Yaesu MH48-a6j XLR Cable

If you are one of the amateurs that have purchased a Yaesu moble radio for use in your home, I'm sure you are getting tired of holding that MH48 DTMF microphone. The other side of this effort is an attempt to get a little control over the sound quality of the transmitted audio. Another amateur had asked me about making an adapter cable for his XLR based microphone for use with a Yaesu FTM400DR that he was using in his home. The cable would allow the use of a foot switch to key the transmitter. He told me that he would have bought a cable retail, had it been available. It's hard to imagine that the commercial world missed this one.

The MH48 mic is the default mic for most if not all of Yaesu's mobile radios in 2015. It uses a condeser element similar to most communications mics. My understanding is that the output impedance is a couple of k ohms. The radio that this project is centered on is the FTM400DR. According to the owners manual specifications, the transmitter audio input impedance is "about 2.5K".

The XLR or "Canon" is an audio industry standard connector for balanced audio. Any balanced microphone will have a male XLR as its output connector. The output impdeance of a balanced mic may be as low as 150 ohms or as high as 600 ohms. The FTM400DR lacks a balanced mic input. Most if not all mobile radios have unbalaced mic connections. There simply isn't a need for a balanced connection.

Connecting a balanced "600 ohm" microphone to the unbalanced input of the FTM400 isn't hard to do. The brute force way is to connect the "cold" or pin 3 (the one at the top of the circle) to pin 1 which is ground. As a quick side note, if you ever need to wire an XLR connector just remember GRUB. Ground (pin1), red or white (pin 2) und black or dark color (pin 3). The brute force method is used here because although there is a difference in impedance, its not a game ender. As a common practice its better to have a low output impedance feed a higher input.

The better (and more expensive way) to make the connection is to use a transformer to both isolate and balance the impedances. Hosa is a company that makes cables and adapters for the audio industry. The MIT-156 is a cable with a built in matching transformer. It's intended to be used for connecting a single balanced mic to a unbalanced camcorder or DSLR camera. One end of the cable is a female XLR and the other is a 3.5mm or 1/8" "stereo" male phone plug. I serioulsy doubt that there are individuall left and right output taps on the transformer. My guess is that the phone plug end of the cable has both the ring and tip connections shorted together. In this application the phone plug would be cut off and discarded.

Tired of reading yet? Well here's one more twist. The mic jack on the radios I tested (FT2900, FTM400 and FT1807) would not allow a standard 6 pin RJ12 (6P6C) connector to lock in place. I compared the MH48 connector to a new RJ12 I had and couldn't see any real difference. Because I had an ace up my sleeve, I really didn't give it much more thought.

The ace I have is a replacement MH48 mic CABLE. It can be purchased for around $6. Just Google "MH48" on Ebay or Amazon. It is the same cable that is used in the MH48 mic. One end has the RJ12 connector attached. The other is a single header connector for use inside the mic. The single header isn't used for this effort so it gets cut off! Only three wires are needed. They are ground, tx audio and ptt. I can't say that the wire colors will always be the same from cable to cable so I will use the RJ12 pin number instead. IF you choose to go the transformer route.....The "hot" or center conductor and the shield from the cutoff end of the Hosa cable would be directed connected to the audio and ground wires from the MH48 cable.

Parts needed for this cable are:
Replacement MH48 cable
XLR female (Neutrik NC3FXX)
1N4148 diode
Foot switch
12" shielded small diameter wire
1/4" TRS (stereo) phone connectors
Cable Diagram (png version to the right)

Notice the use of a 1N4148 diode in the PTT path. Yaesu used a surface mount switching diode in the MH48 so I included a through hole version ie 1N4148. The diode is installed in the XLR female body. IF you are only using your footswitch for MH48 Yaesu radios, you can install it inside the footswitch. I can't say what if any issues may arise from placing the diode inside the footswitch and then using it on other radios.....

The best choice for the footswitch connections is to use TRS connectors. They are kinda beefy and will take some abuse. Leaving the "sleeve" or ground tab isolated and using just the ring and tip is a matter of personal preference. A TRS phone is considered a "shorting" connector. That's due to the fact that when the male is inserted into the female, the tip momentarily shorts against the sleeve of the female. IF the sleeve is isolated (not used), the tip doesn't short to any part of the circuit. If a TR or "mono" set of connectors is used, the sleeve HAS to be used.... This is the case with the Heil FS3 footswitch.

In this build I didn't have the female TRS connectors available so I used what I had, RCA male. It helped that the cable I used for the keying circuit was a shortened length of shielded audio cable that was already terminated with a male RCA.

Here's a few pictures of the cable: Click on the pictures for a larger version.
   
KB2UMJ at Yaa Whoo Dot Com
January 16th, 2016