From: lstowell@pyrnova.mis.pyramid.com (Lon Stowell)
Newsgroups: rec.audio
Subject: Re: Impedence and Loudness control - explain, please...
Date: 31 Dec 92 17:15:38 GMT
Reply-To: lstowell@pyrnova.pyramid.com.pyramid.com (Lon Stowell)
Organization: Pyramid Technology Corporation

In article <1992Dec31.043409.8133@dunix.drake.edu> kjt001@dunix.drake.edu (Devil Bunny!) writes:
>I have a couple of things I'm curious about.  I'm looking for relatively
>basic definitions, but not the kind of definition you'd have to give
>your grandparents....
>
>
>1>  What exactly is impedence?  I know it has to do with speaker
>resistance, but how do amplifiers react to it? What are common
>impedences?  I know that my Infinity speakers are rated at 6 ohms, but
>what does that mean, exactly?  Is it better to have an amp hooked up for
>a higher or lower impedence than the speakers if you can't match it
>exactly?  What are the effects of either one?

  a. As you've guessed, impedance is opposition to the flow of  
     current.  The main difference between impedance and resistance
     is that resistance is constant over the frequency band.
     Impedance is a bit more complex, if you'll pardon the term.
     Impedance is the combination of resistance, capacitance, and
     inductance.                            

     In a purely resistive circuit, the current and voltage are
     always in phase...changes in one are directly correlated with
     changes in the other. This is known as resistive impedance.    

     In a capacitive or inductive circuit, the voltage and current
     lead or lag each other by an amount known as the phase angle.
     This phase angle will change with frequency, as the effective
     opposition to current flow and/or voltage changes is not
     constant with frequency.   

     Both inductive and capacitive opposition to current/voltage
     changes are termed "reactive" impedance.  A circuit in which
     the voltage and current are not in phase is termed "reactive".

     The total impedance of a circuit or circuit element (say a
     speaker) is the vector sum of the reactive impedances
     (inductive and capacitive impedances have opposite signs) and
     resistive impedances......and it is ALWAYS frequency dependent.
     A circuit can be characterized by the phase angle, or how far
     the voltage curve for a sine wave leads or lags the current
     curve for that sine wave.  If the phase angle is 0 degrees, at
     THAT frequency the circuit acts as a pure resistor.  
     The POWER in a circuit depends on the phase angle of the
     circuit and the impedance.   

     Practical speakers always have reactive impedance as well as
     resistive impedance.  The phase angle of the speaker will vary
     with frequency...some speakers more than others.  Some of the
     more exotic speaker types have either very low ( 1 ohm or less)
     and/or very reactive impedances.  

     Some amplifiers are designed to produce their outputs into
     purely resistive 8 ohm loads.  They may oscillate or even self
     destruct if presented with loads well under 8 ohms OR loads
     with high phase angle.  Good amplifiers can drive reactive
     loads without harm.  

     Speaker manufacturer's rate their impedance in different ways.
     Some take the safe approach and rate the speaker at the minimum
     impedance across the audio band, most tend to give a rating
     for a weighted average across the bass region...where the most
     power is consumed.  On the more common brands, the crossovers,
     etc. tend to keep the impedance more or less resistive in the
     upper frequencies anyway...but there are a few with highly
     capacitive impedances in the tweeters that some [IMHO poorly
     designed] amplifiers cannot tolerate.  
     cannot tolerate.                 



>
>2>  Is the loudness control on receivers anything more than a bass
>booster?
>
>
  Depends on the receiver.  Some boost both the bass and treble.
  Some actually vary the amount of boost in inverse proportion to
  volume level.  Whether any of them are worth a darn is a religious
  flame-bait issue.

    
From: kirk@hpcc01.corp.hp.com (Kirk Lindstrom)
Date: Wed, 23 Dec 1992 19:07:23 GMT
Subject: Re: rec.audio FAQ (part 3/3)
Organization: Shredding the water of SF Bay, HP-OCD