Icom HM-151 Microphone Upgrade Background

 

I tried all the obvious quick-fixes on my HM-151 mic. None of them really made it sound like it should. I realized that the mic element had to be changed. Being the original mic element is a condenser-type, the replacement would also have to be condenser. As well, there is too much bass output. This would require a change of the surface-mount blocking capacitor on the mic's circuit board. There is also a bit of hollow-ness from the mostly empty plastic mic housing. These are addressable problems.

I find I actually use the function buttons on the HM-151. You can't even access the stacked VFO registers unless you use the 151's buttons. It's a better idea to upgrade the 151 and keep all that remote functionality, rather than using another type mic which doesn't have the remote controls.

The technical goal is to find a balance between driving the 1st audio op-amp in the rig so that it is nice and linear - and having that deliver a signal to the DSP which is what it wants to see....so as to produce the best signal the rig is capable of.  Feeding the DSP Codec excess audio in the bass range just uses up valuable bits - and doesn't land up representing sound that enhances understandability.

 

I had to find the compromise between FM and SSB drive requirements. Finding that specific point took a lot of experimentation. I obtained and tried dozens of elements from the big parts suppliers. None of them worked correctly. I realized that these elements were made for consumer electronics and not radio mics. I contacted a small US manufacturer of mic elements and they agreed to make a custom element to my specifications (with a significant minimum order) My specs were definitely non-standard.

 

My goal was to have the option of nice clean audio for rag chewing and then using the rig's TBW to tighten it up for DX chasing. FM and AM needed much more drive to function correctly. With my combinations of mods, that's exactly what this upgrade does.


With the custom element and changed SMD coupler cap, I obtained a smooth balanced response and the optimum drive level. I left just enough bass at the low end to sound really great at the "Wide" SSB setting.  TBW at 200-2900 is perfect for regular QSO's. Set the TBW to 300/2500 and you have that pile-up busting sound.


The new setup also gives  FM  a brighter - cleaner sound. Now it easily goes out to full deviation with average speaking levels (15KHZ filter setting / 50% mic level). With the stock mic, I got continuous reports of low audio on FM. (especially on simplex)

For SSB, my favorite setting is 50% mic level with 200hz on the TBW low setting, 2900hz on the high. This makes the  ALC go nearly to the end of the bar... perfect. Mic level settings on SSB will be much lower than the old mic element (from increased drive). We've boosted the highs and trimmed the bass - shifting the center of the audio energy spectrum up. This results in more understandable audio on SSB. It's much cleaner and more present. It sounds like an excellent desk mic with tone adjusted just right.

100% mic level on the old mic is equal to 25% mic level on the new element. Now, 50% puts you right at the sweet spot.
Depending on your voice, you have the adjustment range to dial it right in now. In the mobile, dial it down a bit, at home, bring it up a bit. For mobile operation you will be using mic level settings under 25%!

I added a shaped acoustic foam insert in the rear of the mic that helps deaden the circuit board and case (all studio mics use this). I slightly enlarge the mic opening on the case and align the new element directly in back of it. To protect from close-talking "popping P's", a small white silk wind screen is mounted just in back of the mic opening with a 1/8th slice of foam behind it. Lastly, I add about one ounce of metal inside the rear shell (potted with glue)- to give the mic a better feel in the hand. It was just too light. Now it feels just right. The potted metal ballast also deadens the mic shell resonance that causes the plastic hollow sound.

Remember, I do each mic separately on the bench. I take a surgeon's care to do each one perfectly. Many mics have been shipping back to their owner in literally one hour from their arrival here (on slow days). Maximum time here is 24 hours. Average is 3 hours.

What a pleasant change! This is more like it. This is exactly what I originally expected the mic to sound like. SSB is smooth and clean with full output at anything over 50% on the mic level setting, and if you add the compressor, it does what a compressor should do. On FM I get great audio reports all-around now. No more having to change the mic level setting when changing modes. Relax, speak naturally and have great audio. What a revelation.

Good News: The type #2 element makes AM usable for the first time. The stock element couldn't even get to 25% modulation. With the type two on 75% mic level or more, you now have good modulation. That's excellent news.


I've got over 7500+ upgrades out there now and have a 5-Star rating on Eham.com reviews.

http://www.eham.net/reviews/detail/6580

Sound Samples

Conditions- Recorded in MP3 format on hand-held recorder using Icom 756 Pro as receiver...recorder held in front of speaker (not ideal - but gives a fair idea)

Created by Bob Nagy - AB5N