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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 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 filtered/tinny audio is used for FM.

People who own the 880 and 2820 know what this sounds like on FM and Dstar.

Not good.


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 mod package (FM rigs), it's $55

as usual.  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:

Bob Nagy

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.

Icom HM-133 & HM-133V Mics

Stock HM-133 Dstar
Stock HM-133 FM
Upgraded HM-133 Dstar
Upgraded HM-133 FM
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