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Fitting a Dynamic Microphone to an Icom HF radio
(and some other radios too!)
By Brian, MW0GKX.
From a question by Tom, MM6ATU.

With this page I hope to answer a question that Tom asked via email and, although it deals with the the Shure 550L and an Icom IC-756, can be adapted to other dynamic microphones / Icom radios by checking the page(s) for the mic / radio you're trying to connect. I've also included a small section on how to check if you need to follow this guide with a different model radio, as a result of a follow-up question by a, slightly confused, reader.

The original question was:
"… I've been looking to find out how I can connect a Shure 550L base mic to an Icom IC-756 (mk1)… Can you help?"

Well, those of you that know me will know that I do like to help in any way I can. This page is that help as changing to a dynamic mic on many Icom (and other) radios is not as straight forward as just wiring the plug correctly!

Firstly a bit of background about the Icom Radios

There are two types of Icom HF radio insofar as microphone interfacing is concerned:

  1. Low-Gain models. These tend to be the earlier designs, such as: IC-735, 745, 751, 761, 765, 781, the early IC-706 and the non-Pro versions of the IC-746 & 756.

  2. High-Gain models. These are the more modern designs, including: IC-746 Pro, 756 Pro, 7000, 7700, 7800 and the IC-706 MkIIG (to name just a few).

The difference between the types means that a pre-amplifier may be required for the Low-Gain types, whereas the High-Gain types should be fine.

With both types you will need to adjust the front panel (or menu) controls (Mic Gain and Compression, maybe even using the Compression Level as a secondary Mic Gain) to provide sufficient audio levels. If the radio controls do not provide sufficient audio drive then a mic pre-amp will be required.
Please Note: When I say a pre-amplifier I don't mean something like a 7 Watt audio amp kit, as was attempted by a certain, local to me, MW3 not too long ago. This would (did) over-drive the audio circuits in the radio and, if it works without damaging the radio, will make you sound horrible on-air! More about pre-amps later.

So you just hook up the dynamic microphone as per the information in the Microphone Wiring Area (opens in a new window), cross referencing the relevant Radio and Microphone schemes and all is well then?

NO!

Icom (and some other radio models) supply around +8 volts DC of power up the audio line from the radio to the microphone as well as the mic audio signal going down the line into the radio. Great for powering the electret type microphone, that requires power to work, but not for a dynamic type. De-coupling (or blocking) this DC is a must or you will probably release the magic smoke somewhere as you connect the 8 volts straight to ground through the dynamic element!

My radio is not listed in your examples;
How do I find out if it has a DC voltage on the Mic Audio line?

It is as easy as connecting a volt meter (set for, at least, 15V DC scale) between the Mic Audio pin and Ground pin of the Mic socket on the radio, with the radio switched on. BE CAREFUL not to touch any of the other pins with the test probes, you may find that inserting an un-wired plug into the radio will give you some assistance with this.

If you have no DC voltage present then either there is no power supplied up the Audio lead or someone has been there before and blown the regulator! If the radio works fine with it's own Mic then you can stop reading this page and wire up your dynamic Mic following the pin out for your radio. If the radio does not work on it's own Mic then further investigation will be required and is outside the scope of this page.

If you have a DC voltage on the Mic Audio pin then you will need to read, understand and follow the rest of this page.

Getting rid of the "unwanted" DC

De-coupling the DC is as easy as fitting a 1µf non-polarisied tantalum capacitor in the mics' hot audio lead.capacitor fitted You may get by with a .68µf or a .47µf, but anything less (say .01 µf, .005 µf, etc.) will not pass any speech audio worth listening to.

If a non-polarisied tantalum capacitor is not available then you could use an ordinary tantalum capacitor. The working voltage of the capacitor should be at least 11V if 8V supplied but ensure the capacitor voltage is greater than the supplied voltage by at least a third, higher voltage does not matter but don't go lower. Ensure that the positive lead of a polarised capacitor is to the radio side of the circuit. The negative lead will connect to the mic element (see image, right).

With the mic in the question (Shure 550L) I would:

  1. Remove the base plate of the mic.
  2. Disconnect the white wire from the PTT switch.
  3. Solder the negative lead of the capacitor (if polarised type) to the place vacated by the white wire.
  4. Slip a piece of heat-shrink tube over the white wire and solder the wire to the positive side of the capacitor (if polarised type), using the heat-shrink to insulate the joint.
  5. Replace the base plate of the microphone.

Which wire to where

So, back the original question: "how I can connect a Shure 550L base mic to an Icom IC-756". Well the table below shows which coloured wire goes to what numbered pin on the mic plug:

Shure 550LIcom 756
ColourPlug Pin No
White1
Green &
Shield / Braid
7
Red5 or 6
Black5 or 6

As the Red and Black wires are not connected to anything except the PTT switch they can be connected to either pin 5 or pin 6 (one wire to each) without problem.

Adjust the radio settings

Once it's wired up (you didn't forget that capacitor, did you?), plug it in and, using a general coverage receiver or a trusted friend, check your audio for quality, level etc. You may need some patience with trying different settings for the Mic Gain and Compression, but only alter one setting at a time, assess the result, then twiddle a bit more until you have the result you desire.

Microphone pre-amplifier

If you find that you can't get a high enough audio level after twiddling with the settings then you will need a microphone pre-amplifier. There are suitible circuits around on the internet for you to build from scratch, if you search for them, or mic pre-amp kits are available from various suppliers, like the K1803 Universal Mono Pre-Amplifier from Maplin (Order Code: VE21X) which, although the specs say requires a 10 - 30 V supply, can be powered from a 9 V PP3 battery.

With a little ingenuity, you could even power your circuit from the 8 V supplied up the mic lead that was blocked out earlier.

The pre-amplifier (depending on its size and the room available in the base of the mic) could be built into the mic, ensuring that nothing can short out, otherwise you could build it into a project box. You could even fit the capacitor you added to the mic in the box as well and, with a suitable socket fitted to the box, this would allow you to use other dynamic mics on the same radio, via the box, without the capacitor modification.

I hope that answers your question Tom.

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