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Author Topic: Sunair microphone impedance  (Read 880 times)
WY3X
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Posts: 768




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« on: December 13, 2002, 06:13:21 PM »

I may be answering my own question as to why I can't get a desk microphone to work, but I don't really understand what's going on. I'd like to use a desk-type microphone with my Sunair GSB-900DX, and I purchased a new-in-the-box ElectroVoice model 622 Low-Z 150 ohm microphone for this purpose. The shop manual for the GSB-900DX mentions no specification for microphone impedance. I opened the factory Sunair microphone and found a magnetic something-or-other element. A lot of the inputs (phone patch, etc.) on the radio are 600 ohm. I don't really want to remove and use the Sunair hand-mike element because I'd like to keep it factory stock, but I think the problem may be the microphone element in the ElectroVoice. Is there a way to check the impedance of the factory Sunair microphone? Will a lo-z microphone give low or no audio when used with a radio expecting a high impedance microphone? In my way of thinking, a Lo-Impedance element would overdrive a radio looking for a high-impedance element. I've triple-checked all the connections, so I know that part is right. Any ideas?

Thanks, -Web Williams in Myrtle Beach, SC
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AC5UP
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Posts: 3812




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« Reply #1 on: December 13, 2002, 07:24:09 PM »

Speaking in broad terms, a Hi-Z dynamic microphone will develop higher voltage at lower current than a Lo-Z equivalent. If the Sunair mic does indeed use a dynamic element you can measure the coil resistance with an ohm meter against the EV for a ballpark comparison. A Lo-Z element may come in around 6 ohms (DC) while a Hi-Z element may be closer to 20 ohms. These readings may vary considerably depending on the sensitivity of your meter and this is just a rough check.

A more likely scenario is that the Sunair has phantom DC voltage on the mic connector (typically 8 volts at 10 mils) to power an amplified electret mic. Your DC voltmeter will tell you if there's any voltage on the hot audio pin, but a Q & D method would be to put the EV to your ear then plug & unplug the connector. If you hear a click in the mic, a DC voltage is holding the diaphragm back from developing full output. A blocking cap around .47 uf (or larger) in series with the mic element will cure that, but you may find it still needs a pre-amp to drive the rig to full output.

Aside from the mic element and switch, are there any other parts inside the Sunair mic? If not, it may have a crystal element and you will need a little pre-amp to bring the EV up to that level.

- AC5UP
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AC5E
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Posts: 3585




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« Reply #2 on: December 13, 2002, 07:34:43 PM »

Hi Web: For starters, most microphones are "sound powered." Their output voltage is derived entirely from the sounds they pick up.

Since normal speech sound pressures are surprisingly uniform, microphone output power in microwatts does not change that much between types, but their output voltage varies over a wide range.

A low impedance mike expects to provide a low voltage at a comparatively high current to its intended load. A load that may have be equivalent to a 100 to 1,000 ohm resistor - followed by a very high gain amplifer.

But a high impedance mike expects something like a classic vacuum tube input circuit, with a half megohm or more of resistance and essentially no current draw -followed by a low to moderate gain amplifier.

Since the output power is much the same, low Z mikes are generally low voltage output critters at a current of a few hundred microamps: while a high Z mike may have up to a volt or more of peak voltage output at less than two or three microamps.

Now - mikes are not very finicky about what they drive. A 150 ohm impedance mike will almost always do a fine job driving a 600 ohm load. Or a 1500 ohm load. But hook one up to a high impedance amplifier that's designed for a much higher voltage and they fall flat.
They just don't drive the amplifier hard enough to do much good.

And the other way, a high impedance mike into a low impedance amplifier is usually loaded down to the point that its output is also very low.

If you really want to know what your mike matches - an AM radio tuned to a talk show for a sound source, a potentiometer and an o'scope will tell you. Just hook the mike between the arm and one end of the pot, put the 'scope across the mike element, and adjust the pot for maximum voltage across the mike element. When you get max deflection on the scope, measure the load resistance and you have the optimum load for that mike. After that - it's yours to match!

73  Pete Allen  AC5E
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WY3X
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« Reply #3 on: December 13, 2002, 10:09:18 PM »

OK, took a look at the schematic, and the microphone "hot" pin is isolated from everything by capacitors, so I don't think it can be an electret type- caps block DC. I opened the microphone back up, and on the rear of the microphone element, it says "controlled magnetic transducer". Not condenser, not dynamic, not carbon. I removed the element from the housing, and it looks very much like an old rotary-dial telephone microphone element. I'm thinking those things were on the order of 600 ohms impedance? Further, I took my DVM and checked for voltage on the microphone jack (in the receive mode) and found none. The only voltage was on the key line- 11.16VDC. So whatever a "magnetic transducer" is must be the secret to unravelling the puzzle. For now, I guess I'm going to stick with the hand mike :-( . Maybe I'll try to buy another one of these and borrow the element out of it. Or perhaps I can get an old D-104 amplified mike to work with it.

Thanks all, -Web
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AC5E
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« Reply #4 on: December 14, 2002, 08:26:21 AM »

Chuckle: Web, in reference to a microphone, a "magnetic transducer" is essentially our old friend the loudspeaker. It has a coil attached to a cone that moves in a magnetic field to produce an EMF. Audio output. I suspect the impedance is nominally 600 ohms, since the mike is going to need the resistance for damping.

Now, back in the old days I solved many a difficult audio problem with a .001 tubular capacitor. Firmly pinch one lead between your thumb and forefinger and use the other lead as a test prod. Do a "ring around" the input until you get a blast of audio out. That's the mike input!

And it's surprising how many times I have found some idiot at the factory wired the frazzling thing counterclockwise instead of clockwise! A simple problem that can be a booger to find.

If you don't get audio out anywhere something is wrong internally. You can go from stage input to stage input until it comes to life. The previous stage is the dead stage. Needless to say - this is best done with an audio amplifier on the output instead of a transmitter - and if you must use a transmitter use low output power and a dummy load.

73  Pete Allen  AC5E
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