Same headphone, two scanners - problem

I have a a headphone plug with two male 1/8" plugs that I am plugging into the headphone jacks of a 536HP and another scanner, either a PRO-197, WS-1080, or 996XT. The audio from both plugs goes to one pair of headphones using the other end ot the cable with a single female 1/8" jack. However, I did this only for a while then stopped. The reason: Started to get popping sound in 536HP, the scanner actually shut off and restarted, the P25 audio was distorted, and I went "Oh s..."
When pulled the dual set up headphone jack, no problem with the 536HP. Seems like I am getting some type of DC voltage loop in the 536HP when I have the ground connection of the plug physically connected to another scanner at the same time. This happened one time about 3 months ago, but forgot, and plugged the dual headphone cable into tonight with same problem.
Obviously, I won't do that again. But does anyone know what might be going on to cause this behavior. The problem is that I would like to use one pair of headphones to monitor more than one scanner at a time when my wife is sleeping. Is there some way to do this properly where I would not get this type of behavior?
Would an audio mixer be good in this type of situation?
Any advice would be appreciated.

Steve AA6IO

Yes, an audio mixer is the proper solution, and yes you should not continue your current configuration.

The headphone jack is an output-only circuit, not intended to receive input. A mixer prevents audio signals from improperly flowing between devices, in this case between scanners. Otherwise, you get one scanner's headphone jack trying to input signal into the other scanner's headphone jack. That will always cause problems. Hopefully, you noticed the symptoms soon enough to prevent permanent damage.

Mixers also prevent damage to output-devices like headphones/speakers, etc. by managing the combined input-signal from multiple scanners so that the output-signal sent to the headphones is the electrical equivalent of only one signal, even though multiple sound-tracks have been merged..

In this way, the mixer protects not only input-devices, but output-devices, too.

Hope this helps,

Hi there. I have used this product with up to 4 scanners (a HP1, a WS1040, Pro106 and a PSR100) plugged into it with no issues at all.

Amazon.com: Belkin Rockstar Multi Headphone Splitter (Light Green): Electronics

sjgostovich said:

Hi there. I have used this product with up to 4 scanners (a HP1, a WS1040, Pro106 and a PSR100) plugged into it with no issues at all.

Amazon.com: Belkin Rockstar Multi Headphone Splitter (Light Green): Electronics

Click to expand...

Good morning,

Unless it has protective circuitry like a mixer, you have simply been fortunate, and are still at risk for the same problem the OP described.

I have not yet found a technical-spec's document to explain what is inside the Belkin Rockstar adapter, but it seems to be equivalent to nothing more than several "Y--adapter" cords/adapters chained together.

EDIT:
Quoted from the above Amazon link:

Mix music by connecting multiple music players at once. Mixing and fade-ins controlled by the standard controls of each MP3 player

Click to expand...

Please note, that this is referring to D.J.-style "mixing", where only one source is being played at a time, like music at a dance in a school's gym. There might be slight overlap as one source is faded-out/stopped and another faded-in/started.

But this is not intended to be an always-on multiple-input "mixer", like a true semi-pro/professional mixer, as illustrated above on the Mackie website, where multiple scanners are "playing" simultaneously to the same output device -- speakers/headphones, etc..

Hope this helps,

Thanks QDP and sjgostovich
Yes, I am interested in an alway-on mulitple-input mixer. QDP, do you have any recommendations with 1/8" inputs/outputs. Was just looking at the Rolls MX44S Mini-Mix IV Mini 4 Channel Audio Mixer on Amazon. About $70. Do you think something like this would be satisfactory, or do you have other recommendations. Thank you.
Steve AA6IO

sshermanmd said:

...Was just looking at the Rolls MX44S Mini-Mix IV Mini 4 Channel Audio Mixer on Amazon. About $70. Do you think something like this would be satisfactory, or do you have other recommendations. Thank you.

Click to expand...

I have never seen that mixer before. It is a real mixer, even though it is very small. As mixers go, it is also a very basic mixer, with few features, just the fundamentals. (It's the first one I've seen that can be powered by a 9Volt battery.) I won't try to encourage you to buy it nor discourage you from buying it.

I will say, regardless of brand, when buying mixers, the buyer should answer for himself/herself the question "How many mixer channels/slots do I need/want now and later?"

You might have only four scanners/devices now and would get by with a four-channel mixer, now. And if that's all you need for a long time, great. But, if you might get additional scanners, MP3 players, or other devices later that you wish to connect to your mixer as signal-sources, you might wish to consider now a mixer with more than four mixer-channels. If four mixer-channels is enough, then the product you found on Amazon might well do the job.

P.S. Here's the link to Rolls' .pdf file on that product: Rolls MX44s 4-CHANNEL STEREO MIXER, and the general product page: MX44s Mini-Mix IV

Hope this helps,

QDP
Thank you

Steve AA6IO

Also you should never plug a mono plug into a stereo headphone jack. Doing so shorts out 1/2 of the audio circuit.

You can "split" a set of phones for almost nothing.

When I went to the races a lot, I would (at first) buy a junk set of earphones that had a larger than average cord diameter, so there would be wires inside that were a little thicker than average. This was so I could split the wiring and send the audio from one device, a tiny AM/FM radio that I would tune to the track PA frequency, and the other would go to my scanner. One set of earphones would last about a year before the foam rubber would rot, and I would toss them. Later on, I bought a set of decent ones that I split the same way, it just wasn't as easy because the wires were very thin, but it worked, and they still do, about 15 years later. I made several of these split phones for friends and they loved them.

ratboy said:

You can "split" a set of phones for almost nothing.

When I went to the races a lot, I would (at first) buy a junk set of earphones that had a larger than average cord diameter, so there would be wires inside that were a little thicker than average. This was so I could split the wiring and send the audio from one device, a tiny AM/FM radio that I would tune to the track PA frequency, and the other would go to my scanner. One set of earphones would last about a year before the foam rubber would rot, and I would toss them. Later on, I bought a set of decent ones that I split the same way, it just wasn't as easy because the wires were very thin, but it worked, and they still do, about 15 years later. I made several of these split phones for friends and they loved them.

Click to expand...

If you modified the wiring of a stereo headset so that you branched it into two separate mono-plugs, one for the left and the other for the right, which you then used one in the AM/FM radio and the other in the scanner, then that likely would be fine. There are also splitter adapters that will achieve this, for those people who are not comfortable with modifying the headphone's wiring.

If you wired the "split"-ends in such a way that an audio signal from one radio/scanner's headphone jack tried to enter into the other unit's headphone jack, then that would be a problem; even though it might not immediately create permanent damage.

Thanks everyone for your feedback. UPMan, the plugs I was using were stereo plugs/cables.
Anyway, I have received the Rolls MX44s 4-CHANNEL STEREO MIXER and tried the set up.
Unfortunately, my results are not much better. Using stereo cables for all connections. When plug my 536HP and 996XT from headphone jacks into inputs 1 and 2 on mixer, still get loud popping on the 536HP. Does not sound much different than my OP. No problems with the 996XT.
This is not a problem between my PRO-197 and 996XT, or between any other combo of scanners except when I put the 536HP into the mix. But this was the same as with my original set up as mentioned in my OP. Only with the 536HP plus any other scanner.
A little bit hesitant to try any other combos using the 536HP regarding mixing of audio.
Thanks everybody. If any other suggestions, please advise.
Steve AA6IO

Could be a grounding issue. If the audio out is floating (which Unidens supposedly are), you'll need an isolation transformer between the 536 output and any other audio device.

QDP2012 said:

If you modified the wiring of a stereo headset so that you branched it into two separate mono-plugs, one for the left and the other for the right, which you then used one in the AM/FM radio and the other in the scanner, then that likely would be fine. There are also splitter adapters that will achieve this, for those people who are not comfortable with modifying the headphone's wiring.

If you wired the "split"-ends in such a way that an audio signal from one radio/scanner's headphone jack tried to enter into the other unit's headphone jack, then that would be a problem; even though it might not immediately create permanent damage.

Click to expand...

yes, I put two plugs on the cord(s), so I each ear was totally independent of each other. I've been doing this for almost 30 years, going back to the Bearcat BC205XLT days. I tried a splitter adaptor a couple of times, but both of them seemed to be pretty junky and the jack where the headphones plugged in soon got loose to the point the sound cut in and out when the cords moved around, so I went to the split cord setup. The only negative is the time involved and the loss of the stereo mode of the headphones. And the cost of the extra plug, of course. Cost is why I used "junk" earphones for it. If the phones die, toss them, and buy another set for $1.99.

ratboy said:

yes, I put two plugs on the cord(s), so I each ear was totally independent of each other. I've been doing this for almost 30 years, going back to the Bearcat BC205XLT days. I tried a splitter adaptor a couple of times, but both of them seemed to be pretty junky and the jack where the headphones plugged in soon got loose to the point the sound cut in and out when the cords moved around, so I went to the split cord setup. The only negative is the time involved and the loss of the stereo mode of the headphones. And the cost of the extra plug, of course. Cost is why I used "junk" earphones for it. If the phones die, toss them, and buy another set for $1.99.

Click to expand...

I've seen the same problem with adapters becoming loose, and can certainly understand why you would pursue a better-quality and more permanent solution. Manually-split headphones can work well, giving the user a convenient, comfortable headset to wear instead of two mono earpieces, which are rarely comfortable and often don't stay put. For $1.99 and a little time, it sounds like a good solution.

Good job,

jonwienke said:

Could be a grounding issue...

Click to expand...

I'm beginning to suspect a grounding problem, too.

I have my antenna hooked up through an MCA204M coupler. But I do not have the scanners actually connected to any ground per se, as I do my ham transceivers to an outside grounding system. Thus, the antennas, when hooked up to the scanner configuration, are not seeing any grounding system.
Do you think this would make a difference. I could easily run a ground wire to the casing on my scanners.
Steve AA6IO

That's irrelevant.

With many stereo headphone outputs, the common conductor is connected to the radio's chassis ground, and there is no voltage potential between them. Uniden scanners audio outs do not work this way; the common conductor is not connected to chassis ground. Instead, it is connected to an inverted version of the audio out to increase the maximum possible output voltage. When you connect this output to an input that has the common connected to ground, part of the audio is shorted out, and all sorts of audio artifacts can occur, as well as damage to the radio.

An audio isolation transformer breaks the connection between the inverted common out and ground, and makes everything happy again.

jonwienke said:

That's irrelevant.

With many stereo headphone outputs, the common conductor is connected to the radio's chassis ground, and there is no voltage potential between them. Uniden scanners audio outs do not work this way; the common conductor is not connected to chassis ground. Instead, it is connected to an inverted version of the audio out to increase the maximum possible output voltage. When you connect this output to an input that has the common connected to ground, part of the audio is shorted out, and all sorts of audio artifacts can occur, as well as damage to the radio.

An audio isolation transformer breaks the connection between the inverted common out and ground, and makes everything happy again.

Click to expand...

Why how Motorola of them...

W5PKY said:

Why how Motorola of them...

Click to expand...

It is not motrola of them, many audio amps have a push pull transistor configuration where you have a left and right output with the one amp and a common ground or you can use both sides for a high power mono output where both conductors are above ground potential and float.

This is where the change is from the old scanners where you could run a jumper from audio shield to the bnc ground tab to bypass the headphone limiting resistor and you have to go inside to jumper it now.

Easiest way to do this is to get the stereo to twin mono adapter and use a ground loop isolator on each radio.

http://www.amazon.com/Pyle-PLGI35T-...&keywords=ground+loop+isolator#productDetails

David Kb7uns

OK everybody. Thanks for the advice. I was going to say thanks for the "feedback," but that actually seems to be the problem, with regard to ground loops. Looks like the next step will be an audio isolation transformer.
Regards
Steve AA6IO