for Rout? It's a non linear amplifier property as wel, diminishing with output power.
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jaap 
5th October 2012, 03:28 PM  #20 
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Join Date: May 2007

There are not two values for Rout but an infinite set of values. The distinction between variables and constants is a fuzzy one. The only things in electronics which are really constant are things like the charge on an electron. However, for a welldesigned circuit Rout will be roughly constant so we can calculate using it. It will, of course, vary with signal level and frequency and won't necessarily be a pure resistance.
The small signal model is fine provided you have small signals. Part of the art of engineering is knowing which approximation to use, and not complaining that 'theory is wrong/useless' when the wrong approximation has been used instead of the right approximation. It is not the fault of the approximation if an engineer naively uses it outside its domain of applicability. 
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