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Amplifier input/output impedance |
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Chivas
Senior Member Joined: 09 Sep 2008 Status: Offline Points: 110 |
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Posted: 16 Sep 2009 at 2:16pm |
Okay, here goes...let's pray I am not hitting any open nerves with my very naive (and perhaps stupid) question....refer to AC/DC coupling thread.....
The caveat:
I am trying to educate myself in the world of hi-fi systems and it started with a love for music, so hopefully the right reason, the rest I find bloody interesting and knowledge will hopefully help me to get closer to the music that I so love. I am not questioning any designs!
PS. (tg, I still owe you a review on my newly acquired vinyl rig with GSE2, I know, my apologies - on the horizon)
I gather that a high input inpedance is desirable for an amp (any) in order not to load (ito current) the source (any) based on P=I^2*R (i.e. , permitting that you have sufficient gain (V) (P=V^2/R) from the source. Right?
Then why is a low output impedance desirable if a low input impedance is desirable for the next stage (e.g. speakers). Are the output and input impedances of two subsequent stages not in series?
I understand that impedance includes (capacitance and inductance (C&L)), so high impedance (magnitude - R) makes it easier to drive a load (current wise) but this also includes higher C&L which will mess with the phase? Guess the trick is to find a medium with high resistance and low reactance (imaginery part of impedance). So according to this logic, high output impedance is good...?
PS. I noticed that GS amps all have a relative high output impedance?
Any light on the subject would be greatly appreciated.
Chivas
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Chivas
Senior Member Joined: 09 Sep 2008 Status: Offline Points: 110 |
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Correction:
[Then why is a low output impedance desirable if a high input impedance is desirable for the next stage (e.g. speakers). ]
Chivas
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tg [RIP]
Moderator Group Joined: 19 Jan 2008 Location: Sydney Status: Offline Points: 1866 |
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Go play some records and stop making my head hurt
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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take a look at http://en.wikipedia.org/wiki/Impedance_matching
which gives a good explanation apart from where it says "With modern audio electronics, impedance matching degrades audio performance" which is a misleading generalisation. then take a look at http://en.wikipedia.org/wiki/Maximum_power_theorem This gives one of the best explanations IMO, and it saves me going to the lengths of explaining enabling me to get more work done... Both address transmission lines to a degree, but the assumption is made that reflections only count when the cable is at a length of one or multiples of the signal wavelength. However, in high fidelity (or lifelike) audio reproduction we are all seeking perfection approaching the infinitely small which tends or seems to reveal further depths to the music. I therefore think that at much less than a wavelength there must be either partial reflections, or if that argument is unsound then how about the simple fact that music consists of complex waveforms that are not sine-waves, but can be constructed of sine-waves, and in such construction some of sine frequencies are incredibly high, and therefore even a short cable could be at a wavelength or more. That is my reason for driving headphone cables with a resistive source pitched around the mid-point of headphone transducer impedances, such that reflections are possibly reduced, even so, not knowing the cable characteristic to be used by the consumer, some part of this has an effect. I also reinforce it by providing a secondary negative feedback loop to correct for the fluctuating impedance (which many including me have measured - but manufacturers response charts tend to be "smoothed out") every octave or part of, above upper mid frequencies. In effect the output impedance is actually reduced through negative feedback (a known fact) except where it is ironing out the response - and these frequencies are nearer those where reflections can take place anyway. I will end now with an observation of Peter Belt theory from a hi-fi magazine in the late 70's or early 80's (I don't recall exactly when). It taught me a lot about "damping factor"... "Prof" Belt reckoned that a short across the pins of a mains plug on a vacuum cleaner would stop the motor "micro-moving" (even if not in the same room) so as not to upset the music... The special wire was reef knotted for RF reasons I assume (it made it inductive at RF) and there were croc clips to attach it to the prongs of the plug. It could only work if the vacuum cleaner switch was set to the on position (think about it), when the damping factor was maximised such that it prevented the motor turning by microscopic amounts. By that argument and the damping factor argument in general, it should have been impossible to turn the motor. However, it was possible to turn the motor.... And obviously there was some resistance to turning because that's what you get with electric motors especially on vacuum cleaners, except that I could feel no difference when turning the motor with the short in place or not. The short represented a zero impedance source. A speaker is a motor too. It was around that time I stopped worrying about damping factor. Edited by Graham Slee - 17 Sep 2009 at 2:04am |
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Chivas
Senior Member Joined: 09 Sep 2008 Status: Offline Points: 110 |
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Hi Graham
Thanks for your reply and the links. I will print these articles out and follow tg's advice and listen to a few records, study them and most probably come up with more questions....
Good times!!
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