After looking around the web for interface circuits for VOX or PTT interfaces between Soundcards and radios I came up the following circuits. I'm no EE genius but these worked very well, and all parts were available at Radio Shack.
First a simple VOX circuit.
WARNING
1. Do NOT connect the Soundcard output directly to the Audio in of the
microphone.
2. Turn OFF sounds in your win95/98 or NT system. I can't tell you how
many times I've hear CHIMES.WAV and DING.WAV on the air!
3. Sound Cards expect a stereo jack to be inserted. Using a mono jack can
short the connections. Radio Shack Part No. 274-374B converts a
1/8" (3.5mm) mono mini-plug to fit a 1/8" (3.5mm) Stereo jack
4. Use shielded cables.
5. Radio Shack Part No. 270-283A is a small Project enclosure with a PC
board included. Be sure to mount the isolation transformer under the board
or the enclosure cover wont fit. Learn from my mistake!
6. Use the Line-Out jack not Speaker Out.
1. VOX Circuit
The VOX circuit worked well, but the VOX, Anti-VOX settings on my IC-706MKII were really testy, and constantly messing with the audio levels drove me nuts. Also, on the air my IMD was fair but not great. So I went to PTT using a com port.
2. PTT Circuit
NOTES:
1. 1N4148 = Radio Shack Part No. 276-1122
2. 10K Ohms = Radio Shack Part No. 271-1126
3. NPN = Radio Shack Part No. 276-2009
The audio input goes through the same isolation transformer as the VOX circuit. Additionally, this circuit allows for either DTR or RTS to key the rig, as some software allows for either. I didn't use the Audio out from the accessory jack to drive the soundcard since my audio goes through a TimeWave DSP which, quite frankly beats the pants off the DSP in the Icom706MKII.
From comments on the air, this setup produces a much cleaner signal. As a bonus, it can be used for AFSK RTTY as well as SSTV !