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Mixing together two mics

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Radioflyer

Joined: 04 Feb 2009
Posts: 43
Location: Boston

Posted: Sat Jun 11, 2011 7:46 pm Post subject: Mixing together two mics

Is it workable to mix two mics together for input into a Com's mike jack by using impedance matching resistors? Here' my situation...



My two place aircraft is fitted with one headphone jack and one mic jack. The mic is keyed via SPST PTT switches in the control sticks. The intercom I  have is a portable unit that inconveniently gives me two mic output plugs: one for pilot and one for copilot.



Ostensibly, the two mic ouputs are to allow use of external DPDT PTT switches and a "Y" harness which would then plug into the aircraft's one mic jack. I understand that the normally open DPDT switches essentially shield one mic from the other and allow the "Y" cord to "mix" the mics into one output. However,  I want to avoid the resulting cabling mess if I were to do this. So, I'm looking for a good way to combine the mic outputs into one output without causing too much signal deterioration.



--Jose

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nuckolls.bob(at)aeroelect
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Posted: Sun Jun 12, 2011 5:33 am Post subject: Mixing together two mics

At 11:46 PM 6/11/2011, you wrote:

 	  Quote: 	
		  



Is it workable to mix two mics together for input into a Com's mike 

jack by using impedance matching resistors? Here' my situation...



My two place aircraft is fitted with one headphone jack and one mic 

jack. The mic is keyed via SPST PTT switches in the control sticks. 

The intercom I  have is a portable unit that inconveniently gives me 

two mic output plugs: one for pilot and one for copilot.



Ostensibly, the two mic ouputs are to allow use of external DPDT PTT 

switches and a "Y" harness which would then plug into the aircraft's 

one mic jack. I understand that the normally open DPDT switches 

essentially shield one mic from the other and allow the "Y" cord to 

"mix" the mics into one output. However,  I want to avoid the 

resulting cabling mess if I were to do this. So, I'm looking for a 

good way to combine the mic outputs into one output without causing 

too much signal deterioration.


	


    Not enough information to offer cogent advice.

    I've not encountered this kind of intercom before

    and it's difficult to deduce the rationale behind

    its design.



    Are you planning to permanently install this

    intercom? If so, it seems practical to eliminate

    the y-cable and simply hardwire the harness

    to emulate the 'portable' wiring configuration.



    Without seeing internal schematics for the intercom,

    we have no way of knowing how the push to talk

    switches influence connection of the microphones.



    A simple y-cable can do no more than parallel

    the three conductors. It's a low risk experiment.



   Bob . . .

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Radioflyer

Joined: 04 Feb 2009
Posts: 43
Location: Boston

Posted: Sun Jun 12, 2011 1:12 pm Post subject: Re: Mixing together two mics

The Intercom is an RST 442B. Old stuff, but it works well and I can't afford a replacement right now. So, I'm planning to install it as a permanent intercom and am trying to hardwire the "portable harness" as you suggest. The only problem is how to make the two output mic signals as one. If I follow to the letter the way the two mic outputs are to be used with a DPDT PTT switch and Y-cable, I wind up with 



I think the rationale was to allow each pilot to install his own PTT switch without involving the aircraft wiring and to allow easy portability to any other aircraft. 



Each mic that plugs into the intercom sees a resistor network that eventually sums all inputs into an LM386.  Before each mic signal gets very far into the network it is looped back via a 27 Ohm resistor as a mic output. I can send a schematic.



My assumption is that it is not OK to simply tie the two mics together to make one mic output. I'm hoping some simple resistors can be used to mix the two mic outputs. I f I picked up a "mixed signal" after the LM386, then I would get all audio (music, nav, marker beacon, etc) possibly into the COM and I think I want to avoid this.



--Jose

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Jun 12, 2011 3:18 pm Post subject: Mixing together two mics

At 05:12 PM 6/12/2011, you wrote:

 	  Quote: 	
		  



The Intercom is an RST 442B. Old stuff,


	


   I found the schematic in my archives . . .



 	  Quote: 	
		  Each mic that plugs into the intercom sees a resistor network that 

eventually sums all inputs into an LM386.  Before each mic signal 

gets very far into the network it is looped back via a 27 Ohm 

resistor as a mic output. I can send a schematic.



My assumption is that it is not OK to simply tie the two mics 

together to make one mic output. I'm hoping some simple resistors 

can be used to mix the two mic outputs. I f I picked up a "mixed 

signal" after the LM386, then I would get all audio (music, nav, 

marker beacon, etc) possibly into the COM and I think I want to avoid this.


	


   Yup. But as a permanent installation, you could do something

   like the 'relay' version of PTT and mount the relays in an enclosure

   along with the intercom board. A sort of re-packaging effort

   that would meed Jim's design goals while keeping all

   the mickey-mouse wiring corralled in one enclosure.



   Bob . . .

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Radioflyer

Joined: 04 Feb 2009
Posts: 43
Location: Boston

Posted: Sun Jun 12, 2011 3:49 pm Post subject: Re: Mixing together two mics

Bob, Thanks for taking the time to look at the schematic. I guess you're right and that I should just implement a relay. I was hoping to avoid yet another device, but it is probably the path of least resistance given the situation.



Or....maybe I could install a 2 channel isolation op-amp (unity gain) in the intercom to combine the pilot copilot mics lines. Hmmmm.  Decisions, decisions.

--Jose

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Jun 12, 2011 4:16 pm Post subject: Mixing together two mics

At 07:49 PM 6/12/2011, you wrote:

 	  Quote: 	
		  



Bob, Thanks for taking the time to look at the schematic. I guess 

you're right and that I should just implement a relay. I was hoping 

to avoid yet another device, but it is probably the path of least 

resistance given the situation.



Or....maybe I could install a 2 channel isolation op-amp (unity 

gain) in the intercom to combine the pilot copilot mics lines.


	


   But that would put both mics into the keyed

   radio at the same time . .  probably not good.



 	  Quote: 	
		    Hmmmm.  Decisions, decisions.


	


   There's a host of useful experiments that would

   probably lead to a useful configuration. It's all

   a matter of time and return on investment. I can't

   count how many times I would have been better off

   just following the instructions . . . or pitching

   the thing and starting from scratch.



   Nobody ever said that education was cheap.

   Bob . . .

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