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Two coms mic switching

 
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cofford



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

PostPosted: Wed Mar 11, 2020 3:29 pm    Post subject: Two coms mic switching Reply with quote

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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PostPosted: Thu Mar 12, 2020 4:01 am    Post subject: Two coms mic switching Reply with quote

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
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PostPosted: Thu Mar 12, 2020 7:34 am    Post subject: Two coms mic switching Reply with quote

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

PostPosted: Thu Mar 12, 2020 9:27 am    Post subject: Re: Two coms mic switching Reply with quote

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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PostPosted: Thu Mar 12, 2020 9:44 am    Post subject: Two coms mic switching Reply with 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

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
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PostPosted: Thu Mar 12, 2020 10:15 am    Post subject: Two coms mic switching Reply with quote

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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PostPosted: Thu Mar 12, 2020 10:36 am    Post subject: Two coms mic switching Reply with quote

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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PostPosted: Thu Mar 12, 2020 10:37 am    Post subject: Two coms mic switching Reply with quote

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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PostPosted: Thu Mar 12, 2020 1:04 pm    Post subject: Two coms mic switching Reply with quote

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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