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Question for Brian Lloyd

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Dennis Johnson

Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.

Posted: Thu Aug 10, 2006 6:28 pm Post subject: Question for Brian Lloyd

Hi Brian (and others),

  

 A couple of weeks ago you posted a suggestion for connecting a stereo0 IPod-type device to a mono audio panel.  You said to insert a 100 ohm0 resistor in series in each of the left and right channels, at the stereo jack,0 before combining the left and right channels together.  

  

 I'm now wiring my panel and I have a stereo jack that0 I'll connect to my mono audio panel.  I was planning to just jumper0 the left and right (high) terminals together at the stereo jack so I can0 listen to my IPod in flight.  Can you expand a little on the need for the0 resistors?  I actually have two concerns:  One is that I'd like to0 understand the issue a little bit, and the other is that I've heard that people0 don't get enough volume from portable entertainment devices, and the resistors0 might make that problem worse.

  

 Thanks so much for your help,

 Dennis Johnson

 Legacy #257, hoping to fly in three months

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bob(at)bob-white.com
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Posted: Thu Aug 10, 2006 6:47 pm Post subject: Question for Brian Lloyd

On Thu, 10 Aug 2006 19:25:59 -0700

"Dennis Johnson" <pinetownd(at)volcano.net> wrote:



 	  Quote: 	
		   Hi Brian (and others),

 

 A couple of weeks ago you posted a suggestion for connecting a stereo IPod-type device to a mono audio panel.  You said to insert a 100 ohm resistor in series in each of the left and right channels, at the stereo jack, before combining the left and right channels together.  

 

 I'm now wiring my panel and I have a stereo jack that I'll connect to my mono audio panel.  I was planning to just jumper the left and right (high) terminals together at the stereo jack so I can listen to my IPod in flight.  Can you expand a little on the need for the resistors?  I actually have two concerns:  One is that I'd like to understand the issue a little bit, and the other is that I've heard that people don't get enough volume from portable entertainment devices, and the resistors might make that problem worse.

 

 Thanks so much for your help,

 Dennis Johnson

 Legacy #257, hoping to fly in three months


	


Hi Dennis,



The resistors are needed to isolate the two outputs from each other.

Basically you want to add the voltages from the left and right

channels.  The input to the audio panel will be a high impedance, so

the 100 ohm resistors won't have much effect on the volume.



Bob W.



-- 

http://www.bob-white.com

N93BD - Rotary Powered BD-4 (first engine start 1/7/06)

Custom Cables for your rotary installation -

http://www.roblinphoto.com/shop/

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brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Thu Aug 10, 2006 7:26 pm Post subject: Question for Brian Lloyd

On Aug 10, 2006, at 10:25 PM, Dennis Johnson wrote:



 	  Quote: 	
		   Hi Brian (and others),



 A couple of weeks ago you posted a suggestion for connecting a  

 stereo IPod-type device to a mono audio panel.  You said to insert  

 a 100 ohm resistor in series in each of the left and right  

 channels, at the stereo jack, before combining the left and right  

 channels together.



 I'm now wiring my panel and I have a stereo jack that I'll connect  

 to my mono audio panel.  I was planning to just jumper the left and  

 right (high) terminals together at the stereo jack so I can listen  

 to my IPod in flight.  Can you expand a little on the need for the  

 resistors?  I actually have two concerns:  One is that I'd like to  

 understand the issue a little bit, and the other is that I've heard  

 that people don't get enough volume from portable entertainment  

 devices, and the resistors might make that problem worse.


	


Well, it gets to the way solid-state audio amplifier output stages  

are built. They are designed to have very low output impedance. This  

way they can generate a voltage output that isn't much affected by  

the thing receiving the voltage (a speaker or earspeaker in this  

case). In addition they use negative feedback, i.e. part of the the  

output signal is inverted (flipped over) and fed back to the input in  

order to cancel out errors (distortion) generated within the amp  

itself. This further reduces the output impedance. So if you tie one  

output to the other, the output that is NOT generating the signal you  

want will look almost like a dead short to the output that is.



Now many of you have tied the left and right outputs together and you  

get sound. That is because most of the audio is in-phase and a lot of  

the signal is identical in both the left and right channels. You hear  

sound. The problem comes when there is something on the left channel  

that doesn't appear on the right. The right output will then act as  

almost a short to that part of the signal. Very little of what is on  

the left channel will make it to your audio panel or intercom.



So we add the 100 ohm resistors. These keep one output from loading  

down the other. The two signals, left and right, then mix and you get  

the best possible output with less distortion and less load on the  

two amplifier outputs.



Brian Lloyd                         361 Catterline Way

brian-yak AT lloyd DOT com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery

nuckollsr(at)cox.net
Guest

Posted: Fri Aug 11, 2006 5:28 am Post subject: Question for Brian Lloyd

At 04:59 AM 8/11/2006 -0400, you wrote:

 	  Quote: 	

On Aug 10, 2006, at 10:25 PM, Dennis Johnson wrote:

>Hi Brian (and others),

>

>A couple of weeks ago you posted a suggestion for connecting a

>stereo IPod-type device to a mono audio panel.  You said to insert

>a 100 ohm resistor in series in each of the left and right

>channels, at the stereo jack, before combining the left and right

>channels together.

>

>I'm now wiring my panel and I have a stereo jack that I'll connect

>to my mono audio panel.  I was planning to just jumper the left and

>right (high) terminals together at the stereo jack so I can listen

>to my IPod in flight.  Can you expand a little on the need for the

>resistors?  I actually have two concerns:  One is that I'd like to

>understand the issue a little bit, and the other is that I've heard

>that people don't get enough volume from portable entertainment

>devices, and the resistors might make that problem worse.

Well, it gets to the way solid-state audio amplifier output stages

are built. They are designed to have very low output impedance. This

way they can generate a voltage output that isn't much affected by

the thing receiving the voltage (a speaker or earspeaker in this

case). In addition they use negative feedback, i.e. part of the the

output signal is inverted (flipped over) and fed back to the input in

order to cancel out errors (distortion) generated within the amp

itself. This further reduces the output impedance. So if you tie one

output to the other, the output that is NOT generating the signal you

want will look almost like a dead short to the output that is.

Now many of you have tied the left and right outputs together and you

get sound. That is because most of the audio is in-phase and a lot of

the signal is identical in both the left and right channels. You hear

sound. The problem comes when there is something on the left channel

that doesn't appear on the right. The right output will then act as

almost a short to that part of the signal. Very little of what is on

the left channel will make it to your audio panel or intercom.

So we add the 100 ohm resistors. These keep one output from loading

down the other. The two signals, left and right, then mix and you get

the best possible output with less distortion and less load on the

two amplifier outputs.

    Brian's most lucid explanation is illustrated and further

    supported in the audio chapter of the 'Connection which one

    may presently access at:

http://aeroelectric.com/articles/Rev11/18Audio_R11.pdf

    Bob . . .

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Hopperdhh(at)aol.com
Guest

Posted: Sun Aug 13, 2006 6:03 am Post subject: Question for Brian Lloyd

There have been some excellent replies to this thread.

  

 The only thing that I can add is that due to the extremely low output  impedance of some amplifier circuits, the result is OVERHEATING of the output  devices of both sources as they each attempt to drive the output  node to their own desired voltage -- instant by instant.  This is  accompanied of course by increased current drawn by each amplifier.   In some cases the output devices could fail.  The 100 ohm resistors limit  the current that can flow between the two amplifiers.

  

 Dan Hopper

 RV-7A

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