FTDX10 headphone receive audio hiss problem and solution

 The FTDX10 is a great radio, but the audio through the front headphones socket is awful. Listening through the headphones, if you turn the volume to minimum there is an annoying hiss within my hearing range (approx 200Hz -> 12KHz). When the volume is turned up slowly you start to hear the real receiver noise come up, but you can still hear the false hiss above the real audio. If you keep turning the volume up the receiver noise then completely overcomes the false hiss, but the volume is now too loud for comfortable listening.

My problem is that I am predominantly a CW operator and am therefore only interested in sound <1KHz in frequency coming from the headphones (normal specification 20Hz to 20KHz). Non music, communication headphones (20Hz to 10KHz) help with this problem, but I couldn't find a pair of communication headphones that would get rid of the annoying false hiss from the FTDX10, suggesting that the false hiss frequency content was below 10KHz.

I thought about audio filtering solutions out there like Timewave DSP-599ZX or MFJ-751B but the last thing I wanted to do was spend more money on another box after shelling out over a grand for the radio. I also wanted to use the radio portable and wanted to keep any solution as simple as possible. I therefore opted to make one myself. I wasn't completely sure what part of the audio spectrum was causing my discomfort, but it was likely that an annoying hiss would be caused by frequencies above 1KHz and as I only needed frequencies below this for CW this seemed like a good target frequency to begin attenuation from.

Remembering that headphone audio is stereo, I would need 2 identical low pass filter circuits, one for the left and one for the right audio channel. Most headphone leads consist of 2 wires for each channel audio (L&R) each of which has an individual screen, but the screen is common from the 'S' part of a TRS (TipRingScreen) audio jack.

I wanted to be able to swap headphones but the keep the low pass filter in line, so the solution was to create a 15cm stub of audio cable terminated at one end with a male TRS audio jack (3.5mm male TRS audio jack in the case of the FTDX10) to go into the radio. The other end of the stub would be open wire for connection into the twin low pass filter circuit, the output of which would then go to a female TRS audio socket which my headphones would plug into. The picture above shows my headphones jack plug on the left hand side, the twin low pass filter circuit in the centre and the open wire end of the stub going to the radio on the right hand side.

The picture has enough detail to get the component values, but the calculation for the cut off frequency goes as follows:

f = 1/2 x pi x R x C

therefore

1/2 x 3.14 x 1500 x 0.1uF

= 1061Hz

Its only a single stage audio filter so the roll off is gentle as the audio frequency goes up, but it achieves total attenuation of the annoying audio hiss without too much wanted audio drop.

Remember, avoid directly grounding either of the audio channels to the screen. Please test with a DVM to make sure you haven't made a mistake before connecting to your expensive radio.

Try it, however you do so at your own risk. I think you will like it!