A few years ago, I built a crack with C4S which I used with HD600 cans. It worked splendidly and was dead quiet with no discernable hum even at full volume. Here is how it was hooked up.
Computer--- digital optical cable----Modi Multibit DAC---Shielded RCA cables---Crack
Recently, I completed a Sex 3.0 (OT wired for 4 ohms) and placed it where the Crack had been. Immediately I heard an objectionable hum which sounded like 60Hz. It was apparent with the volume turned all the way down and increased as the volume was turned up.
But, with the inputs shorted, it was dead silent even at full volume. So, I realized the problem was not the amp itself but rather external noise getting in.
So, I spent days cleaning the tube pins and sockets, retightening the chassis screws, and cleaning all connecting surfaces of RCA cables/jacks with Deoxit. Afterwards the hum was only a little better. I was just about to post to the forum.
But then I noticed something.
Even though I was not using it as an input, the USB cord was still connected from the computer to the DAC. After unplugging it, The Sex amp became dead silent even at full volume.
So, problem solved!
But then I got to thinking. If the Sex amp itself was not the problem and it was external noise getting in by way of the USB cord, why was the Sex amp affected but not the Crack?
I had done the double diode ground breaking mod on the Crack. Would that have explained the difference? The noise I was hearing was a steady hum, not a clicking or static noise that the mod was described to remedy.
Just curious
John
