Saturday, October 1, 2011

Receive (RX) Audio Cable

Receive (RX) Audio Cable

Radio RX Audio to Sound Card "Line In" or Microphone Jack
This page has advice for those building their own interface. The RX audio cable connects the radio's Microphone jack or data jack or speaker jack to the sound card's LINE IN or MIC jack.
On the sound card, the typical color coding of the jacks is::
    • LINE IN =  Blue
    • MIC in = Pink
At the radio, it is best to use the packet or data jack if one exists. Otherwise use the microphone jack or speaker jack.
Does your radio have a 6-pin mini-DIN data jack? Then you might want to read this page about such jacks.
At the sound card, you want to use the LINE IN jack. It is normally a good match to a typical radio's RX audio voltage. If you need to use the sound card's more sensitive MIC jack,  see the MIC schematic below for more information including a schematic that adds in a voltage divider circuit. You may also have success using the Volume Control program (instead of the divider circuit) to limit RX audio into the MIC jack (requires some fine adjustments), but the divider circuit will make it easier to regulate.
In the RX audio cable, I strongly recommend the use of an in-line transformer to break the ground path between the radio and computer grounds for two reasons:
  • to eliminate any potential ground loop hum on the signal,
  • and more importantly, to reduce the risk of any damage to the computer or radio from any difference in voltage potential
Note that the simple RX cables that may come with some commercial interfaces, for example, the Rigblaster Plus and Nomic (and maybe other models), do not have isolation transformers. These simply cables are generally shielded but do not have any additional circuitry or transformers, so you may want to build a cable that does have the transformers.
I see that Buxcomm now offers a simple add-on isolation line if you don't want to build your own. You simply plug in the Buxcomm isolation cable between your sound card and your audio in and your audio out lines. The item part is called a ISOL8R and you can order it for $9.97 ($17.97 for 2) at You can also find other vendors on eBay by doing a search for "Ground Loop Isolator" and/or "Hum noise filter".
If you're curious, there's an interesting discussion of proper connections of shields and grounding at:
See also Buck's page at

Note for 9600 baud packet: There's a general consensus that that the RX and TX audio lines in your interface must NOT have audio isolation transformers for 9600 operations. Generally, small, inexpensive audio or power transformers do not have the correct frequency response range. For example, the typical 600 ohm 1:1 audio line transformer has a 300 Hz to 5 kHz frequency response range which is not low enough. Until someone sends me an isolation circuit with suitable components (I wish you would!, take these precautions when running without isolation transformers:

  • make sure the computer and and the transceiver's power supply are connected to the same AC power circuit (i.e. sharing the same ground).
  • connect/disconnect the sound card interface to the transceiver or computer only when both devices are unplugged from the AC power circuit.
You should also use shielded cable to reduce the risk of any RFI (Radio Frequency Interference) getting on the RX signal.
Here's the schematic for an isolated RX cable:
To build your RX cable:
  • Cable material: Use a cable with a single insulated wire and a braided shield.  Small coaxial cable like RG-174U should work well; even RG-58U would work. The shield can act as ground line if you are using a transformer. If you aren't using a transformer, attach the shield at the radio ground only, and not at the computer/sound card ground.
  • Sound card connector: You will need a plug that will fit the sound card LINE IN/Microphone jack, probably a 1/8" (3.5mm) stereo 3 conductor male mini-plug  (e.g. Radio Shack part #274-284). Do not use a mono 2 conductor plug.

  • Radio connector:If you need your radio's "pinout" information, you'll probably find it at  or  Buck's packet site (on Buck's site, go to the page for TNC-to-radio cables and find your radio. Click on the "Add to Cart" button for the cable for your radio and this will reveal a schematic of the radio's pin out information.) 

    Remember that in addition to the RX line, you will also need to attach the
     TX and PTT lines to the radio, so all three inputs could be sharing the same plug.

    • Mobiles or Base Radios: You can use either the microphone connection or preferably the radio's special "data" plug jack, if it has one. Or you can use the speaker jacks.

      For Receive (RX) audio it's best to use the pin on the radio's data jack, since it provides a constant audio output level and -- usually -- no squelch (AGWPE needs to hear an un-squelched signal) . If you use the radio's microphone or speaker  jack instead, remember the audio output level will be affected by both the radio's "volume" and "squelch" settings and not having these set correctly is often a source of problems for AGWPE.

    • Handhelds: Most HTs have a 'Speaker' jack that requires a 1/8" (3.5mm) mono (2 conductor) mini-plug. If your radio's user manual says to use a stereo plug instead of a mono plug, be sure you do.

      Check either Buck's packet site for pin-out information or
    • 9600 baud use: The newer 9600-ready radios often have a data jack that has a special "9600 baud receive" pin or a combination "9600/1200 receive pin" that is different from the 1200 receive.

  • Transformer: The type of transformer is determined by the source of the RX audio. If you take it from the radio's microphone or data jack, you'll use a 600 to  600 Ohm (1:1) transformer. If you take it from the radio's speaker jack (8 Ohm impedance), you'll need a 1000:8 transformer. (No isolation transformers for 9600 baud operations.)
    Note for 9600 baud users: see notes at top of page. Generally, you should NOT use isolation transformers for 9600 baud operations.

    Sources: Isolation or audio transformers can be obtained from most electronics parts stores, for example, Radio Shack #273-1374 for a 1:1 or #273-1380 for a 1000:8. Peter PE1MHO/G7ECN  says another source for 1:1 transformers is an old internal telephone modem that a computer store might be glad to give you.
    Attaching the transformer: Be sure you use the proper lead wires on the audio transformer as identified by any accompanying schematic for the transformer or by using an Ohm meter to determine the leads for the same coil (windings) of the transformer. Note that if the transformer has 6 leads, two will be center tap wires that won't be used. (Don't cut them until you know you have the 4 other leads correctly wired.)
    One lead from the primary coil of the transformer will connect to the center conductor (tip) of the sound card plug. The other lead from the primary coil will connect to the sleeve (outer conductor) of the sound card plug. In this circuit, it doesn't matter which lead connects to the tip and which connects to the sleeve, so long as they are from the same coil. (If you are using a 1000:8 transformer, it is important to connect the 1000 Ohm primary coil of the transformer to the sound card and not the radio.)

    On the secondary coil of the transformer, one lead will attach to the RX pin of the radio.  The other lead will attach to radio ground. Again, it doesn't matter which of the connects to the radio RX and which to radio ground, so long as they are from the same coil. (Again, if you are using a 1000:8 transformer, it is important to connect the 8 Ohm secondary coil of the transformer to the radio and not the sound card.)
  • The middle conductor (ring) of the sound card plug should be left unwired, unless you are building a RX cable for a second radio.
  • When you're done, tape a "Line In" ( label on the sound card end of the RX audio cable so that you don't confuse it with the TX "Line Out" cable.

Circuits without an isolation transformer:

You will find other schematics (at various web sites and even in AGWPE Help) that do not use isolation transformers. The danger with any circuit directly connecting the computer and radio is that the devices could develop different ground potentials which could lead to damage to the devices. This risk is inherent for both a dual wire connection (RX-to-RX and Ground-to-Ground) or a single wire connection (RX-to-RX, no common ground wire). Either method provides a route for current that could develop because of different potentials. For this same reason, never connect the shield of a wire to both both devices; ground it only to one device.
If you're curious, there's an interesting discussion of proper connections of shields and grounding at:
Also visit

RX Audio to the "Microphone" jack on your sound card

If you want or need to use the sound card's Microphone jack instead of the LINE IN jack, then you will probably need to add an attenuation circuit on the RX audio line so that you do not overdrive the sound card. The exact attenuation will depend on the maximum output voltage of your radio RX line and the maximum input voltage on the MIC jack of your sound card. (The Sound Blaster card I have has a maximum of 200 mVpp on the Microphone jack, where the Line In jack has a maximum of 2 Vp-p.  Other cards accept a maximum of 100 mV.)
Warning: On most sound cards, the Microphone (MIC) jack does not allow stereo input,  i.e. a left and right channel. Typically, the ring in the MIC jack is used to provide a +5 bias voltage to power electret microphones. It is not used for stereo (right channel) input. For that reason, you can't use MIC input if you want to interface one sound card to two radios.
If you do use the MIC jack, it is better to use a stereo plug instead of a mono plug. Just leave the ring unwired. A mono plug can be used, but it will short circuit the ring (+5V) to the sleeve (0V), although the card's circuitry is designed to cope with this --  a resistor severely limits current flow.
Here's a schematic for a 10:1 attenuation circuit that should work fairly well in many microphone jack setups. The 1:1 audio transformer provides an electrical separation of the computer from the radio and may be especially important on cables used in microphone jacks and laptops.
When you are done, tape a "MIC" label on the sound card end of this RX audio cable so you don't confuse it with the TX "Line out" cable.
 I see that Buxcomm now offers a simple add-on attenuator line if you don't want to build your own. You simply plug in the Buxcomm attenuator cable between your sound card and your radio's speaker line. The item is called a 40DBPAD and you can order it for $7.95 at (look at the very bottom of the page).


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