Koss SB45 headset adapters for some amateur radio & GMRS transceivers

Koss SB/45 Headset

At about $30, the Koss SB45 is a well-regarded headset for amateur radio use. It has an electret microphone element, which is compatible with many handheld transceivers. (For those transceivers which expect a dynamic microphone, for example some Yaesu models, the Koss SB40 is a similar model in appearance, price range, and in good reviews received.)

Koss SB45
Microphone Electret
Impedance 100Ω
Sound Output 103dB SPL / 1mW
Mic Sensitivity -36dB ± 3dB, V/Pa
(more sensitive)
DC Power Required Yes

Koss SB40
Microphone Dynamic
Impedance 120Ω
Sound Output 96 dB SPL / 1mW
Mic Sensitivity -56dB ± 3dB, V/Pa
(less sensitive)
DC Power Required No

The headsets do not simply plug in to the radios, however. In this post I describe some adapters I made to use the Koss SB45 headset with a few different radios.

Each adapter includes a push-to-talk (PTT) button, a toggle switch to lock PTT on, and a 1/4" phone jack for a foot pedal or hand-held PTT button.

Adapter for Wouxon KG-805G

The Wouxon KG-805G is a handheld GMRS transceiver.

Wouxon KG-805G Adapter Wouxon KG-805G Adapter Wouxon KG-805G Adapter

From observing the availability of speaker microphones that are compatible with both Kenwood and Wouxun handhelds (e.g., like this), I looked for documentation for adapting headsets to the “Kenwood standard.”.

Kenwood speaker/mics have the following pinout:

2.5mm Speaker Jack
Tip Speaker
Ring Program
Shield Ground

3.5mm Microphone Jack
Tip 5V
Ring Mic+
Shield PTT

N1GY has a nice article, A Headset Adaptor for Kenwood, Baofeng and Wouxun HTs, describing such an adapter. That adapter shown there specifies shorting the program connection to ground. I found when I did that, however, that the transmit LED on the KG-805G glowed dimly. That didn't seem right, so I opted to just leave the Program line disconnected. Here is the schematic that I used in the end.

Wouxon KG-805G Adapter Schematic

Note that the shield of the Mic line to the KG-805G is not grounded! Grounding that shield is the way that PTT is actuated. I used a piece of acrylic plastic to mount the jack for this connection, to avoid it being grounded through the metal project box.

Adapter for Alinco DJ-500T

The Alinco DJ-500 is a handheld 2 Meter / 440 MHz transceiver.

DJ-500T Adapter DJ-500T Adapter DJ-500T Adapter

Some digging produced a pinout for the DJ-500t.

3.5mm Speaker Jack
Tip Speaker+
Ring n/c
Shield Speaker-

2.5mm Microphone Jack
Tip Mic+
Ring 5V
Shield Ground

The DJ-500T handheld is compatible with the Alinco EMS-76 Speaker/Mic and EME-56A Earphone Mic. I was able to get a look at the circuit diagram for the EME-56A (no link available), and based the following similar circuit on that.

DJ-500T Adapter Schematic

Adapter for Alinco DR-590T

The Alinco DR-590T is a much older and discontinued 2 meter / 440 MHz mobile transceiver.

Alinco DR-590T Adapter Alinco DR-590T Adapter Alinco DR-590T Adapter

The pinout for the DR-590 microphone jack is available from the schematic in the service manual, available here. Similar information is available here.

Alinco Mobile Microphone Jack
1 Mic Input
3 Down
4 Up
5 +5 VDC
7 Mic Ground & Shield
8 Ground

The DR-590T comes with a speaker/mic with a DTMF keyboard, so I thought I might want to switch easily back and forth between the headset and the speaker/mic. I added a toggle switch for that. The rig also has separate speaker jacks for VHF and UHF; I included a switch to choose between combining both outputs and sending the result to both earpieces, or feeding VHF to one earpiece and UHF to the other.

I cribbed most of the adapter circuit from the EMS-52 Service manual. The VHF and UHF outputs are too high for safe headphone use, so I tried out various resistances to drop the outputs to a reasonable level. I did not want to adjust the internal mic gain pot for different mics, so the 47kΩ resistor in the mic line was also chosen by experimentation, to give an audio level close to that produced by the speaker mic.

Alinco DR-590T Adapter Schematic



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