Hi all, I'm contemplating connecting a low noise mic with a 60k? output impedance to a low noise preamp with a 47M?/20pf input impedance. Is this proper or normal for audio. I want to save that 3db of signal if mismatching has no negative effect.
Mikek
Transformer impedance matching makes sense but wouldn't be practical in your case. The 47M is a sensible load for a 60K source.
Adding 60K shunt resistance in the preamp would just make the s/n worse.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
20pF shunt capacitance counts for nothing at audio frequencies.
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The "normal" standard pro audio mic interface is a low Z mic into a preamp that has about 1K input Z via n XLR connector.
What kind of mic do you have with a 60k Z?
m47M input impedance sounds like a FET preamp; the thermal noise of the 47M is pretty well-shunted by the 60k source impedance and the FET draws negligible input current thru the ~60k thevenin impedance it sees so I don't see any reason to "match" anything with a 60k source impedance when at audio you're only interested in voltage gain, you just hurt your noise performance here. The high-gain noise performance will depend almost entirely on the characteristics of the FET preamp circuit
Thermodynamic musings:
If you connect two resistors through, say a copper twisted pair, each will generate a Johnson noise voltage, so each will heat up the other one. Conservation of Energy demands that the heat flows be equal in an isothermal box, regardless of resistor values.
The electrical power transfer creates an equivalent thermal conductivity; a hotter resistor delivers net power to a cooler one. That's real but the thermal conductivity of copper wires is enormously bigger.
A ideal dynamic mic has an equivalent ohmic impedance determined by the coupling of the diaphragm to the air. The only noise source is Brownian motion of molecules hitting the mic. Connect that to a resistor, as above, and COE demands that the noise caused by Brownian motion must be the same as from a resistor of the same impedance.
That's weird. Mother Nature thought this over some.
Just the single sentence statement of Conservation of Energy forces thousands of effects and equations into line.
An ideal transformer can make voltage gain without adding noise, so is the optimum way to maximize power transmission and s/n between different ohmic impedances. Or put enough hi-z amps in parallel; somebody here is good at that.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Right. And many jfets could be paralleled before there were significant loading of the 60K source. Gate current noise will be trivial as long as the fets run at low drain voltage.
(I learned that from AoE.)
-- John Larkin Highland Technology, Inc lunatic fringe electronics
I wouldn't think brownian motion of the air could overcome the inertia/stiction of the mechanical diaphragm of a dynamic mic to generate a signal. Electet/condenser/ribbon, probably.
Though I suppose a diaphragm with stiction is not "ideal."
Hmm, I'm not sure. Putting on physics hat... If I can treat the mic as a harmonic oscillator*.. Some mass, restoring force (spring) and then some damping. Then the fluctuation dissipation theorem says, that the damping will lead to the fluctuation (noise).
The Brownian motion in the air is another separate noise source. (I think)... removes physics hat,
So here's one for you. Why do I need impedance matching when using a transformer?
George H.
*I think it's B. Pippard in "Physics of Vibration" where he looks at the noise of a torsional oscillator with various damping.
Brownian motion is sound. If it can't move the diaphragm, sound can't either.
In a really quiet room, after a while, most people can hear Brownian noise. I did that once an a giant anechoic chamber at Bell Labs.
Putting an ideal dynamic mic in a variable-vacuum chamber is interesting too.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Inertia and stiction are two different things. In the absence of stiction/ friction. The mic has to move, maybe just not very much, depending on the mass.
George H.
I did specify an ideal dynamic mic. That wouldn't have a frequency resonance. The only damping/ohmic component would be the bidirectional coupling to air. In an open field, with no echoes, radiated acoustic energy is lost to the universe. Sort of like a wideband antenna floating in space. Both have an impedance and a noise temperature.
You don't.
In the OP's case, a step-up transformer would give noise-free voltage gain so improve s/n. The ideal limiting case is a transformer that matches the complex impedances, but that's not practical into 47 megs. The turns ratio would be 885:1.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Well, I don't have any mic yet, I'm in the "Yahoo micbuilders group" and they suggest running Condenser mics at higher voltages with a larger resistor for lower noise. However, I ask, "Why do I need 48V for the 5024, doesn't it only require 3V?"
"Response, The aim is to make a mike with SOTA noise performance using smallest number of bits. 48V gives a LOT better performance"
So that may have more to do with A to D conversion. Someone might clear that up for me.
I'm looking at this mic setup as they suggest.
Then I want to feed this preamp, second schematic on the page.
I realize I still need to develop the 48V phantom power to the mic.
End result is the high gain, directional mic mentioned previously. Still in discussion about parabolic reflector vs Parabolic horn. I'm likely to try this reflex horn just for my own edification. Only 13 inches deep but 20 inches in Diameter.
They have some low noise preamps on their site and some discussion about multiple mics in parallel to reduce noise. Maybe later, I'll fool with that. Thanks, Mikek
I don't see how that follows at all, mic diaphragms have inertia and sound is a pressure wave in a fluid like any other it comes in large pressure variations and small variations.
I find this reference for Brownian motion effects on an electret mic, but an electret mic diaphragm is really thin, like a couple of uM.
with a real-world dynamic mic why would it not be the thermal noise of the magnetic transducer coil itself that dominates vs. air molecules
I'm trying to find a reference that Brownian motion of the air impacting the diaphragm generates a measurable signal at all with a dynamic mic or at least one that's distinguishable from the thermal noise generated from the windings themselves.
The noise floor from Brownian motion of the air seems to be a studied effect for an electret/condenser capsule but I can't find a similar reference for dynamic.
Muse away. I can only add, I do have a 15k to 1Meg transformer rated
UTC O-15 pg 16, >
I can't see why a dynamic mic would have less Brownian noise relative to sensitivity, unless maybe it has a larger diaphragm. I'm thinking the Brownian-based s/n increases with the square root of area. Tiny MEMS mics should have high relative Brownian noise. Looks like the noise is typically enough to be measurable.
Unless they include a powered amplifier, both electret and cap mics are bidirectional transducers, just like a speaker or a dynamic mic.
If a dynamic mic has static friction at the nano-inch level, it will ignore Brownian motion. But I can't see why a metal diaphragm would have stiction. It would sound awful if it did.
Looking into the terminals of a dynamic mic, there are two noise sources: Johnson noise of the coil resistance (zero for my ideal mic) and Brownian noise. Those will of course add the way that uncorrelated noises add. Similarly, there are two sources of impedance. The net ohmic impedance must have the same Johnson noise as any resistor at a given frequency and temperature.
You could measure the impedance of a dynamic mic. Measure the ohms of the coil and calculate its Johnson noise. Then measure total noise and tease out the Brownian component.
Or measure noise and impedance at atmospheric pressure and in vacuum. and separate the contributors.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
In RF design you try to achieve a power match i.e. load impedance is the same as source impedance for best S/N. Why isn't this principle applied to audio ?
A dynamic microphone has a coil DC resistance and hence a Johnson noise source. The noise voltage depends on the resistance and there is not much you can do about it in a voltage amplification stage. This resistance also generates a current noise. When the noise current flows through the load resistance (amplifier input resistance) it also generates a noisy voltage signal. The higher the amplifier input resistance, the higher voltage will be generated.
I know that microphones are usually designed to give a flat frequency response when the load impedance is about 10 times the microphone source impedance.
Why not load the microphone for best power match and fix the frequency response with an equalizer after the preamp ?
That gives you an 8:1 noise-free voltage gain, if you keep it away from magnetic fields.
Cool catalog. Do they still make that stuff?
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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