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Will this mic work?

@KF4ZGZ
 

Or will it be to much and hurt something?
I have a TenTec 706 electret desk mic that goes with my Jupiter.
Since I use something else with the Jupiter, I have wired an adapter for the Ubitx.
But will it be too much?
According to the TenTec paperwork .....
The Mic needs7-14 v @ 5 milliamps, and adds 13 db of gain to the output.
works into any circuit of 100 ohms or more
the type is: CMOS PTT switch , signal amplifier, NPN T?R transistor
pulls 2 milliamps current on transmit

Safe or not?

Evan Hand
 

As far as I can tell without the schematic of the mic, it will not work.  The input of the uBitx has approximately 5 K ohms for the bias of the electret element.  If the current draw of the mic is 5 ma, then the drop across that resistor would be 25 volts.  At most the supplied voltage is 12 volts (13.8 with a "normal" ham supply).  Not enough to run the mic.  You would need to drop the resistor in the uBiTx mic circuit (R60) to 680 ohms  That would give a drop of (680+220)*.005=4.5 volts.  That leaves 7.5 volts for the mic.  NOTE: there is a 220 ohm resistor ahead of R60.

The PTT line on the uBiTx is a direct input on the Nano, so the 2 ma max draw should not be an issue.  If there is an issue, a pull-up resistor (2.2 K) should fix it.

Please double-check the above before you implement it.  My recommendation is to go with another solution like a 9 volt battery in the mic or wire a separate supply at the mic connector in the uBiTx.  This will depend on if you want to use an electret without a built-in amp in addition to the amplified one.  If you are going to dedicate the mic and rig together, then swap the resistor after you do more verification.

Just suggestions.  The short answer is that it will not work.
73
Evan
AC9TU

@KF4ZGZ
 

After I posted this I was looking back and figure out that I probably would not have enough to run the mic.

Dale Parfitt
 

Spot on Evan,

There is a hand mic version of the Astatic  D-104 that uses an internal battery. I have used them on my vintage rigs and get excellent audio reports.  All this assumes the mic gain stages (as designed) of the BitX are inadequate- I have not confirmed this myself.

 

Dale W4OP

 

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Evan Hand
Sent: Friday, May 29, 2020 4:08 AM
To: BITX20@groups.io
Subject: Re: [BITX20] Will this mic work?

 

As far as I can tell without the schematic of the mic, it will not work.  The input of the uBitx has approximately 5 K ohms for the bias of the electret element.  If the current draw of the mic is 5 ma, then the drop across that resistor would be 25 volts.  At most the supplied voltage is 12 volts (13.8 with a "normal" ham supply).  Not enough to run the mic.  You would need to drop the resistor in the uBiTx mic circuit (R60) to 680 ohms  That would give a drop of (680+220)*.005=4.5 volts  That leaves 7.5 volts for the mic.  NOTE: there is a 220 ohm resistor ahead of R60.

The PTT line on the uBiTx is a direct input on the Nano, so the 2 ma max draw should not be an issue.  If there is an issue, a pull-up resistor (2.2 K) should fix it.

Please double-check the above before you implement it.  My recommendation is to go with another solution like a 9 volt battery in the mic or wire a separate supply at the mic connector in the uBiTx.  This will depend on if you want to use an electret without a built-in amp in addition to the amplified one.  If you are going to dedicate the mic and rig together, then swap the resistor after you do more verification.

Just suggestions.  The short answer is that it will not work.
73
Evan
AC9TU

Vince Vielhaber
 

TenTec puts 9vdc on pin 4 of the mic connector. Not having a schematic of the mic, you may need to put a DC blocking cap on pin 1 since the ubitx puts voltage there for a condenser mic. You'll need to supply your own 9vdc to pin 4 to run the mic. Looking at your first post again, I see that mic will take 7-14. Put a 560 ohm 1/4 watt resistor from your 12v supply to the 706's pin 4. That should power the mic.

Will it be too much audio? Don't know until you try it, but if it is you can add a pot to adjust it. I did that on a bitx40.

Vince - K8ZW.

On 05/29/2020 07:28 AM, @KF4ZGZ wrote:
After I posted this I was looking back and figure out that I probably
would not have enough to run the mic.