ramjet
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#1
7th March 2007
Old 7th March 2007
  #1
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imedance matching

hi

this arose out of one of my other posts so here goes.

it has to do with the pa that i work with daily

the output of the desk is 75 ohm the input of the graphic is 20k ohms(not a miss read) output of the graphic is 300 ohms and the input of the powered speakers is 44 ohms. is this a mess and am i damaging the signal or should i not worry about this. it just got me curious when i read that is i make a pad for something that i would need to know the input and output impedances of what it was going between.

any info gladly recieved
#2
7th March 2007
Old 7th March 2007
  #2
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Where did you get the 44 ohm input figure from for the powered monitors? That just doesn't sound right. List the brand/model, and whether they are balanced or unbalanced. You may also be able to do a quick check with a meter, provided there are no caps on the inputs.

Paul
ramjet
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#3
7th March 2007
Old 7th March 2007
  #3
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hi

yes my fault again i typed it all incorrect.

brands are as follows: Allen and Heath ML4000 output imp 75ohms
ARX EQ60 input imp 20k ohms output imp 300ohms
ARX SPL12 & SPL18 input imp 44k ohms

yes these are all correct have even double checked my typing.

so is this causing me trouble, i just find all the different numbers interesting as in the manufactures of differest equipment are all different and was wondering what the theory of impedance in this instance is and what effect it has

all of this is balanced
#4
8th March 2007
Old 8th March 2007
  #4
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Impedance matching is important when you are dealing with high frequencies like your CB, cable TV, and Ethernet....

At audio frequencies, you want a low output impedance and a high input impedance so that most of the voltage shows up across your inputs. The only place where you might not want input impedances high (above 10k) is mic pre-amps and when there is a transformer coupled output (and even then, its usually just fine).

The mic pre-amp sometimes needs to have a lower input impedance because there is less noise that way - the mic is a very low level signal.

The tranformer outputs sometimes will have too low a current in the secondaries if the input impedance of the next stage is too high.

The time you really need to worry about matching impedance for line-level audio is when you are the phone company - or have some other reason for having 10 miles of wire between devices.....



-tINY

#5
8th March 2007
Old 8th March 2007
  #5
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The general rule-of-thumb for matching inputs to outputs is found by multiplying the output impedance by 15. A ratio of 15:1 or higher is usually acceptable.

In your case, 300 X 15 = 4,500 (or 4.5K) = minimum load impedance = 15:1.

Insert the powered speaker's input impedance (44,000/300), and you get a ratio of 146:1. That's a factor of 146, much higher than 15, and probably acceptable.

This is a general quesstimation for doing things. Look for optimal impedance matching in the equipments' manuals.

Paul
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