Author Topic: measuring microphone impedance?  (Read 3144 times)

labcoat man

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measuring microphone impedance?
« on: June 18, 2007, 02:22:54 AM »
Hi ,

this seems on the surface to be an obvious question but Im yet to find an answer.  I need to measure the actual output impedance of a microphone. Was wondering if  there was a simple way with an oscilloscope or DMM and a tone etc.

Can the voltage drop method used to determine line amp output impedance
 (finding the resistance needed to halve the output voltage at a certain frequency) be applied to determining a mic's output impedance?

Any help would be great and much appreciated!

thanks

Tim


pstamler

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Re: measuring microphone impedance?
« Reply #1 on: June 18, 2007, 02:55:16 AM »
Quote from: "labcoat man"
Can the voltage drop method used to determine line amp output impedance
 (finding the resistance needed to halve the output voltage at a certain frequency) be applied to determining a mic's output impedance?


Yes, with some caveats. You'll need a sound source which can be put into a completely isolated space along with the microphone. And that sound source has to cover all the frequencies at which you're interested in finding the mic's impedance, which is difficult when you get to the bass frequencies. But if you have an isolated room, you should be okay.

Unless...what kind of microphone are you testing? If it's condenser and phantom-powered, you'll have to factor in things, including the loading effect of the phantom resistors and coupling capacitors. Doable, but more hassle.

Peace,
Paul

labcoat man

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measuring microphone impedance?
« Reply #2 on: June 18, 2007, 03:15:22 AM »
Thanks Paul , thats good to know!

What I discovered on trying the voltage drop  method is that the voltages are too low to read on a DMM and then on an oscilloscope (Im a bit new to these) they are too low to get a decent wave form that can be measured Im talking around 1-2 millivolts here from what I can gather.

I tried it with a sm57 to start and a 1k tone through  a speaker turned up to a  point just below discomfort  and with the mic against the grill and the voltage readings were very low but the mic works fine.

I actually want to measure output impedance of a  ribbon mic (Im trying to avoid having to measure the actual ribbon impedance with everything hooked up and risking ribbon damage from the DMM etc otherwise I'd use the transformer turns ratio to estimate the impedance).

If there is any way around this that would be great, unless my measuring is all out of whack. Im measuring ac current across  the output pins of the mic .

 Perhaps some kind of enclosure around the source and the mic might
add to the amount of tone it picks up for a better current? Swallowing the mic and 'singing' a tone brought ok readings  but Im not a very good oscillator.

I even hoped  there might be a way to calculate with the DB meter on a mic pre but I guess there is many complication there...

Cheers


Tim

rodabod

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measuring microphone impedance?
« Reply #3 on: June 18, 2007, 07:14:39 AM »
Quote from: "labcoat man"

I tried it with a sm57 to start and a 1k tone through  a speaker turned up to a  point just below discomfort  and with the mic against the grill and the voltage readings were very low but the mic works fine.

I actually want to measure output impedance of a  ribbon mic (Im trying to avoid having to measure the actual ribbon impedance with everything hooked up and risking ribbon damage from the DMM etc otherwise I'd use the transformer turns ratio to estimate the impedance).


Have you considered working things the other way around? With a dynamic microphone, try driving the mic with a small voltage as you would with a loudspeaker.
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Rossi

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measuring microphone impedance?
« Reply #4 on: June 18, 2007, 01:58:40 PM »
Measuring microphone impedance is one of the few useful applications for a variable impedance preamp.

Place the mic in front of a loudspeaker, run a test tone through it. Turn the impedance switch to its highest setting. Adjust gain to a certain point on your VU meter (software may be more accurate than a low resolution hardware meter), then set the impedance switch to its lowest setting. The gain loss on your meter allows you to calculate the mic's output impedance (at the frequency of your test tone).

A transformerless input variable impedance preamp is better for measuring than a transformer coupled pre.

What kind of ribbon mic are you measuring?
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pstamler

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measuring microphone impedance?
« Reply #5 on: June 18, 2007, 02:09:28 PM »
Forget the DMM; as you've discovered, it's not sensitive enough. But you should be able to plug the microphone into the line input of your soundcard and get a clean enough signal to measure. It'd be way too noisy to record music, but a 24-bit soundcard should be quiet enouigh to get pretty accurate measurements.

Peace,
Paul

labcoat man

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measuring microphone impedance?
« Reply #6 on: June 18, 2007, 09:42:11 PM »
Thanks for everyones help I may get somewhere yet!

Im reluctant to apply voltage to the ribbon mic for fear of damaging the ribbon. Unfortunately I don't have a sound card or a computer or a multi impedance mic pre so I may have to look into borrowing at least one of those.

One thing I was thinking was perhaps with a single impedance mic pre I could load the input with a variable resistor/pot and somehow achieve voltage drop on the db meter by determining what db drop corresponds to a voltage drop of half then measure resistance across the pot.

Just a stab in the dark... any thoughts?

PS the ribbon mic is an old lustraphone with a new lundahll transformer.

Thanks

Tim

PRR

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measuring microphone impedance?
« Reply #7 on: June 18, 2007, 10:09:29 PM »
> try driving the mic with a small voltage as you would with a loudspeaker.

Risky advice. Common dynamics will stand many dozen millwatts this way; one is even rated for speaker use. Ribbons tend to be an order of magnitude more fragile, especially to "thump". They have higher electromechanical efficiency, but much softer suspension.

> the voltage readings were very low but the mic works fine.

Output level of a ribbon may be 0.1mV at 74 dB SPL, 1mV at 94 dB SPL which can be uncomfortable.

> the voltages are too low to read on a DMM

DVMs are for mechanics.

A good 'scope will resolve ribbon levels, but many general purpose 'scopes won't show enough wiggle to read clearly.

You NEED a preamp.

Wire a 10K audio pot as a 2-terminal rheostat, 10K at max, 1K in the middel, zero at minimum. Add a paper scale, use the ohmeter to calibrate the knob.

Take a shell off a mike cable and tack the pot to the hot wires. Set the pot to 10K.

Plug this to your mixing console mike input. Verify that meters move, and that the zero-ohm setting kills the signal.

Put that mighty-loud Ricky Martin or Ramones CD in the boombox, turn it up to clipping, set the mike near.

Set the mixer so that racket hovers near reference level; zero VU or -15dBfs.

Turn the 10K pot until meter shows 6dB less. Read the pot scale.

This is approximately the broadband audio impedance of the mike.

For more accuracy, you should determine the input impedance of the mike amp and do math. Most inputs have Zin much higher than common dynamics, so it is hardly worth the effort. Some ribbons are cheated-up to high impedance. If the pot reading is 300 ohms or less, it's good enough. If the pot reading is higher than 300, you may want to correct. If the pot must be set to 1K for 6dB drop, and the mike input is 2K (a common value), then it is really a 667 ohm load and a 667 ohm mike. Since you never need to know mike impedance better than 20%, this is just hardly worth caring about.

Mike impedance is not constant with frequency. But it will tend to be very nearly flat. But narrow peaks or dips can upset sonic balance when used with lo-Z preamps. If that's what you are really asking, an electrical measurement is a lot easier to set up than an acoustic measurement.

Condensers need to be powered-up, obviously. Most condensers are padded-out enough that Zout is mostly resistive, with some transformer or cap error at extremes. But a few have near-zero Zout. These will not load-down, they will just distort, which is not what you want to know.

labcoat man

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measuring microphone impedance?
« Reply #8 on: June 18, 2007, 10:24:11 PM »
Thanks PRR for a great post!
Ill give it a try as soon as I get a chance and let you know how I go.

cheers

Tim


 

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