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Two coms mic switching

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cofford

Joined: 01 Jan 2018
Posts: 9
Location: Puget Sound, WA

Posted: Wed Mar 11, 2020 3:29 pm Post subject: Two coms mic switching

Hello all,



I'm sure this has been answered before, but I'm planning to use two com radios without an audio panel, so I need to switch the mic input.  My current plan is to use two PTT switches on the stick, and tie the COM 2 PTT to a relay to switch the mic automatically.  Does only the "MIC HIGH" side need to be switched, or do I need to switch the "MIC LO" side as well?  I think "MIC LO" is just a ground reference for the mic input, but I'm looking for the lowest noise solution.



Thanks,

Casey

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alec(at)alecmyers.com
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Posted: Thu Mar 12, 2020 4:01 am Post subject: Two coms mic switching

I don’t think you need to switch either side. Run the mic in parallel into both radios, and allow the appropriate PTT to select which transmits.



 	  Quote: 	
		   On Mar 11, 2020, at 19:31, cofford <cofford(at)gmail.com> wrote:

 


	






Hello all,



I'm sure this has been answered before, but I'm planning to use two com radios without an audio panel, so I need to switch the mic input.  My current plan is to use two PTT switches on the stick, and tie the COM 2 PTT to a relay to switch the mic automatically.  Does only the "MIC HIGH" side need to be switched, or do I need to switch the "MIC LOW" side as well?  I think "MIC LOW" is just a ground reference for the mic input, but I'm looking for the lowest noise solution.



Thanks,

Casey





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=495242#495242

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Mar 12, 2020 7:34 am Post subject: Two coms mic switching

At 06:59 AM 3/12/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: Alec Myers <alec(at)alecmyers.com>

 I don’t think you need to switch either side. Run the mic in parallel into both radios, and allow the appropriate PTT to select which transmits.

 > On Mar 11, 2020, at 19:31, cofford <cofford(at)gmail.com> wrote:

 > 

 --> AeroElectric-List message posted by: "cofford" <cofford(at)gmail.com>

 Hello all,

 I'm sure this has been answered before, but I'm planning to use two com radios without an audio panel, so I need to switch the mic input.  My current plan is to use two PTT switches on the stick, and tie the COM 2 PTT to a relay to switch the mic automatically.  Does only the "MIC HIGH" side need to be switched, or do I need to switch the "MIC LOW" side as well?  I think "MIC LOW" is just a ground reference for the mic input, but I'm looking for the lowest noise solution. 	

   A DPDT switch routes PTT and MI HI to each radio

   independently. An example of this is shown in

   Figure 18-11 of the 'Connection

  http://www.aeroelectric.com/Books/Connection/18_Audio.pdf

   Bob . . .

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cofford

Joined: 01 Jan 2018
Posts: 9
Location: Puget Sound, WA

Posted: Thu Mar 12, 2020 9:27 am Post subject: Re: Two coms mic switching

 	  nuckolls.bob(at)aeroelect wrote: 	
		  At 06:59 AM 3/12/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: Alec Myers <alec>

 I don’t think you need to switch either side. Run the mic in parallel into both radios, and allow the appropriate PTT to select which transmits.

 > On Mar 11, 2020, at 19:31, cofford <cofford> wrote:

 > 

 --> AeroElectric-List message posted by: "cofford" <cofford>

 Hello all,

 I'm sure this has been answered before, but I'm planning to use two com radios without an audio panel, so I need to switch the mic input.  My current plan is to use two PTT switches on the stick, and tie the COM 2 PTT to a relay to switch the mic automatically.  Does only the "MIC HIGH" side need to be switched, or do I need to switch the "MIC LOW" side as well?  I think "MIC LOW" is just a ground reference for the mic input, but I'm looking for the lowest noise solution. 	

   A DPDT switch routes PTT and MI HI to each radio

   independently. An example of this is shown in

   Figure 18-11 of the 'Connection

  http://www.aeroelectric.com/Books/Connection/18_Audio.pdf

   Bob . . . 	

Thanks for the answers.  Not switching at all would cause an impedance mismatch in my case as both radios have a built-in intercom so the mic input would always be live.  Sounds like switching the MIC HI line is the way to go.

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alec(at)alecmyers.com
Guest

Posted: Thu Mar 12, 2020 9:44 am Post subject: Two coms mic switching

I’m pretty sure microphones aren’t impedance matched; a mic input is a high impedance input and the mic is (?) 600 ohms, so running one into two inputs doesn’t load the source. 

Sent from my iPhone



 	  Quote: 	
		   On Mar 12, 2020, at 1:27 PM, cofford <cofford(at)gmail.com> wrote:

 

 

 

 

 nuckolls.bob(at)aeroelect wrote:

> At 06:59 AM 3/12/2020, you wrote:

> 

>> 

>> 

>> I don’t think you need to switch either side. Run the mic in parallel into both radios, and allow the appropriate PTT to select which transmits.

>> 

>>> On Mar 11, 2020, at 19:31, cofford  wrote:

>>> 

>> 

>> 

>> 

>> Hello all,

>> 

>> I'm sure this has been answered before, but I'm planning to use two com radios without an audio panel, so I need to switch the mic input.  My current plan is to use two PTT switches on the stick, and tie the COM 2 PTT to a relay to switch the mic automatically.  Does only the "MIC HIGH" side need to be switched, or do I need to switch the "MIC LOW" side as well?  I think "MIC LOW" is just a ground reference for the mic input, but I'm looking for the lowest noise solution.

> 

> 

>   A DPDT switch routes PTT and MI HI to each radio

>   independently. An example of this is shown in

>   Figure 18-11 of the 'Connection

> 

>  http://www.aeroelectric.com/Books/Connection/18_Audio.pdf

> 

> 

> 

> 

> 

>   Bob . . .

 

 

 Thanks for the answers.  Not switching at all would cause an impedance mismatch in my case as both radios have a built-in intercom so the mic input would always be live.  Sounds like switching the MIC HI line is the way to go.

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=495264#495264

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ceengland7(at)gmail.com
Guest

Posted: Thu Mar 12, 2020 10:15 am Post subject: Two coms mic switching

That's kinda like adult diapers. (It Depends.)



Most audio circuits are a low impedance feeding a high impedance, but 

you can't assume it's true in every case. You may well find high gain 

inputs that have their load impedance intentionally kept low, to reduce 

noise susceptibility when there's no source connected.



On 3/12/2020 12:40 PM, Alec wrote:

 	  Quote: 	
		   



 I’m pretty sure microphones aren’t impedance matched; a mic input is a high impedance input and the mic is (?) 600 ohms, so running one into two inputs doesn’t load the source.

 Sent from my iPhone



> On Mar 12, 2020, at 1:27 PM, cofford <cofford(at)gmail.com> wrote:

>

> 

> nuckolls.bob(at)aeroelect wrote:

>> At 06:59 AM 3/12/2020, you wrote:

>>

>>> 

>>>

>>> I don’t think you need to switch either side. Run the mic in parallel into both radios, and allow the appropriate PTT to select which transmits.

>>>

>>>> On Mar 11, 2020, at 19:31, cofford  wrote:

>>>>

>>> 

>>>

>>> Hello all,

>>>

>>> I'm sure this has been answered before, but I'm planning to use two com radios without an audio panel, so I need to switch the mic input.  My current plan is to use two PTT switches on the stick, and tie the COM 2 PTT to a relay to switch the mic automatically.  Does only the "MIC HIGH" side need to be switched, or do I need to switch the "MIC LOW" side as well?  I think "MIC LOW" is just a ground reference for the mic input, but I'm looking for the lowest noise solution.

>>

>>    A DPDT switch routes PTT and MI HI to each radio

>>    independently. An example of this is shown in

>>    Figure 18-11 of the 'Connection

>>

>>   http://www.aeroelectric.com/Books/Connection/18_Audio.pdf

>>

>>

>>

>>

>>

>>    Bob . . .

>

> Thanks for the answers.  Not switching at all would cause an impedance mismatch in my case as both radios have a built-in intercom so the mic input would always be live.  Sounds like switching the MIC HI line is the way to go.


	




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alec(at)alecmyers.com
Guest

Posted: Thu Mar 12, 2020 10:36 am Post subject: Two coms mic switching

I know. The nerdy part of me really really really wants to try it, to see if it works. 

On Mar 12, 2020, at 2:10 PM, Charlie England <ceengland7(at)gmail.com> wrote:

That's kinda like adult diapers. (It Depends.)

Most audio circuits are a low impedance feeding a high impedance, but you can't assume it's true in every case. You may well find high gain inputs that have their load impedance intentionally kept low, to reduce noise susceptibility when there's no source connected.

On 3/12/2020 12:40 PM, Alec wrote:

 	  Quote: 	

 I’m pretty sure microphones aren’t impedance matched; a mic input is a high impedance input and the mic is (?) 600 ohms, so running one into two inputs doesn’t load the source.

 Sent from my iPhone

> On Mar 12, 2020, at 1:27 PM, cofford <cofford(at)gmail.com> wrote:

> 

> 

> 

> 

> nuckolls.bob(at)aeroelect wrote:

>> At 06:59 AM 3/12/2020, you wrote:

>> 

>>> 

>>> 

>>> I don’t think you need to switch either side. Run the mic in parallel into both radios, and allow the appropriate PTT to select which transmits.

>>> 

>>>> On Mar 11, 2020, at 19:31, cofford  wrote:

>>>> 

>>> 

>>> 

>>> Hello all,

>>> 

>>> I'm sure this has been answered before, but I'm planning to use two com radios without an audio panel, so I need to switch the mic input.  My current plan is to use two PTT switches on the stick, and tie the COM 2 PTT to a relay to switch the mic automatically.  Does only the "MIC HIGH" side need to be switched, or do I need to switch the "MIC LOW" side as well?  I think "MIC LOW" is just a ground reference for the mic input, but I'm looking for the lowest noise solution.

>> 

>>   A DPDT switch routes PTT and MI HI to each radio

>>   independently. An example of this is shown in

>>   Figure 18-11 of the 'Connection

>> 

>>  http://www.aeroelectric.com/Books/Connection/18_Audio.pdf

>> 

>> 

>> 

>> 

>> 

>>   Bob . . .

> 

> Thanks for the answers.  Not switching at all would cause an impedance mismatch in my case as both radios have a built-in intercom so the mic input would always be live.  Sounds like switching the MIC HI line is the way to go.

-- 

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trigo(at)mail.telepac.pt
Guest

Posted: Thu Mar 12, 2020 10:37 am Post subject: Two coms mic switching

It’s kinda cool to assist these technically intrincated discussions 😉



Enviado do meu iPhone



 	  Quote: 	
		   No dia 12/03/2020, às 18:22, Charlie England <ceengland7(at)gmail.com> escreveu:

 

 

 

 That's kinda like adult diapers. (It Depends.)

 

 Most audio circuits are a low impedance feeding a high impedance, but you can't assume it's true in every case. You may well find high gain inputs that have their load impedance intentionally kept low, to reduce noise susceptibility when there's no source connected.

 

> On 3/12/2020 12:40 PM, Alec wrote:

> 

> 

> I’m pretty sure microphones aren’t impedance matched; a mic input is a high impedance input and the mic is (?) 600 ohms, so running one into two inputs doesn’t load the source.

> 

> 

> Sent from my iPhone

> 

>>> On Mar 12, 2020, at 1:27 PM, cofford <cofford(at)gmail.com> wrote:

>> 

>> 

>> 

>> 

>> nuckolls.bob(at)aeroelect wrote:

>>> At 06:59 AM 3/12/2020, you wrote:

>>> 

>>>> 

>>>> 

>>>> I don’t think you need to switch either side. Run the mic in parallel into both radios, and allow the appropriate PTT to select which transmits.

>>>> 

>>>>> On Mar 11, 2020, at 19:31, cofford  wrote:

>>>>> 

>>>> 

>>>> 

>>>> Hello all,

>>>> 

>>>> I'm sure this has been answered before, but I'm planning to use two com radios without an audio panel, so I need to switch the mic input.  My current plan is to use two PTT switches on the stick, and tie the COM 2 PTT to a relay to switch the mic automatically.  Does only the "MIC HIGH" side need to be switched, or do I need to switch the "MIC LOW" side as well?  I think "MIC LOW" is just a ground reference for the mic input, but I'm looking for the lowest noise solution.

>>> 

>>>   A DPDT switch routes PTT and MI HI to each radio

>>>   independently. An example of this is shown in

>>>   Figure 18-11 of the 'Connection

>>> 

>>>  http://www.aeroelectric.com/Books/Connection/18_Audio.pdf

>>> 

>>> 

>>> 

>>> 

>>> 

>>>   Bob . . .

>> 

>> Thanks for the answers.  Not switching at all would cause an impedance mismatch in my case as both radios have a built-in intercom so the mic input would always be live.  Sounds like switching the MIC HI line is the way to go.

 

 

 -- 

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Mar 12, 2020 1:04 pm Post subject: Two coms mic switching

 	  Quote: 	
		  

 Most audio circuits are a low impedance feeding a high impedance, but you can't assume it's

 true in every case. You may well find high gain inputs that have their load impedance

 intentionally kept low, to reduce noise susceptibility when there's no source connected. 	


   Keep in mind that the MIC HI input to an intercom

   or transmitter is designed to interface with a

   powered, amplified microphone. This is a legacy

   hold-over from the days when aircraft microphones

   were not unlike those in your telephone . . .

   carbon granules behind a foil diaphragm.



  https://tinyurl.com/htnn6nq



   This microphone is essentially a resistor whose

   resistance varies with audio stimulation. Hence

   it must first be biased up with some current

   so that the electronics can tap the resulting

   voltage variances that represent speech.



   Here's an exemplar MIC input circuit for a

   King transceiver . . .



 [img]cid:.0[/img]



   Looking into the DC power path, the DC source

   impedance is on the order of 600 ohms, the AC

   load impedance is on the order of 390 ohms

   while the load impedance of the first audio

   stage is about 48,000 ohms.



   So while the voltage INPUT network may indeed

   be 'high' impedance, the microphone BIAS network

   is not. Further, paralleling two such inputs

   doubles the bias to any one microphone.



   The using DPDT "TX SELECT" switch for PTT and

   MIC HI lines is pretty much mandatory.



 

 

   Bob . . .

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