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How exactly do you impedance match mics to rigs?

d1g1man

Member
Feb 25, 2008
50
0
16
Hi everyone. I have always wondered how to do this.
I have a reallly nice microphone at 150 ohms impedance, trying to match it to a rig at the standard 600 ohms.

Everything I have read says that it really should not matter.
However, I get very low output from the mic. When I connect it to a pre-amp I get a lot of squeal.

What is the method or circuit you have to build to match a mic to a rig?

Thanks!
 

W2IHY iBox
Saw this link on qrz earlier today.
It's a match box, line level, RF filter, and more.
I'm not saying you can't just use a line transformer or preamp/transformer; I just thought the link provided a kind of trick setup for those who havn't seen it yet. The built-in RF filter and audio attenuator is so much icing on the cake - so tho speak.
Hope this helps some...
 
150 ohms to 600 ohms will not matter at all.In fact since the load impedance is only a bit higher than your mic,the levels should not be a problem unless the mic output is not enough to drive the radio in the first place.Where the real problem is is when using a high impedance like 10-50 thousand ohms and trying to drive it into a radio that has 150/600 ohms input impedance.The levels will really suck as you are forcing the mic to try and deliver more power (current) then it is capable of.The freq response will also suffer. A similar problem is when using a low impedance mic to drive a high impedance radio.The freq response will suffer and the radio will not be driven properly because the mic cannot deliver the voltage required.

BTW what type of radio and mic are you trying it with? It's not an Icom is it? They require a really high mic level.
 
Yeah D1G1Man1Adam12 ....guy

Anyway, If the output is low from the mic, something might be wrong with it. What radio are you using? Does the radio have a mic gain circuit on it? Turned the mic gain all the way up and still low output? ....probably something wrong with the mic.

And the W2IHY boxes are great and do the job, put have a fat wallet on standby, they be pretty spendy. Too spendy for me for a mic box.

good luck
 
rig and mic

The rig I am using is a disposable Magnum 257.

The microphone is the one in the pdf below.
([FONT=Arial,Bold][FONT=Arial,Bold]MODEL 515SBG-18XF[/FONT][/FONT])

Rob, the box you are talking about is nice, but I don't want to buy anything (except parts).
I am trying to learn something. If just wanted to talk, I would just plug in the stock mic.

This problem has come up a half dozen times over the past several years, every time I research it I come up empty handed. I was hoping to be pointed in the general direction of some basic theory or just a first step.

There must be a way to interface it with a pre-amp to make it work properly. I will search for some schematics and experiment.

Thank you everyone for your input so far, it is appreciated!

http://www.shure.com/stellent/group.../documents/web_resource/us_pro_515sbgx_ug.pdf
 
So - you liked that little box I found?
Cool - huh?
Yeah, I'm sure that this box doesn't have any corner on the market of the engineering for what they've done to make it. I don't think it is so unique as it is practical. It does allow you to hook up just about any mic that one may have - along with a side jack that can be plugged into a footswitch that can key the mic 'on'.

There must be something similar out there that does almost the same thing.
BTW-didn't that same company have a brochure that showed the inner workings of it? Couldn't be more than a few parts and the right ideas.

I'm no electrical engineer, but I'm leaning twards the idea of a transformer or a different mic.
Didn't I read somewhere on this site somewhere - that someone was usnig an Astatic 636 mic for their Mag 257? I could be wrong.
BTW - you said 'a nice mic'; what mic would that be - out of curiosity?
My bet? A 4:1 audio transformer might work. If nothing else it might be a good place to start...
Two cents worth...
 
It's pretty easy to find transformers to match up a 200-600 ohm impedance mic to a hi impedance mic, but I don't know where you'll find a pre-made one that will match a 200 to a 600 ohm impedance. That's because (as QRN said) it usually isn't a problem. It sounds like with that mic you have it is a problem. If you really want to do this, you're probably going to have to build it yourself. Here's the type of transformer you will need:

http://www.jensen-transformers.com/datashts/16a.pdf
 
I bought a few of the 636L's and have played with them on a few rigs.

One was an FT-7800R and the audio was super thin. I thought it might be an issue as the rig has a 2k input impedance. It turned out to be simpler than that. The audio range of that mic is very communications grade and just doesn't have any bottom. It rolls off around 300Hz from what I can tell.

Works great into a 600 Ohm input FT-857D, but it's very communications grade in sound quality...thin. No noise at all from my stereo playing or windows down in a Jeep though so for the purpose of mobile, it works well for me. I would prefer a response down to 60Hz or so though.
 
I agree with the statements that a communications microphone is limited as far as transforming the impedance to a rig with a different input without causing problems.
A gadget which I have used for a long time both in sound on stage, as well as recording and ham radio is the PreSonus tubePre. It runs about a hundred bucks new but is a very handy gadget. PreSonus TubePRE | Sweetwater.com
Look up " PreSonus tubePre" on google for more details. The tube only uses 16-18 volts on the plates which is a bit odd but I have been able to match both low to high and high to low
with great results. I hope I didn't get too far off the subject. I don't have any relationship with Sweetwater and if you look around you will find many sources for the pre-amp.
 
Last edited:
The microphone is the one in the pdf below.
([FONT=Arial,Bold][FONT=Arial,Bold]MODEL 515SBG-18XF[/FONT][/FONT])

D1G1 -

The SHURE mic that you have is balanced, and the radio requires an unbalanced mic. If you have a 3-wire cable coming from the mic, make certain that the black wire and the shield are connected together. Also the mic output (unloaded) should be around 180mv p-p on a scope with an input of 94db SPL. This is approx. equal to saying "aaaaah" in a normal speaking level with the mic about 1 inch from your mouth. It's a quick way of determining if your mic output is ok.

- 399
 
Hi everyone. I have always wondered how to do this.
I have a reallly nice microphone at 150 ohms impedance, trying to match it to a rig at the standard 600 ohms.

don't bother. the microphone you're looking at is not designed for communications purposes. even though the frequency response is from 80hz. - 16Khz. the 257 tx speech circuits will rolloff below 300hz. and above 3000hz..

in other words, you're looking at a microphone with a wide frequency response that will never be noticed when used with the 257. if the 257 didn't restrict the audio bandpass range to 2700hz. your am signal would be 3 channels wide. the slight impedance mismatch is the least of your problems, it's a lot to do about nothing. the 515 is designed for public address and paging purposes for these reasons.
 

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