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<div style="color:black;font:12pt Arial, Helvetica, sans-serif;"><span style="font-style:italic;">Power</span> is irrelevant here! Mic preamps and virtually every electronic amplifier I'm aware of is a <span style="font-style:italic;font-weight:bold;">voltage </span>amplifier. Power transfer had meaning when signal sources and loads were passive ... in the earliest days of telephone systems - there were no amplifiers, so power transfer was an issue. We certainly don't add an 8 Ω resistor in series with the output of a so-called audio "power" amplifier so it can have its maximum power transfer! The "power" in audio power amplifier is a long-standing misnomer ... technically, it should be called a voltage amplifier with high output power capability. The reason IEC and many other standards specify low device output impedances and high input impedances (for example, <2.2 kΩ and >22 kΩ respectively for consumer gear) is to transfer 90% or more of the signal <span style="font-weight:bold;font-style:italic;">voltage</span>. This is one of the most commonly heard misunderstandings when I teach my seminars on signal interfaces and grounding! Right next to "balanced lines require equal and opposite signal swings" ... which is completely indefensible BS!
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<div>Bill Whitlock</div>
<div>AES Life Fellow</div>
<div>Ventura, CA<br clear="none">
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<div style="font-family:arial, helvetica;font-size:10pt;color:black;"><font size="2">-----Original Message-----<br clear="none">
From: Dan Lavry via ProAudio <proaudio@bach.pgm.com><br clear="none">
To: Bill Whitlock <engineer_bill@verizon.net>; proaudio@bach.pgm.com; chris@chriscaudle.org <chris@chriscaudle.org>; 6807.chris@pop.powweb.com <6807.chris@pop.powweb.com><br clear="none">
Sent: Sat, Jun 12, 2021 4:25 pm<br clear="none">
Subject: Re: [ProAudio] Microphones question<br clear="none">
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<div>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.</div>
<div>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.</div>
<div>I do agree that peak power transfer is not the only consideration, and different tradeoff is in order.</div>
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<div>Dan Lavry</div>
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<div style="font-size:85%;color:#575757;">Sent from Samsung Galaxy smartphone.</div>
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<div>-------- Original message --------</div>
<div>From: Bill Whitlock via ProAudio <proaudio@bach.pgm.com> </div>
<div>Date: 6/12/21 3:12 PM (GMT-08:00) </div>
<div>To: proaudio@bach.pgm.com, chris@chriscaudle.org, 6807.chris@pop.powweb.com </div>
<div>Subject: Re: [ProAudio] Microphones question </div>
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<div style="color:black;font:12pt Arial, Helvetica, sans-serif;">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
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<div>See Motchenbacher & Fitchen, "Low Noise Electronic System Design" page 62 and others.</div>
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<div>Bill Whitlock<br clear="none">
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<div style="font-family:arial, helvetica;font-size:10pt;color:black;"><font size="2">-----Original Message (in part)-----<br clear="none">
From: Scott Dorsey via ProAudio <proaudio@bach.pgm.com><br clear="none">
To: proaudio@bach.pgm.com; chris@chriscaudle.org; 6807.chris@pop.powweb.com<br clear="none">
Sent: Sat, Jun 12, 2021 2:42 pm<br clear="none">
Subject: Re: [ProAudio] Microphones question<br clear="none">
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<div dir="ltr"><br clear="none">In a perfect world the lowest noise would be when the input impedance of the<br clear="none">preamp matches the output impedance of the microphone, ....</div>
<div dir="ltr"><br clear="none">--scott
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