[ProAudio] Microphones question

Dan Lavry dan at lavryengineering.com
Sat Jun 12 16:25:37 PDT 2021

Scott is talking about maximum power transfer. A voltage source V with series source resistance RS is connected to load RL. If RL is open, there is no load current. If the load is a short, there is no load voltage. In either case no power on the load, it takes both current and voltage.So somewhere between zero and open there is a resistance for peak power transfer. It happen to be the peak of a parabola curve at RS=RL.I do agree that peak power transfer is not the only consideration, and different tradeoff is in order.Dan LavrySent from Samsung Galaxy smartphone.
-------- Original message --------From: Bill Whitlock via ProAudio <proaudio at bach.pgm.com> Date: 6/12/21  3:12 PM  (GMT-08:00) To: proaudio at bach.pgm.com, chris at chriscaudle.org, 6807.chris at pop.powweb.com Subject: Re: [ProAudio] Microphones question 
Where does that come from?  The lowest noise at the output of an amplifier results from the source impedance being equal to or less than the "optimum" for a particular amplifier - where e-noise and I-noise are equal contributors to total noise. This is determined by the first-stage device and its operating point.  The amplifier's input impedance is irrelevant.  For example, a transistor with a collector current of 1 mA may have equal e and i noise with a 500 Ω source impedance, but the input impedance of the stage could be 100 kΩ!  In most amplifiers, an intentional resistor is added only to lower input impedance for mic loading purposes 

See Motchenbacher & Fitchen, "Low Noise Electronic System Design" page 62 and others.

Bill Whitlock

-----Original Message (in part)-----
From: Scott Dorsey via ProAudio <proaudio at bach.pgm.com>
To: proaudio at bach.pgm.com; chris at chriscaudle.org; 6807.chris at pop.powweb.com
Sent: Sat, Jun 12, 2021 2:42 pm
Subject: Re: [ProAudio] Microphones question

In a perfect world the lowest noise would be when the input impedance of thepreamp matches the output impedance of the microphone, ....

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