++++++++++++++++++ Date: Fri, 05 Jul 2002 09:07:47 -0300 From: J Ellis To: "Puppy, Poppy" , "Elecraft, List" Subject: [Elecraft] high level audio input to K2 Hi Folks, For those of you who might want a high level (0 dbm) input to the K2, I made the following mod which requires no holes and is very easily removed. It bypasses the internal speech amp so it does not rely on padding the input. It also provides DC isolation thru the use of a xfmr input. It enables you to use, among other things, an external voice processor such as a Symetrix 528E for really fantastic sounding audio. (while these studio quality units are quite expensive new, they are available quite reasonably on Ebay) This mod does, however, use the 2nd RX antenna jack as the input connector, so if you have the amp and the 160M option, the "no-holes" part of this mod won't apply. You will have to bore a hole... Basically all this mod does is provide a DC isolated transformer coupled auxiliary input to the balanced modulator. The functions of the added resistor and capacitor are identical to the existing R2 and C20 on the KSB2. You can use a 3 circuit stereo phone plug to provide a 600 ohm balanced audio input (shield grounded at the source, not at the K2) or you can use a standard 2 circuit mono phone plug for unbalanced input. Remove R2 on the KSB2 (5.6K) and install another 5.6K at R2 but leave the lead on this resistor next to the carrier balance pot about 1/4 inch long above the board. This facilitates attaching a second 5.6K resistor vertically to that junction point. The other end of this second 5.6K will go to a .33 uf cap. The values of the resistor and capacitor are not that critical. Tie the free end of the capacitor to the center conductor of a 7" piece of RG-174 and attach the shield to the center pin of the 78LO6 regulator (gnd) on the KSB2. Use small heat shrink tubing or other suitable insulation on the shield of the RG-174 to make sure it doesn't short to anything. Obtain a 3 circuit 1/4" stereo jack, preferably with a plastic sleeve. You might be able to get by with one with a metal sleeve, but a plastic one will give you additional protection against ground loops or "rf in the audio" by providing a single point ground connection to the audio input of the K2 right at the KSB2 sideband adapter. Fit the jack in the K2 and you will see that you will have to file a flat spot on the mounting sleeve of the jack to make it fit. Using a jack with a plastic sleeve will ease the filing process and minimize the possibility of damaging the mounting threads. Position the jack squarely to avoid problems with toroidal xfmr T3. To the tip and the ring of the jack, NOT THE SLEEVE, attach the primary of a small 600 ohm to 600 ohm xfmr. (Radio Shack model 273-1374). This xfmr is small enough to mount on the jack. To the other side of the transformer, attach the free end of the RG-174 coax. I put a piece of heat shrink over the jack and the xfmr to hold the xfmr in place. A few things to remember: The speech amp, the compressor and the vox circuitry in the K2 are bypassed when using this input, but the regular microphone input is unchanged. Make sure your mic is disconnected before using the this input from a computer, TNC, etc. as the mic might be live when you go into xmit, even if you are using this auxiliary input. PTT control is easily handled thru the key jack on the back or thru the front panel connector. You will need around a volt of audio. If this is a problem, you might either need to use another xfmr to step up the voltage or you could use an xfmr with a sufficient step up ratio in place of the 600 ohm to 600 ohm unit as long as you can find one small enough! You will need to be able to make audio level adjustments from the device that you are using to drive the K2. Since the internal speech amp and compressor are not used when using this auxiliary input, you will have no control over audio level control from the K2. For what it is worth, I ran this mod past Wayne and he blessed it. I have also installed it in two K2's, one with the KPA-100. I experienced no problems with it, even with the KPA-100 running full tilt. Enjoy, 73, John, NP2B K2/2281, K2-100/2573 -- Jeanette and John Ellis (NP2C & NP2B) P.O. Box 24492 Gallows Bay Station Christiansted, St. Croix, U.S. Virgin Islands 00824-0492 tel (340) 773-9643, fax (340) 773-6574 website www.viaccess.net/~jellis when responding to e-mail, please send to jeanette at atthehelm.com or john at atthehelm.com +++++++++++++++++