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eHam Forums => Elmers => Topic started by: NU9J on November 11, 2012, 07:18:16 AM



Title: Mic voltage for my rig?
Post by: NU9J on November 11, 2012, 07:18:16 AM
I am modifying an Alinco DR-130 VHF FM TXRX. I am replacing the mic input with another, but that means I need to know the correct input voltage. Here are all the relevant facts I can get.

1) Measured input impedance ~12kOhm @ DC
2) Service manual states 1kHz -30dBm signal should give 4.7 kHz of deviation
3) Using an oscilloscope at it's lowest voltage setting (0.01 V/div), I can just barely see a ripple when I whistle into the mic it came with.

My best guess is that the input should be ~0.8mVrms, since -30dBm is 0.001 mW, and I'm guessing that dBm is referenced against 600 Ohms. That means 0.001 mW = V^2/600 => V = 0.774 mV.


Title: RE: Mic voltage for my rig?
Post by: K9YLI on November 11, 2012, 08:10:38 AM
Two basic kinds of  microphones..  low impedance.. about 600 ohms.
high  impedance  10k and above.
What does the book say?? I am guessing  since you measured  12 k  that it is a high impedance  mic needed.
The book also will state if it is   electret, (needing  voltage)  or   dynamic, crystal, or other type.
I think  just  match the impedance and the   element type and you are good to go.

dont get too technical....
 


Title: RE: Mic voltage for my rig?
Post by: AC5UP on November 11, 2012, 08:15:15 AM
I'm assuming you found the Alinco web site and the page for manuals.... http://www.alinco.com/pdf.files/
 (http://www.alinco.com/pdf.files/)
Electret microphone capsules tend to be similar in terms of output voltage and may differ in size, shape, and frequency response curve. If your mic is worn out and you're thinking of adapting a similar mic, it may be possible to migrate the old capsule to the new mic and then you'll know it's the correct part. If that's not an option I wouldn't worry too much. Chances are the original was nothing exotic and Alinco probably spec'd a common part. Translation: Cheap and easy to come by.

You'll find pinouts for the 8-Pin plug but no internal wiring details at the link given above.


Title: RE: Mic voltage for my rig?
Post by: NU9J on November 11, 2012, 09:00:05 AM
I'm not replacing it with a new mic. I am constructing a new input and need to know how to scale the voltage.


Title: RE: Mic voltage for my rig?
Post by: K8AXW on November 11, 2012, 09:12:58 AM
THS:  I think I understand the question as, "you want to make a different input (connector - input jack ?) for your rig?

If this is correct, then you simply duplicate the existing connections to your new connector.  Nothing else needs to be considered.

If this isn't correct, I apologize for my ignorance.


Title: RE: Mic voltage for my rig?
Post by: AC5UP on November 11, 2012, 09:53:23 AM
If you can't get a spec from Alinco, the other option is to look at what's typical for an electret mic capsule...

http://www.jlielectronics.com/spec.htm (http://www.jlielectronics.com/spec.htm)

Not much variation in sensitivity numbers and with a rating of +/- 4 dB I'd say they're all about the same. Which should give you a ballpark level and impedance to design for. Then you can add a potentiometer with a DC blocking cap and set the source level to wherever it needs to be.

Also....... If this is a digital interface, this is what Alinco suggests for packet:  http://www.alinco.com/pdf.files/Tech/Packet/DR-130T_PKT.pdf (http://www.alinco.com/pdf.files/Tech/Packet/DR-130T_PKT.pdf)



Title: RE: Mic voltage for my rig?
Post by: KA4POL on November 11, 2012, 10:45:49 AM
I am modifying an Alinco DR-130 VHF FM TXRX. I am replacing the mic input with another, but that means I need to know the correct input voltage.
Looking at the schematic you see a simple bipolar transistor (2SC4081) amplifier for the signal coming from the microphone. Is it that amplifier you want to replace? What is your intention? Do you want to change the frequency response of the input?


Title: RE: Mic voltage for my rig?
Post by: NU9J on November 11, 2012, 10:54:40 AM
If you can't get a spec from Alinco, the other option is to look at what's typical for an electret mic capsule...

http://www.jlielectronics.com/spec.htm (http://www.jlielectronics.com/spec.htm)

Not much variation in sensitivity numbers and with a rating of +/- 4 dB I'd say they're all about the same. Which should give you a ballpark level and impedance to design for. Then you can add a potentiometer with a DC blocking cap and set the source level to wherever it needs to be.

Also....... If this is a digital interface, this is what Alinco suggests for packet:  http://www.alinco.com/pdf.files/Tech/Packet/DR-130T_PKT.pdf (http://www.alinco.com/pdf.files/Tech/Packet/DR-130T_PKT.pdf)

Hm, the sensitivity is in V/Pa. This website (http://www.engineeringtoolbox.com/sound-pressure-d_711.html) suggests, for human voice levels, 50-60 dB = 2-20 Pa. Sensitivity of -42 dB would then give 0.1-1 mVrms. Looks like the 2 estimates so far line up at about 1mVrms. Seem reasonable? Maybe could add the trimmer as you suggest to give something between 0.5 and 2 mV.

BTW, this is for a bluetooth radio. Should be able to control the radio from the computer or smartphone without any wires :)