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rd2(at)dejazzd.com
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 Posted: Tue Nov 03, 2020 5:35 am Post subject: isolation resistors |
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Hi group,
I need to merge 3 audio signals (all low power) and feed them to pin T (non-switched audio input) of a King KMA 24 audio panel. The purpose is to sufficiently isolate the signals without significant attenuation.
The KMA 24 input impedance is listed as 500 ohm. The impedance of the sources is not known except that they are intended to be amplified. The sources are: Warnings from Monroy Traffic Scope, Traffic Warnings from GNX 375, and AoA warnings from uAvionix AV-20-S.
Should I use resistors in series with each source and what value would you recommend? 4.7 kohm ?
Thanks
Rumen
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Ceengland
Joined: 11 Oct 2020
Posts: 381
Location: MS
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| Posted: Tue Nov 03, 2020 7:00 am Post subject: isolation resistors |
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On Tue, Nov 3, 2020 at 7:40 AM rd2 <rd2(at)dejazzd.com (rd2(at)dejazzd.com)> wrote:
| Quote: | --> AeroElectric-List message posted by: rd2 <rd2(at)dejazzd.com (rd2(at)dejazzd.com)>
Hi group,
I need to merge 3 audio signals (all low power) and feed them to pin T (non-switched audio input) of a King KMA 24 audio panel. The purpose is to sufficiently isolate the signals without significant attenuation.
The KMA 24 input impedance is listed as 500 ohm. The impedance of the sources is not known except that they are intended to be amplified. The sources are: Warnings from Monroy Traffic Scope, Traffic Warnings from GNX 375, and AoA warnings from uAvionix AV-20-S.
Should I use resistors in series with each source and what value would you recommend? 4.7 kohm ?
Thanks
Rumen
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Since all are aviation devices, they *should* be capable of driving a 600 ohm load at adequate volume. So the goal is to keep the load they see above 600 ohms. Each source impedance will likely be either a resistor in series with its output, or the very low impedance of the output stage itself. If you assume that source impedance is zero for each device (it'll more likely be 50-100 ohms), then the load each device will see will be the series-parallel total resistance of its own series resistor (which you're adding) and the audio panel input (600 ohms), and the series resistors of the other two sources. The signal level will be attenuated by around 50%, which will usually be OK.
Being lazy & avoiding the math, if you start with around 1K ohms for each series resistor, it'll likely work fine. You might be able to go as low as 300-400 ohms. You're balancing signal loss to series resistance, vs signal loss due to overloading the output with too low a combined load resistance. (Little risk of damage; just a signal that's too weak.)
The above may or may not work if you add consumer electronics to the mix. There are zero 'standards' to how consumer gear does things, and the output levels are almost always too low to work without 'help'.
Charlie
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rd2(at)dejazzd.com
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| Posted: Tue Nov 03, 2020 8:43 am Post subject: isolation resistors |
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Thanks Charlie.
R
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dj_theis
Joined: 28 Aug 2017
Posts: 56
Location: Minnesota
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| Posted: Tue Nov 03, 2020 10:36 am Post subject: Re: isolation resistors |
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| rd2(at)dejazzd.com wrote: | Hi group,
I need to merge 3 audio signals (all low power) and feed them to pin T (non-switched audio input) of a King KMA 24 audio panel. The purpose is to sufficiently isolate the signals without significant attenuation.
The KMA 24 input impedance is listed as 500 ohm. The impedance of the sources is not known except that they are intended to be amplified. The sources are: Warnings from Monroy Traffic Scope, Traffic Warnings from GNX 375, and AoA warnings from uAvionix AV-20-S.
Should I use resistors in series with each source and what value would you recommend? 4.7 kohm ?
Thanks
Rumen |
This may be overkill but I had a similar need and used the DIY board from Proffessor Nuckolls.
http://www.aeroelectric.com/DIY/Audio_Iso_Amp_9009-700L.pdf
Wired it up and tested it on the bench with the wiring shown in the attached and shown in the lower right corner. All tested well with good volume control and excellent isolation. Have not yet run the engine on the plane but don't expect any problems. the PM3000 has an auto music silence fetaure when recieving or PTT is depressed. All worked well.
Very pleased with the outcome.
Dan Theis
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Z16M Revmaster 2300 r9-sht 2.pdf |
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Dan Theis
Scratch building Sonex #1362
Still working on the Revmaster Alternator improvement |
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rd2(at)dejazzd.com
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- so adding another iso amp would really be overkill in this case.... besides it's a certified bird... and I was looking for a lazy non-complex solution. Thanks anyway. R