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Author Topic: Audio panel  (Read 1084 times)
WB4DHC
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Posts: 16




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« on: June 26, 2005, 05:06:36 PM »

One of my "lessons learned" during Field Day is that we need to provide headphones for the operator and Logger.  Otherwise the noice in the room gets rather distracting.  With 2 rigs going and a couple of other conversations underway it becomes rather difficult to concentrate.  My idea is to provide 2 headphone jacks with separatly adjustable audio and a switchable speaker for each operating position.  DOes anyone have info about a similar setup (homebrew or commercial).  I don't want to "reinvent the wheel" unless necessary.
THanks for any info.
Robert
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HA5RXZ
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Posts: 380




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« Reply #1 on: June 27, 2005, 11:43:08 AM »

I'm pretty sure that there is the schematic for a headphone splitter in the ARRL Handbook. I can't look right now as I am at work.

HA5RXZ
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N3ZKP
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Posts: 2008




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« Reply #2 on: June 28, 2005, 11:10:58 AM »

An amplified, isolated splitter isn't necessary.

I did this a couple of years ago for my club's rigs and all it took was two jacks for the headphones. a jack for the speaker, an SPST switch, a cable with plug for the external speaker jack on the rig and a plastic box to put it all in.

Parallel all three jacks and connect them to the cable from the rig. Put a switch in the line for the speaker.

There is more than enough drive from the most rig's external speaker jack to handle this and separate volume controls are really unnecessary.

Total cost was about $6 per rig with all new parts and it works very nicely.

Lon

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WB2WIK
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Posts: 20595




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« Reply #3 on: June 29, 2005, 12:55:29 PM »

>RE: Audio panel  Reply  
by N3ZKP on June 28, 2005  Mail this to a friend!  
An amplified, isolated splitter isn't necessary.<

::I agree.

>I did this a couple of years ago for my club's rigs and all it took was two jacks for the headphones. a jack for the speaker, an SPST switch, a cable with plug for the external speaker jack on the rig and a plastic box to put it all in.
Parallel all three jacks and connect them to the cable from the rig. Put a switch in the line for the speaker.
There is more than enough drive from the most rig's external speaker jack to handle this and separate volume controls are really unnecessary.<

::With that, I don't really agree.  If you use all the same brand and model headphones, this will probably work okay, but if you plug in different types of headphones, without independent level controls it's easy to blast one operator's head off while the other one is straining to hear anything.  Plus, different people have different levels of "inherent sensitivity" to audio, with some having great ears and others not.  I'd definitely use independent level controls.  But, this can be done without amplifiers, just using a multi-output box with an adjustable L-pad (low-Z variable attenuator) on each output.

::Telex and others sell such items commercially, and I own about five of them.  My Telex boxes each have one input and six outputs (standard 1/4" headphone jacks) and an L-pad on each one.  I think they cost about $25 each many years ago, they might cost a bit more today, but I couldn't build one for so little.  

WB2WIK/6

Total cost was about $6 per rig with all new parts and it works very nicely.

Lon
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WILLY
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Posts: 286




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« Reply #4 on: June 30, 2005, 08:13:58 AM »

 by WB2WIK on June 29, 2005     

"
RE: Audio panel Reply
by N3ZKP on June 28, 2005 Mail this to a friend!
An amplified, isolated splitter isn't necessary.<

::I agree.

>I did this a couple of years ago for my club's rigs and all it took was two jacks for the headphones. a jack for the speaker, an SPST switch, a cable with plug for the external speaker jack on the rig and a plastic box to put it all in.
Parallel all three jacks and connect them to the cable from the rig. Put a switch in the line for the speaker.
There is more than enough drive from the most rig's external speaker jack to handle this and separate volume controls are really unnecessary.<  "


Is impedance matching ignorable?

It would seem that paralleling several speakers would present an impedance that is drastically different than the design of the audio out of the rig.
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N3ZKP
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Posts: 2008




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« Reply #5 on: June 30, 2005, 08:36:41 AM »

Steve:

I understand what you are saying, but in our case varying volume levels not been a problem that I am aware of.

Lon
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AA4PB
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« Reply #6 on: June 30, 2005, 08:47:27 AM »

The impedance mismatch by running several headsets in parallel is not really an issue as long as you can get enough audio to drive them. Most modern audio amps don't attempt to match impedances anyway. You quite often find a resistor of 20 ohms or so in series with the headset jack on the radio anyway.
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WB2WIK
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« Reply #7 on: June 30, 2005, 01:07:23 PM »

>RE: Audio panel  Reply  
by WILLY on June 30, 2005  Mail this to a friend!  
Is impedance matching ignorable?
It would seem that paralleling several speakers would present an impedance that is drastically different than the design of the audio out of the rig.<

::That's why I recommended L-pads.  The Telex box I referenced keeps a pretty constant load impedance on the source (receiver) no matter how many sets of phones are plugged into it.

WB2WIK/6  
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AA4PB
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Posts: 12840




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« Reply #8 on: June 30, 2005, 03:09:59 PM »

L pads have a constant input impedance but a variable output impedance. They permit you to change the signal level going to one set of phones without affecting the level of the other phones. However, if you connect four 8 ohm L pads in parallel you have a 2 ohm load on the receiver output. It remains constant but it is still only 2 ohms. You still have to provide a transformer or something to provide the impedance match - or I suppose you could wire them in series/parallel to get the impedance if you don't need a common ground on the headsets.
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N3ZKP
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Posts: 2008




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« Reply #9 on: June 30, 2005, 04:43:43 PM »

<< You quite often find a resistor of 20 ohms or so in series with the headset jack on the radio anyway. >>

That's why I used the external speaker jack on the rigs to feed the headphones and a small external speaker.

I do the same thing at home: the headphones plug into a jack on the external speaker housing and a switch cuts out the speaker when necessary.

Lon
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WILLY
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Posts: 286




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« Reply #10 on: July 03, 2005, 10:57:51 AM »

 by AA4PB on June 30, 2005  
"The impedance mismatch by running several headsets in parallel is not really an issue as long as you can get enough audio to drive them. Most modern audio amps don't attempt to match impedances anyway. You quite often find a resistor of 20 ohms or so in series with the headset jack on the radio anyway. "
 


by WB2WIK on June 30, 2005    
">RE: Audio panel Reply
by WILLY on June 30, 2005 Mail this to a friend!
Is impedance matching ignorable?
It would seem that paralleling several speakers would present an impedance that is drastically different than the design of the audio out of the rig.<

::That's why I recommended L-pads. The Telex box I referenced keeps a pretty constant load impedance on the source (receiver) no matter how many sets of phones are plugged into it.  "


errr - - -   so which is it?    Ignore impedance and present any old impedance to the audio out?    i.e.  it will work fine and will never hurt anything.

or - -   try to present the design impedance to the rigs audio out amp?


Thanks
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KA0GKT
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Posts: 555




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« Reply #11 on: July 11, 2005, 04:59:48 PM »

Audio Amplifier ICs are fairly inexpensive. I can't find my NTE catalog right now or I'd give you a few numbers.

Radio Design Labs makes a whole line of monolithic electronic application blocks. One is the ST-SH2 Stereo Headphone Amplifier. Input Z is 10K bridging, -18 to +8 dBu so an 8-ohm load resistor of suitible rating can be placed across the headphone out of your rig so you don't damage the audio outputs. Output is 250mW into 8-Ohms or 20 P-P into 2K ohms. Gain is adjustable from 3 to 26 dB. It requires 24 to 33 VDC @ 175 mA.

Here's a link to the web page for the ST-SH2:  http://www.radiodesignlabs.com/st-sh2.htm

They're a good company with which to deal...and located in Prescott, AZ, gotta keep the state's economy going :-D


73 DE KA0GKT/7  
       
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