The HM-133 and HM-133V are Icom mics that ship with a number of mobile radios.
Most of these radios are FM only. Some, like the ID-880, ID-2820 and ID-5100 are
Dstar and FM radios.
On the FM only mobiles, some folks complain that the audio
is low and lacks bass. I upgrade these 133's to correct these problems.
Half of the audio tailoring is done inside the radios. What I do is to boost the bass
and levels in the mic to result in a more natural response curve in the transmitted signal.
On the Dstar radios, the Dstar encoder (AMBE 2020 chip) wants to see a pretty narrow
audio response. Icom uses high-pass filters in the radio to strip out most of the bass
before it hits the Dstar chip. Problem is, that same tinny audio is used for FM.
People who own the 880 and 2820 know what this sounds like on FM and Dstar.
It's like your GPS has a headcold.
Both FM and Dstar can benefit from more bass in the mic audio passband. By modifying
the 133 alone, several DB of bass response can be added back. We don't need a level boost
here- as these radios have a good mic sensitivity adjustment. We just need to change the
mic's response. To gain any more, you have to bypass the high-pass filter in the rig. I've
done it - and it's a hair-raising experience (as if I had any hair to raise). Not suggested.
For the 133's that need an element change and the rest of the mid package (FM rigs), it's $52
as usual. For the 133's to be used on Dstar rigs, I'm setting it at $41 (FM benefits too)
I know there are all kinds of mods out there on the web. If you are comfortable
with SMD components, go for it! If not, Get it done by me and have the custom element that is perfect for the job - not a generic one. Turn-around here is same-day.
Just pack up the mic, give me your phone number and email and ship to:
103 Georgian St.
Hot Springs, AR. 71901
Audio Samples: I'm always reticent to add audio samples because computer speakers are
so poor. What you should hear is a increase in bass response. These are samples of the 133
modified for Dstar radios - without the new type #2 element.