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Is this microphone preamp input impedance too low (150 ohms)?
Old 4 weeks ago
  #1
Gear Nut
 

Is this microphone preamp input impedance too low (150 ohms)?

Hello,

The Apogee Duet lists its microphone preamp input impedance at 150 ohms. That seems very low, especially for a modern device.

As the experts here surely know, the common wisdom is to have your mic pre input impedance 10x greater or more compared to the microphone.

The Manley Reference Cardioid lists its output impedance at 250 ohms. This means it should see a mic pre input impedance of 2500 ohms or more. However, it would be seeing *less* than its own impedance!

I'm not sure if this is something to ignore. Competing devices from other companies have mic pre input impedances at 5k and higher. So I don't know why Apogee went with such a low impedance, and whether or not this is something to be concerned about.
Old 4 weeks ago
  #2
Lives for gear
 
JohnRoberts's Avatar
 

Yes mic input "bridging" terminations, designed to maximize voltage transfer, are generally 10x mic source impedance.

A typical mic should still work into a lower than expected termination impedance but will suffer signal output level loss and likely altered frequency response.

JR
Old 4 weeks ago
  #3
Lives for gear
From Apogee:
Quote:
Mic Pre:
EIN: 128dB (un-weighted) @ 60dB, 150Ohm input
Max input level: +20dBu
Input impedance: 3KOhm
...so the actual impedance is plenty high enough for a modern mic. Being designed for a lower matching impedance might mean a tiny bit less output from your mic's output transformer (as a tubed mic, I'm assuming it has an output transformer), but nothing to worry about.
Old 4 weeks ago
  #4
Lives for gear
Quote:
Originally Posted by gearstudent View Post
Hello,

The Apogee Duet lists its microphone preamp input impedance at 150 ohms. That seems very low, especially for a modern device.

As the experts here surely know, the common wisdom is to have your mic pre input impedance 10x greater or more compared to the microphone.

The Manley Reference Cardioid lists its output impedance at 250 ohms. This means it should see a mic pre input impedance of 2500 ohms or more. However, it would be seeing *less* than its own impedance!

I'm not sure if this is something to ignore. Competing devices from other companies have mic pre input impedances at 5k and higher. So I don't know why Apogee went with such a low impedance, and whether or not this is something to be concerned about.
I think they were just specifying the test conditions for the EIN measurements as having a 150-ohm source impedance. Two bullet points later in the spec sheet says "Input impedance 3k-ohms".
https://apogeedigital.com/products/d...specifications
Old 4 weeks ago
  #5
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JohnRoberts's Avatar
 

Quote:
Originally Posted by MediaGary View Post
I think they were just specifying the test conditions for the EIN measurements as having a 150-ohm source impedance. Two bullet points later in the spec sheet says "Input impedance 3k-ohms".
https://apogeedigital.com/products/d...specifications
+1... 150 ohm is good nominal termination for ein measurements... Short circuit input would be unfair (low noise), open circuit would be unfair (high noise). 150 ohms mimics a typical mic source impedance.

JR
Old 4 weeks ago
  #6
Gear Nut
 

Quote:
Originally Posted by MediaGary View Post
I think they were just specifying the test conditions for the EIN measurements as having a 150-ohm source impedance. Two bullet points later in the spec sheet says "Input impedance 3k-ohms".
https://apogeedigital.com/products/d...specifications
thanks for the clarification. it seemed strange that they would make the input impedance so low. 3k is much more of a normal value.
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