OK, this is driving me nuts! Compounded with the fact that I'm no rocket scientist with electronics, and it's hair-pull time.
Here's the deal. I've built my own 'integrated' headphone amp, using an M3 amp board from amb.org, a STEPS regulated power supply from tangentsoft.net, and the Bugle, mounted in a Par Metals aluminum chassis, all DIY. The power supply is 24vdc. It is split into +/- 12v on the M3 board with a TLE2426 rail splitter, so I added one on a small protoboard to do the same for the Bugle. The virtual ground is tied to the input ground in both the M3 and Bugle, and both are connected to the chassis at a brass screw to which I connect the ground clip from the tonearm, so all grounds connect at the same point. The M3 amp uses a separate ground channel on the output, and so is not connected the the virtual ground in any way. The ground from the 120vac input is not connected the the chassis, only to the PS board. Also, I installed a selector switch and have three inputs: 1 to the Bugle, then to the selector, and 2 line.
So here's the problem! The 2 line inputs are quiet as a mouse, even at full volume with no signal. The Bugle is quiet at normal listening levels, but as I turn the volume up past half way with no signal, it becomes increasingly noisy, with hissing and a little hum. I've using batteries for power, but the problem persists, so ??????????. Do I need to shield the input wires from the input jacks to the Bugle board? The PS is on one side of the case and the Bugle is on the other, with the amp in between, with a home made shield between the PS and amp, so I don't think it's transformer hum. Should I try using the PS made for the Bugle separately? I'm at my wits (not much of those) end! By the way, I'm using OPA2134's for U1/2, and a pair of OPA627PA's for U3.
Don't get me wrong, the Bugle sounds great, and at normal levels there's no noise. But I can't help but think I've done something wrong to cause the noise to appear in the upper end of the scale. Also, I'm running the M3 at the default gain of 11.
