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Thread: Streaming audio off the RSM 3.5mm jack: loud hum while charging

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    Default Streaming audio off the RSM 3.5mm jack: loud hum while charging

    I have a PMMN4062A that I've plugged in a 3.5mm cable to my computer, looking to stream audio from it. It's fine off the charger, but there's a very loud 60hz hum in the audio when the radio is on a charger. Has anyone been able to work around this? Thanks!


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    replace battery on the radio with a battery eliminator... two problems solved (hum and charging the radio)

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    I haven't run into this problem, but you might want to try powering the handheld with one of these. https://shopwiscomm.com/Battery-Elim...-P5667710.aspx

    Guess someone beat me too it. Took too long to find the link to post.

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    The hum is from a ground loop (ac coupling). Check that the AC side of everything is properly grounded and that the AC source is referenced to the ground on the AC source (green wire is ground). That may fix the issue immediately. I have seen similar noise to varying degrees on poorly connected AC outlets or cheap hardware where the '0 volt crossover' reference is not at ground 0 volts.

    If thats not practical to fix, tie the chassis ground of the portable to the chassis ground of the computer. This should fix the humm issue.

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    It might also be your charger is shot. If the main filter caps are going it may be putting out lots of AC on the (supposedly) DC lines going to the battery. Try a different charger. If you get the same results, then it may be just the normal amount of hum on the charger, and a battery eliminator should fix the problem. Or, it's a ground loop like Notorola said...

    If you measure the output of the charger the AC component should be well under 1 volt, a good value would be 1/10 -1/4 volt of AC on the DC line max. Anything more and you're looking at a bad rectifier diode or a weak main filter cap.
    Last edited by Alpha; Mar 24, 2019 at 05:38 AM.

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    You could also try using a clamp on ferrite core on the 3.5mm audio cable running to your computer, that may possibly reduce or eliminate any hum or noise while the radio is charging.

    Here is one on Mouser Electronics that has an inside diameter of 3.5mm that would fit perfectly around the audio cable going to your computer.

    https://www.mouser.com/ProductDetail...XHN2E0Fw%3D%3D

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    You are probably getting a ground loop. If the radio has balanced audio as many do, then you don't want to be returning either side of the circuit to ground (via sound card) as you can damage the radio and you will get a hum. You need a small audio "hum buster" isolation transformer like radio shack used to sell, or similar. (Capacitor gimmick isolators from Amazon wont work)

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    Fully differential means balanced audio output. The designers never meant for the audio to be fed to a sound card and to have one side of the differential amp grounded. DONT DO THAT!

    From an APX service manual:

    "The external speaker path is almost identical to the internal speaker path. The digital audio data is
    sent from the OMAP1710's McBSP1 port to the TI AIC33 CODEC for
    digital to analog conversion and volume control. The external speaker path us
    es the MAKO IC's class-AB audio power amplifier to drive 16 Ohm to 28 Ohm external speakers. The
    input to MAKO IC's audio power amplifier is fully differential and comes from the TI CODEC's
    RIGHT_LOP and RIGHT_LOM. The output of the MAKO IC's audio amplifier is
    also fully differential and available on pins EXT_SPKR_P and
    EXT_SPKR"

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    RFI makes a extremely important point. All AUDIO connections should be DC isolated. This includes the 'ground' shank of the 3.5mm connector. If you need a common ground between the Audio source and destination use the chassis as suggested in post 4. Many of the newer radios use the balanced audio method today. Looking at the schematic will let you know for sure what method is in use on your hardware.

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    Any recommendations for those transformers? I have a Wouxun hooked up the same way for personal use for logging and even though it doesnt hum, I had wondered about this.

    Since im not of the baoturd mentality of calling something good enough when its not, id like to improve my setup. And before someone suggests throwing out the chinese radio, I have couple XTS'es laying around and ive been planning to replace the wouxun with one of them, just never got around to researching this very subject. The way i saw it was if it does blow out the radio, its not a big loss. And since it came up here on the forums, theres no time like the present for some upgrades.

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    Thanks everyone... I've tried two WPLN4111 chargers (with the APX charging adapter) and they both exhibit the same hum. So while the Motorola charger doesn't have a ground, the HP Zbook laptop power supply does have a ground pin. Sounds like the easiest way is to buy a battery eliminator!

    I know the mobile radio speakers are all differential and all the speaker cables warn not to ground the speaker to the vehicle ground or something like that. Didn't know that portables would be the same.

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    Quote Originally Posted by krustykrab View Post
    Thanks everyone... I've tried two WPLN4111 chargers (with the APX charging adapter) and they both exhibit the same hum. So while the Motorola charger doesn't have a ground, the HP Zbook laptop power supply does have a ground pin. Sounds like the easiest way is to buy a battery eliminator!

    I know the mobile radio speakers are all differential and all the speaker cables warn not to ground the speaker to the vehicle ground or something like that. Didn't know that portables would be the same.
    How is a battery eliminator going to help? You are going to damage your radio if you don't use an isolation transformer.

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    Radio shack used to sell a "hum blocker isolation transformer". It was meant for stereo equipment and had phono plugs in and out. I don't have a part number handy but it can easily be searched. Amazon sells a cheap Chinese knockoff that uses capacitors. It wont work.

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    R/S also used to sell audio output transformers too. They were about 10K ohm to 8 ohm matching transformers for the audio output stages of amplifiers to couple to a speaker. You can run 2 of them back-to-back with the 10k windings connected to one another so it goes 8 ohm : 10k <-> 10k : 8 ohm - then hook the 2 8 ohm windings to whatever input and output you need. Sometimes these general matching type transformers are easier to find than the 1:1 isolation transformers. They use similar ones for dynamic microphones like the old Shure Highballs (Unidyne/Unisphere, etc) to match the mic element to a low or high impedance (1-10k ohm) amp output on the Cannon connector, switchably.

    You can also run 2 series capacitors in the audio lines, one in series on the "hot" side and one in series on the "neutral" side, and hook the "neutral" one to ground, and take your signal from the "hot" one. This is not nearly as good as transformers, and there will be a initial DC "thump" as the caps charge up and achieve isolation. That "thump" is a momentary shorting of the "neutral" line to ground until the cap charges a few milliseconds later. Depending on the quality and rating of the audio finals, they may not mind that brief shorting to ground. If the safety margins aren't sufficient then it might blow the final, maybe not. It's a matter of cut-and-try, unfortunately. That's why using caps is possible but not recommended for DC decoupling and isolation vs. a proper matching/isolation transformer.

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    OF course, the first thing I thought of with this thread is NOT to run your radio while it is in the charger....
    Apparently NOT a radio professional.

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    Quote Originally Posted by com501 View Post
    OF course, the first thing I thought of with this thread is NOT to run your radio while it is in the charger....
    Agreed. Its mentioned in the manual, although whether people read that is another matter... Use a battery eliminator if you want to run it in a stationary setting.

    That's one of the reasons i give an advancetec or the moto-branded equivalent charger when i loan out a radio since it forces the user to remove the battery to charge it.