SUMMARY: The sound card to radio interface sold as the Donner Digital Interface cannot and does not work with the Icom IC-706mkII transceiver without shorting the microphone.

This page tells what's wrong and how it should be designed. Back in February 2008 when I sent these very clear remarks to the designer, "Whitey" Donner, K8BE, I was dismissed in this way: "We are in, at last count, 30 countries, and have sold several thousand interfaces, with very minimal mentions of shortcomings." ... Allow me to mention one: The thing can't possibly work with the Icom.

The Icom IC-706mkII interface specifications
Donner Digital Interface schematic supplied by the company.
Schematic after extensive repairs

The most serious problem is with the path from sound card line out to transmit audio in. Notice from the Icom spec sheet that the input level is 100 mV and the impedance is 10 K-OHM. With the interface's mike gain pot set near either end, note that the transceiver input is seeing far less than 10 k-ohm. It is seeing a short to ground when the gain is full down, and 600 ohms impedance when the gain is full up. (The capacitor has negligible impedance.) Only when the pot is near the center does it present an acceptably high impedance greater than 10K to the transceiver input. Unfortunately, the 100 mV needed by my transceiver required me to set the pot at its highest position. The resulting 600 ohm source resistance nearly shorted out the audio from the mike while I observed my SSB signal on a good oscilloscope. The major purpose of using an interface to the accessory plug is to allow both the accessory plug and the mike to remain in place and operable. The Donner design shorts out and distorts most of the mike audio. (The mike is followed by amplifiers, and it is the amplifier output that was being shorted.)

I do not rule out that my transceiver may have a fault, but I find the following: Practically, the transmitted SSB power was down to 10% power with a 7500-ohm load on pin 11, the modulator input. In practice 10 k-ohms is a minimum source resistance, the use of which still lowers the SSB output. In my schematic after repairs the resistance seen by pin 11 is 18.5 k-ohms with the pot at either end and 26 k-ohms centered. The SSB level is not noticeably impacted. Of course there is no problem with the Donner interface transmitting data if you never use your microphone or if you go around to the back and unplug your interface when you want to use the microphone. I have never know a ham to accept the first case, and in the second case, avoiding unplugging the interface for voice is mainly why the connection is there and why one buys an interface.

Note the secondary of the transmit (lower) transformer MUST be around 10,000 ohms. With the transformer impedance ratio of 17:1, the voltage gain would be about 4:1. That gain is a MUST. The 600-ohm secondary does not supply enough audio to fully modulate the transmitter, even with the software sliders full up.

(Practical note: The Xicon transformer is larger than the Tamura and will not fit in the box.)N


On the received audio side, the radio's output impedance is 4.7 k-ohms. I inserted a 4.7k resistor in series with the RX audio to provide that match. With that resistor, less signal gets to the sound card line in, but it's quite enough. Without the resistor, the load is slightly above 600 ohms. The practical result of the 600 ohm secondary is that the audio from the radio's speaker may be low or distorted.

I would think a 10,000:600 ohm transformer would work as well, but may still have to be attenuated with a series resistor to put the audio at about the desired level with the pertinent software volume controls near center in all cases. The 10,000 ohm sides would face the radio and would present the proper high impedances to it. A 10k:10k transformer could be used for the sound card line in, which input Z I have measured as 12 K-ohms in one sound card, but 600 ohms is best in case one actually encounters a 600-ohm line in impedance.

The lesson here is one Mr. Donner was unwilling to learn: A single design will not work with all transceivers. The lesson he learned well: A sucker is born every minute.


Where some designs use a Darlington optocoupler, Donner Digital Interface uses a relay, which works fine. It is a Hamlin HE36210500. It has a 500 ohm coil and it's minimum pull-in voltage is 3.75. It is a nominal 5-volt coil and will take a maximum of 14.

Modern USB to serial ports are not high-power, 10-V rail-to-rail like computer serial ports in the old days. They are flea powered, more like CMOS outputs. However two typical USB to Serial adaptors (tested below) work OK with the Hamlin relay. DON'T use the double-throw relays of the same make. They are 120 ohm coils. Low-power USB ports (which is about all of them today) will NOT work with them. See tests below.

Tests of two representative USB to serial ports and voltages for DTR output:

Keyspan 4-port serial adaptor:

Open circuit: 5.8V
500 ohm load: 4.6V (enough)
121 ohm load: 3.24 (not enough)

Iogear GUC232A USB to serial adapter:

Open circuit: 7.0V
500 ohm load: 5.2V (enough)
121 ohm load: 2.70 (not enough)

Travis Hardin, KE3Y
Huntsville, Alabama
June 2009

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