[BC] mic impedance question

[Bob Tarsio]

Message: 14
From: Glen Kippel <glen.kippel at gmail.com>
<CAMF=0yueKUeoKjm-9=R8YbY4r=SBwC6N+GfG=wLW+_Soj7UX1Q at mail.gmail.com>
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I am putting together a basic recording setup to do my audiobooks and possibly some voiceover work and have a question about the load impedance presented to the mic. Most of the mics I have been looking at show an impedance of 200 ohms (a couple are 75 and one is 50 ohms). But the only mics that specify both their source impedance and a recommended load specify that the load impedance should be 1000 ohms or more. The problem is, just about every mixer or USB adapter that I have spec'ed lists an input impedance of 600 ohms.
 
Could this be detimental to the sound I am attempting to achieve? I am kind of thinking of putting a dual, linear 1-K pot in a Bud box along with the appropriate XLR connectors so that I can set the load impedance to the recommended value. Any comments or suggestions?
 
Glen:
 
The source vs. load impedance situation is based on the idea that the preamplifier needs to have the proper load impedance to the transducer for minimum noise transfer. For example, op amps such as the NE5534 offer a load impedance of about 10K ohms so a step up transformer or a super match transistor set are typically used to do two things. First is to do the impedance transfer and to offer voltage gain. Years ago we built microphone preamps that accepted the typical 150 ohm input impedance of a dynamic microphone and used a transformer to bring that impedance up to 10 K to "match" the op amp's load impedance. This provided about 20 dB of voltage gain and minimized the internal source noise of the op amp. I suspect that your mixer already has this taken into account and they are looking to simply have the microphone be slightly bridged at the input so as not to load it down. I think that your setup as described will work fine. A simple test would be to use various fixed resistors as a source to simulate the microphone and measure the noise. You can start with 100 ohms and work up to the recommended 1 K ohm. I would use metal film resistors for this  test as they offer the lowest KTB noise.
 
Regards,
 
Bob Tarsio
 
www.broadcast-devices.com