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Could use help with microphone project

crappykraco

W9WDX Amateur Radio Club Member
Aug 6, 2014
90
40
28
Hello, all. You'll probably laugh at how simple this is, but i don't mind.

I have an old turner microphone body and one of these https://www.adafruit.com/product/1063 little electret mic amp boards. I am attempting to make it work with an icom ic7200 through the ACC socket.

Here is a look at the icom acc socket

I have supplied power using 3 AA batteries.

I used the turner leaf switches.. ptt is working..

Now to the problem. The little adafruit board has a trim pot on it, and even when adjusted as low as it'll go, and the ICOM mic gain on 1, it pegs the ALC meter on the 7200.
 

Dmans

Sr. Member
Jan 22, 2017
1,344
1,697
173
Georgia
A large value resistor (1.5mega Ohm 1/2w or so) in line with the audio output of the microphone would be a good place to start. Even better would be to use a large value variable resistor in the microphone base and adjust as required/needed.

I use the same amp/mic board on my Kenwood TS440, Anytone AT6666 (Astatic 987L with a switch to accommodate the Kenwood or Anytone with different value resistors inline) and my Cobra 139XLR & 138XLR. Also put one in the stock microphone case of my Uniden 980SSB.


73
David
 

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Shadetree Mechanic

Delaware Base Station 808
Oct 23, 2017
5,157
8,419
673
50
The First State (Delaware)
Here is a thread about a different adafruit module that we use. The one that you have just needs a resistor inline to drop the audio level.

 
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brandon7861

Well-Known Member
Nov 28, 2018
287
312
93
The website states that at the lower gain setting, the output is about 200mV pk-pk (with who knows what load attached, if any). My galaxy radio has an input impedance of about 600Ω and wants to see around 30mV across that for 100% modulation. To put 30mV across the internal 600Ω, I would need a 3.4kΩ resistor in series to properly drop the voltage (assuming the output on that board can handle a 4kΩ load, it should though).
 
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crappykraco

W9WDX Amateur Radio Club Member
Aug 6, 2014
90
40
28
Thank you all for the replies.


From the icom manual
MOD
Modulator input.
Input impedance
: 10 k
˘Input level
: Approx. 100 mV rms

Can you offer some assistance with the math involved in calculating the value of resistor needed?
 

brandon7861

Well-Known Member
Nov 28, 2018
287
312
93
100mV RMS is 282mV pk-pk. You should be good connecting directly and adjusting the gain control slightly.

By directly I mean with a decoupling cap because the output of that mic module outputs Vcc/2 (its DC coupled).

Edit: its just a voltage divider (assuming the voltage output of the module is as the website says at the load you connect to it ~ input impedance + series resistor). Just don't forget the decoupling cap! Not all radios will appreciate 2.5vDC into the mic jack.
 
Last edited:

Holydvr

Tony (The Flasher) 1776 NW Arizona
Nov 2, 2018
424
422
73
43
Golden Valley, AZ
This board have too much gain.
I tested it, it's useless.
You can try to make output attenuator, but x25 gain on board make your neighbour rooster louder than you.
Try? It is fairly easy. Given the cost of the boards the resistor needed is a drop in a bucket if it is indeed needed. At times I think you reply hoping someone comes back at you in as much of a negative way as you come at them. I will not feed your demon there, but I will echo what many have said as far as attenuation. These boards are dirt cheap. If I have to, I will work around them by any means necessary, as the parts cost is extremely marginal. I have lucked out so far, as I rebuild my own crystal and ceramic elements. I have three if these adafruit boards. I will test one for the first time in the next day or so, whether I like it or not.
 

brandon7861

Well-Known Member
Nov 28, 2018
287
312
93
Resistor inline will drop down output level to radio, not a gain, which is way too big.
I get what you're saying now... I have a friend with a mic that drives his radio with a good level signal, but if his tv or radio is on while he operates his radio, thats all I hear. The background noise covers him up.

Looking at that mic module mentioned above, it looks like the "gain" pot is directly in line with the output and not between then mic and amplifier ic.

I wonder if there is a piece of foam or other material that can be placed in front of the element, if that would even work...
 

DTB Radio

Well-Known Member
Sep 14, 2005
1,022
90
58
52
Carlisle, PA. USA
www.dtbradio.com
Hello, all. You'll probably laugh at how simple this is, but i don't mind.

I have an old turner microphone body and one of these https://www.adafruit.com/product/1063 little electret mic amp boards. I am attempting to make it work with an icom ic7200 through the ACC socket.

Here is a look at the icom acc socket

I have supplied power using 3 AA batteries.

I used the turner leaf switches.. ptt is working..

Now to the problem. The little adafruit board has a trim pot on it, and even when adjusted as low as it'll go, and the ICOM mic gain on 1, it pegs the ALC meter on the 7200.

This is quite similar to what's already been covered above, but here's what I would start with:

1. Place a 1k-ohm resistor in series with the mic unit's output to prevent possible over-loading of the output circuit.
2. from the other end of the 1k series resistor, I'd put a variable 1k ohm to ground. Simple variable voltage divider. I'd also place a .001uf capacitor across that variable resistor to help bleed off any RF that might show up in that area. It wouldn't hurt to put an RF coke between the divider's output and the radio for some added RF feedback protection.
 
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