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aux music input quiet on flightcom 403mc

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bryanflood

Joined: 13 Aug 2006
Posts: 9

Posted: Thu Apr 05, 2007 8:05 pm Post subject: aux music input quiet on flightcom 403mc

I had the same problem many others did connecting a music input to my flightcom 403mc. The music was so soft at top volume I could barely hear it on the ground and not at all in flight. Turns out in order to save $ there is apparently no pre-amp in this intercom. So if you want to hook up an mp3 player or walkman or whatever you need to supply your own pre-amp. I looked at all the options and decided to try to build my own. I looked at a lot of circuits and borrowed heavily from some like Bob N. I had problems with bobs circuit when coverting from a stereo input to a mono output. It appears that on his amp design where one leg of the stereo input goes to the + input on the op amp and the other leg of the stereo input to the - input on the op amp. So when you hook it up it seems some of the music cancels out any you get some really funky sounds. Very cool, but no music. Maybe I did it wrong, after all Bob is the expert but it certainly looked like bobs design on the breadboard. After lots of testing however I made changes to any existing designs to get the volume to a reasonable level in the headset with the volume control turned to a medium level on my mp3 player. My set-up is simple the mp3 player headphone jack simply goes into the aux music input jack which goes thru this little amp and then to the intercom. Anyway, I'm not an electrical engineer (I'm mechanical) so I am really not all that great at designing circuits, but this one works for me. I thought I would post it in case anybody want to use it as a starting point for their own amp.



http://www.geocities.com/bryanflood2001/AudioAmpfor403MC.bmp



Bryan

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Rv-9A completed.

nuckollsr(at)cox.net
Guest

Posted: Thu Apr 05, 2007 9:01 pm Post subject: aux music input quiet on flightcom 403mc

At 09:05 PM 4/5/2007 -0700, you wrote:



 	  Quote: 	
		  



I had the same problem many others did connecting a music input to my 

flightcom 403mc. The music was so soft at top volume I could barely hear 

it on the ground and not at all in flight. Turns out in order to save $ 

there is apparently no pre-amp in this intercom. So if you want to hook up 

an mp3 player or walkman or whatever you need to supply your own pre-amp. 

I looked at all the options and decided to try to build my own. I looked 

at a lot of circuits and borrowed heavily from some like Bob N. I had 

problems with bobs circuit when coverting from a stereo input to a mono 

output. It appears that on his amp design where one leg of the stereo 

input goes to the + input on the op amp and the other leg of the stereo 

input to the - input on the op amp. So when you hook it up it seems some 

of the music cancels out any you get some really funky sounds. Very cool, 

but no music. Maybe I did it wrong, after all Bob is the expert but it 

certainly looked like bobs design on the breadboar!


	


    Hmmmm . . . stupid mistake. I'm mystified as to how that

    happened because I have two other versions of the board

    that are wired correctly.  I've have a version

    9009-300-2A which IS wired with the correct polarity but

    also adds spaces for R98 and R99 load resistors cited

    in Note 8 on the drawing. I'll get these ordered tonight.

    The flipped polarity for stereo-inputs blew right past

    me on the earlier version.



 	  Quote: 	
		    d. After lots of testing however I made changes to any existing designs 

 to get the volume to a reasonable level in the headset with the volume 

 control turned to a medium level on my mp3 player. My set-up is simple 

 the mp3 player headphone jack simply goes into the aux music input jack 

 which goes thru this little amp and then to the intercom. Anyway, I'm not 

 an electrical engineer (I'm mechanical) so I am really not all that great 

 at designing circuits, but this one works for me. I thought I would post 

 it in case anybody want to use it as a starting point for their own amp.



http://www.geocities.com/bryanflood2001/AudioAmpfor403MC.bmp


	


    Can't argue with success! If folks are interested

    in more detail on the LM386 audio amplifier chip

    the data sheet can be downloaded from . . .



http://cache.national.com/ds/LM/LM386.pdf



    In the mean time, if anyone is interested building

    the isolation amplifier and doesn't mind doing a bit

    of chop-n-tack to fix the layout error, watch for

    the 9009-300-1 boards to go on sale for $10 each

    when I add the newer 9009-300-2A versions at the

    regular price.



    Thanks for the heads-up on this Bryan.



    Bob . . .



        Bob . . .



        ----------------------------------------

        ( IF one aspires to be "world class",  )

        ( what ever you do must be exercised   )

        ( EVERY day . . .                      )

        (                  R. L. Nuckolls III  )

        ----------------------------------------

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nuckollsr(at)cox.net
Guest

Posted: Fri Apr 06, 2007 4:12 am Post subject: aux music input quiet on flightcom 403mc

At 09:05 PM 4/5/2007 -0700, you wrote:



 	  Quote: 	
		  



I had the same problem many others did connecting a music input to my 

flightcom 403mc. The music was so soft at top volume I could barely hear 

it on the ground and not at all in flight. Turns out in order to save $ 

there is apparently no pre-amp in this intercom. So if you want to hook up 

an mp3 player or walkman or whatever you need to supply your own pre-amp. 

I looked at all the options and decided to try to build my own. I looked 

at a lot of circuits and borrowed heavily from some like Bob N. I had 

problems with bobs circuit when coverting from a stereo input to a mono 

output. It appears that on his amp design where one leg of the stereo 

input goes to the + input on the op amp and the other leg of the stereo 

input to the - input on the op amp. So when you hook it up it seems some 

of the music cancels out any you get some really funky sounds. Very cool, 

but no music. Maybe I did it wrong, after all Bob is the expert but it 

certainly looked like bobs design on the breadboar!

  d. After lots of testing however I made changes to any existing designs 

 to get the volume to a reasonable level in the headset with the volume 

 control turned to a medium level on my mp3 player. My set-up is simple 

 the mp3 player headphone jack simply goes into the aux music input jack 

 which goes thru this little amp and then to the intercom. Anyway, I'm not 

 an electrical engineer (I'm mechanical) so I am really not all that great 

 at designing circuits, but this one works for me. I thought I would post 

 it in case anybody want to use it as a starting point for their own amp.



http://www.geocities.com/bryanflood2001/AudioAmpfor403MC.bmp


	


    You might want to consider a couple of resistors in

    series with each of the stereo outputs before you

    bring them together as a monophonic mix. Output

    signals from the two channels are not identical

    and some degradation of audio will occur when the

    outputs "load" each other as described in the chapter

    on audio systems. Something on the order of 47 ohms

    per side is suggested.



    Bob . . .

        ----------------------------------------

        ( IF one aspires to be "world class",  )

        ( what ever you do must be exercised   )

        ( EVERY day . . .                      )

        (                  R. L. Nuckolls III  )

        ----------------------------------------

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rjquillin(at)gmail.com
Guest

Posted: Fri Apr 06, 2007 7:03 am Post subject: aux music input quiet on flightcom 403mc

A couple of comments if I may.  First I have to agree with Bob,

directly connecting two outputs to one another is not a good idea, at

all, and can cause damage to the output circuit of the source device.

47 ohms would work, but at a minimum use a value equal to the

impedance (resistance) of the earphone/speaker normally connected; say

10 ohms minimum.  Also, the 10 ohm 10uF combination will limit low

frequencies below ~1600Hz; Ok if you don't want much low end, but...



Attached is a pdf you may want to try.  Keep the rest as you had it if

it seems to work well.  This one will sum (mix) the L & R signals

without nuking the mp3 player's output, should give you cleaner audio

as well.  If you don't like the 10/100 ohm pair, go to 100/1k instead.

 Also with this combination the LF will be extended down to about

160Hz.



Ron Q.

old audio engineer.



 	  Quote: 	
		   >  <snai> Anyway, I'm not

 > an electrical engineer (I'm mechanical) so I am really not all that great

 > at designing circuits, but this one works for me. I thought I would post

 > it in case anybody want to use it as a starting point for their own amp.

 >

 >http://www.geocities.com/bryanflood2001/AudioAmpfor403MC.bmp



     You might want to consider a couple of resistors in

     series with each of the stereo outputs before you

     bring them together as a monophonic mix. Output

     signals from the two channels are not identical

     and some degradation of audio will occur when the

     outputs "load" each other as described in the chapter

     on audio systems. Something on the order of 47 ohms

     per side is suggested.



     Bob . . .

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bryanflood

Joined: 13 Aug 2006
Posts: 9

Posted: Fri Apr 06, 2007 6:10 pm Post subject: Re: aux music input quiet on flightcom 403mc

Bob and Ron, thanks for the advice, it looks like I may need to do yet another mod... for some reason I am having trouble with the .pdf file from Ron. Could you send it to me directly at bryanflood(at)hotmail.com? I would really appreciate it.



Thanks again, 



Bryan

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Rv-9A completed.

bryanflood

Joined: 13 Aug 2006
Posts: 9

Posted: Fri Apr 06, 2007 6:20 pm Post subject: Re: aux music input quiet on flightcom 403mc

WOOPS, I foud the drawing from Ron. Just needed to read a bit further. 



Sorry,



Bryan

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_________________
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billp(at)wwpc.com
Guest

Posted: Mon Apr 09, 2007 6:33 am Post subject: aux music input quiet on flightcom 403mc

I had this same problem with my intercom. The basic problem is that the 

MP3 player has headphone outputs that are low impedance. That is to say 

low voltage but high current output. The intercom input is high 

impedance and wants a little higher voltage. I fixed the problem by 

making a little box with 2 Radio Shack speaker to line transformers in 

it (one for left and one for right) and a pair of 1/8" stereo jacks. If 

I remember correctly the transformers were like 8 ohms on one side and 

2000 ohms on the other side. That effectively gives a 1:250 increase in 

volume level to the intercom.

I ruined an MP3 player running it at 100% volume and it still wasn't 

loud enough before I built this box. Now I never run above half volume. 

The best thing is that the transformer box doesn't need batteries and 

the battery in the MP3 player lasts a lot longer because it's not 

working so hard.

I think if you need a mono output to your intercom, you could either mix 

the high impedance outputs of the transformers with a couple of 1K 

resistors or mix the inputs with a couple of 10 ohm resistors and then 

take that and put it through a single transformer.

- Bill

Ron Quillin wrote:

 	  Quote: 	
		   A couple of comments if I may.  First I have to agree with Bob,

 directly connecting two outputs to one another is not a good idea, at

 all, and can cause damage to the output circuit of the source device.

 47 ohms would work, but at a minimum use a value equal to the

 impedance (resistance) of the earphone/speaker normally connected; say

 10 ohms minimum.  Also, the 10 ohm 10uF combination will limit low

 frequencies below ~1600Hz; Ok if you don't want much low end, but...

 Attached is a pdf you may want to try.  Keep the rest as you had it if

 it seems to work well.  This one will sum (mix) the L & R signals

 without nuking the mp3 player's output, should give you cleaner audio

 as well.  If you don't like the 10/100 ohm pair, go to 100/1k instead.

 Also with this combination the LF will be extended down to about

 160Hz.

 Ron Q.

 old audio engineer.

> >  <snai> Anyway, I'm not

> > an electrical engineer (I'm mechanical) so I am really not all that 

> great

> > at designing circuits, but this one works for me. I thought I would 

> post

> > it in case anybody want to use it as a starting point for their own 

> amp.

> >

> >http://www.geocities.com/bryanflood2001/AudioAmpfor403MC.bmp

>

>     You might want to consider a couple of resistors in

>     series with each of the stereo outputs before you

>     bring them together as a monophonic mix. Output

>     signals from the two channels are not identical

>     and some degradation of audio will occur when the

>     outputs "load" each other as described in the chapter

>     on audio systems. Something on the order of 47 ohms

>     per side is suggested.

>

>     Bob . . .

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rjquillin(at)gmail.com
Guest

Posted: Tue Apr 10, 2007 8:07 am Post subject: aux music input quiet on flightcom 403mc

At 07:30 4/9/2007, you wrote:

 	  Quote: 	
		  I think if you need a mono output to your intercom, you could either 

mix the high impedance outputs of the transformers with a couple of 

1K resistors or mix the inputs with a couple of 10 ohm resistors and 

then take that and put it through a single transformer.



- Bill


	


Transformers are an excellent solution to do some impedance matching 

and increase the signal voltage available to the intercom and break a 

possible grounding problem between the source device, if powered by 

ships' power, that at times causes unwanted noise in audio systems, 

however a few words of caution and some thoughts.



For reference, from the 603mc IM the aux input Z is 620 ohms, let's 

call it 600 and keep it simple.



While the transformer will break any DC connection, an AC connection 

remains, and source and load resistances (impedances) from the input 

to output, or vice versa, are still a function of the turns 

ratio.  Simply put, given a transformer with a 1:1 TURNS ratio 

connected to a 600 ohm output load, what ever it is connected to the 

input will see that 600 ohm load; 10k on the load, the input sees 

10k.  It's called a reflected load. The voltage and impedance ratios 

are only the same with a 1:1 transformer.  While the voltage change 

is equal to the turns ratio, the impedance difference is the square 

of the turns ratio, or the turns ratio is the square-root of the 

impedance ratio if that's what you have to work with.



For that 8 ohm to 2000 ohm the Z ratio is 250, or 1:250, but the 

turns ratio is just 1:15.8; assuming an ideal transformer.  I'll call 

it 1:16 for a real world value.  Still a good increase in voltage, 

but we still have to consider reflected load impedance.  That 2000 

ohm secondary is loaded by a 600 ohm input.  Divide that load 

resistance by 250, the Z ratio, and the device driving the 

transformer sees a very low 2.4 ohms.

That mp3 player would likely be most unhappy.



On to your solutions proposed above.  Hopefully I've correctly 

understood what you were saying and put it to paper.  Only two sheets 

this time.   Apologies for that entire data sheet before, that was 

not my intent at all !



Sheet one shows what happens with the first solution.  Load Z should 

be fine at ~42 ohms, but the actual output voltage to the aux input 

from a single channel is only 90% of what the mp3 player puts out due 

to the loss in the resistive combining network.  The actual output 

would add L & R algebraically, but would not double unless both 

channels had exactly the same program material, a mono source.



Sheet two is what I think is your second suggestion.  Here the load Z 

an mp3 output channel would 'see' is ~12.4 ohms.  Depending on the 

device that may be just fine if still a tad low.  The overall voltage 

'gain' is again the product of the transformer gain, the turns ratio, 

16, and the loss from the 10 ohm input resistors looking into that 

2.4 ohm reflected load; only 19% of the signal gets through.  So the 

overall voltage gain, from the output of the mp3 to the aux input of 

the 403 is a bit over 3.  That's good, but likely not really enough.



For both of these examples we have the generalized case of the 

voltage 'gain' of the transformer turns ratio (~16) times the 'loss' 

of the [resistor - transformer] combining network ( 0.0566 or 0.1935) 

working into the 600 ohm input load of the 403.  Were the input-Z of 

the 603 higher some additional voltage gain from the transformer 

would be easier to achieve, but then other issues of mixing outputs 

crop back in.



The active summing amplifier solution, for practical purposes, 

eliminates any of those problems since the inverting input of the 

op-amp is essentially at ground and neither output channel sees or 

interacts with the other.  One down side is the inversion of absolute 

signal polarity for both left and right channels.  Using a 

combination of both two transformers and the active summing both the 

inversion could be corrected, the mp3 output could be de-loaded a bit 

more to keep it happier, and you would have the gain of the amp as 

well.  Use circuit one and take the junction of the two 10k resistors 

to the inverting input pin #2. The mp3 now sees a very nice 40 ohm 

load, there is no channel to channel output loading or crosstalk 

problems.  Don't skimp on the transformer, use something good for a 

couple hundred mW.



There just isn't a really good inexpensive passive solution for 

mixing two signal sources (L & R) and getting a good amount voltage 

gain at the same time, at least not that immediately comes to mind, 

what I have left that is.



Ron Q.

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