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API 550a loading 500 Series EQ\'s
Old 29th August 2007
  #1
Gear Maniac
 
Stu Gutz's Avatar
 

API 550a loading

Does anyone know what loads the 550a sees when in a vintage console? I think mine sound different depending on what is patched into them, so I want to replicate in my rack what a 550a would see inside a console.

Thanks.
Old 31st August 2007
  #2
Lives for gear
 
jsteiger's Avatar
API help

I'm also interested in this. I wish I knew. I would tell you!

TRW, any help here??
Old 1st September 2007
  #3
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superburtm's Avatar
 

bump
Old 1st September 2007
  #4
Gear Maniac
 
Stu Gutz's Avatar
 

It looks like the literature of the new 1608 console says that the unbalanced eqs like the 550a is fed by a 2510 opamp. Also the block diagram of the 7600 channel strip shows the same thing. ie. 2510 opamp into the 550a.
I don't know what it is but my guess is that the output impedance of the 2510 is very low.

In the vintage consoles, was it the 325 card that preceded the eqs? If so, the specs on Dan Alexander's site say that the output impedance of the 325 cards is less than 15 ohms for each of the 3 balanced outputs and there was also an unbalanced output available which was taken directly off the 2520 (I think) output. Whatever the output impedance of the 2520/2510 opamps is .. that is our answer.

600 ohms is probably the correct load for the output.
Old 1st September 2007
  #5
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jsteiger's Avatar
Quote:
Originally Posted by Stu Gutz View Post
...In the vintage consoles, was it the 325 card that preceded the eqs?
This is not the case in my console. The 325 cards are only used for booster amps following the buss, 2-mix, & mono faders. Used for make up gain.

The eq is driven by an output transformer on the input module. This is why it doesn't matter if the 550b's are unbalanced or not. Also, why they "sound better in the console" than they do in a rack. (I wouldn't know yet!...getting close though)

Quote:
Originally Posted by Stu Gutz View Post
...600 ohms is probably the correct load for the output.
This may be correct seeing how 600 ohms seems to have been the standard back then. Avedis says it was a standard for telephone and broadcast equipment that continued on into pro-audio.

In replacing my input channel faders, everyone seems shocked to hear that the originals were 600 ohm faders. I guess the 2520 has got the balls to drive a 600 ohm load and more. (Actually less.)
Old 1st September 2007
  #6
Gear Nut
 

At source impedances that low. I would be more apt to think the different power supplies from the rack to the console are doing it.
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