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Does a resistor really have a "broad band impedance" or is more complicated?
Old 1st January 2011
  #1
Gear Maniac
 
mister sunshine's Avatar
 

Does a resistor really have a "broad band impedance" or is more complicated?

I've been wondering about this for a long time.

When someone substitutes a resistor with the intention of modifying impedance I wonder how the two phenomena relate.

I understand that impedance must be specified at some frequency... or averaged over a range of frequencies.

I have often wondered how a resistor reacts across the audio frequency spectrum.

Is it correct to think that a resistor is truly the equivalent of broad band or flat response across the audio range or are there variances to response at various places along the spectrum.

I'd appreciate any insights you can share.


Thanks.

best regards,
mike
Old 1st January 2011
  #2
Lives for gear
 
DaVogi's Avatar
Does a resistor really have a "broad band impedance" or is more complicated?

the audible frequency range is very small if you compare it to other, much broader and higher ranges used in high frequency technics.
Old 1st January 2011
  #3
Gear Addict
 

There should be no difference in impedance over the audio band for resistors.
The only situation where impedance changes with frequency is; if you're feeding radio frequencies through wire wound resistors where, the inductance of the resistor could be high enough to affect impedance.
Old 1st January 2011
  #4
Lives for gear
 

Hi
Resistors do have 'reactance' which will give them a complex response however for most resistor types and values the effect of this is minimal over the expected 'audio' band.
It is of course quantified in the specifications for each resistor and can be reviewed by looking at the info probably on the web but certainly by contacting the manufacturer.
The upshot is that roughly speaking most resistors except perhaps some wirewound types will have minimal 'issues' for audio however you can happily put the expected inductance and even representative capacitive values into circuit modelling software (spice or whatever) and 'work it out'. This will usually reveal practically no response abberations of statistical value.
Matt S
Old 1st January 2011
  #5
Gear Maniac
 
mister sunshine's Avatar
 

Thank You!
Old 4th January 2011
  #6
Lives for gear
 

its a bit complicated.......


resistors usually have very little inductance in series
this "parasitic" inductance varies with resistor material...

...non inductive resistors have a of little capacitance in shunt but not reactive in the audio range (above 500 khz) but can show flaws with active components (hand matching of active components is required in some cases)

but really its the circuits reaction to the componate, by means of reacting to the active component's elements (inter electrode capacitance between emmiter - base, base-collector, and emitter- collector or in tubes grid-plate, cathode-plate, and cathode-grid ) this action changes frequency and phase response.

and capacitor's construction is more audible as they do color the sound a bit more. resistors usually do this when they are pushed past half their dissipating rating in some designs.

the whole is greater than the sum of its pieces when it comes to design.


btw spice doesn't show this behavior as this behavior changes with ambient temperature of the circuit and construction techniques.
Old 5th January 2011
  #7
Gear Maniac
 

A resistor ('old-fashioned' variety with wire leads!) has a stray capacitance of 1-2pF and a stray inductance of about 5nH per cm of lead length. Plug those figures into the usual sums on impedance and you'll find that any resistor in the usual range (a few ohms to a Megohm or so) has a flat impedance in the audio band and in most cases to many octaves above. Only above 1MHz do you have to start thinking about strays, really.
Old 5th January 2011
  #8
Gear Maniac
 
mister sunshine's Avatar
 

Thanks again!

It's really nice to receive so many easy to understand answers.

best regards,
mike
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