I tried all the obvious quick-fixes on the 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 several manufacturers.
The 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.
That's exactly what this upgrade does.
After selecting a superb mic element and the correct value of the
blocking capacitor, 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 300/2400 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).
For SSB, my new 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 have boosted the highs and trimmed the bass -
shifting the center of the audio energy spectrum up. This results in more
understandable audio on SSB.
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%!
A nice bonus is also that the new element is not
a noise-canceling element, and the audio doesn't drop off so
drastically as you move from 1 to 2 inches from the mic.
I added a shaped acoustic foam insert in the rear of the mic that
really helps kill the hollow plastic case sound. 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.
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).
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: I just tried our new type two element on AM TX
and darn if it
hasn't made this mode usable for the first time! The stock element
couldn't even get to 20% modulation. With the type two on 75% mic
level or more, you can now hold an AM QSO. That's good news. I'm
only using the new enhanced type two elements now.
I'm getting reports back
now almost daily from upgrade users -and they are thumbs up! Check
the
Eham reviews section for tons of reviews- a perfect "5"
across the board.
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)
SSB TX
in Wide - 100 - 2900 hz - Compressor off
SSB TX in Mid - 200 - 2900 hz -
Compressor off
SSB in Narrow - 300 - 2500 hz -
Compressor off
SSB in Wide - 100 - 2900 hz
with Compressor ON
FM TX in Wide filter - 15 khz
The type two element has exactly the same
tonal characteristics, but allows even more drive level.
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