Some Audio Adapter Cables

Just a quick note to shown some of the audio adapter cables I made, in case others are interested. I cut up short premade BNC jumpers and line spliced them to a 5 foot length of star-quad stage cable, chosen for it’s flexibility and freedom from interference pickup. I wanted to avoid the potential magnetic sensitivity of widely separated + and - cables.

I in-line spliced the center conductor, slid the braid back over and solder tacked it together, then I slid a 2.5" piece of 1/4" soft copper pipe over it all, flattened slightly at one end to accommodate the two coaxes. It provides additional shielding and mechanical protection. Then covered it all with shrink wrap. I built two of these with 1/4" TRS connectors and two with Neutrik NC3FM-C-B bisexual XLR connectors. They can be either male or female. Quite handy for test cables. I also bought a few BNC to RCA cables for the unbalanced hifi stuff. This should allow me to plug into almost anything.

Then I built a couple of adapters into 2x2" cast boxes from Parts Express. One is the loudspeaker impedance adapter, with a 0.1Ohm sense resistor on the low side of the line, the other is a 20 dB stereo/balanced pad for looking at the outputs of pro power amplifiers with 3 digit voltage and 4 digit power levels. Quite a few of these are self-bridged, driving both sides of the line, neither side grounded, hence the need for two-in-one. I bought the sense resistor and the 0.1% precision resistors for the pad at Newark. The 9K Ohm resistor is a 10K and a 100K in parallel. I measured the pad at 20.06 dB, so I’m very happy.

Dale Shirk


I realized I hadn’t calculated the power absorbed by the attenuator resisters. I’m changing the 20 dB Pad to 40 dB, 100K to 1KOhms. And yes, I thought about it before I actually smoked it.