Horrible noise on startup

I've got a problem with noise just as the KPA enters browse or performance-mode at the end of the boot-sequence. It's a hiss or white-noise that can be quite bad for up to 10-15 seconds. How bad it gets depends a lot on which rig the KPA starts up with. A clean rig produces very little noise, while something distorted with lots of delay and other effects may produce a horrible loud squeal. The noise appear even if there's nothing connected to the input so the noise must originate from within the KPA itself. I'm currently running firmware 2.2.1 8963 but have had the same problem with other releases too.

I can minimise the problem if I'm careful to switch to browse-mode and activate a clean rig before shutting down, but would rather not have to. There's also a problem with what seems as random performance/slot selection the first time performance-mode is activated after boot so booting directly to performance-mode is no solution.

In case there's no permanent fix for the problem, is there a way to choose one rig (clean) in browse-mode that is supposed to be opened on boot regardless of which rig was active at shutdown?

Quote from Kempermaniac

Maybe the noise is not coming from the KPA but caused by computer monitor, mobile phone, wireless system etc.?

It's been on my mind. I've got the KPA head sitting on a shelf in a 6U rack with a wireless receiver (L6 G90) below. A RF-receiver doesn't generate much noise though. The shelf and casing acts as a shield for anything coming from there, and finally; the noise does appear even if the receiver is turned off. The panels on the KPA itself should isolate its interior fairly well from other external RF-sources too. I often have wifi-router, notebook computer, phone and tablet nearby, but the noise does appear even with all known sources of RF-interference turned off. My macbook that does multichannel recording and runs a mixing console normally sits on top of the rack without any signs of noise except what I must expect if I put the guitar right in front of it.

There's a few rigs that will produce noise on boot almost every time the box starts. An example is the one called "TAF Pushed Hair" from the standard set of rigs.

Some of my older guitars are very sensitive to RF-interference, but that's another issue. One of the worst sources of such interference is actually the KPA itself (the LCD-screen I guess). I need to get the inside of those guitars treated with copper foil and/or conductive paint.

Quote from londonjeepster

yes open a support ticket,this sounds very odd.

I probably should. However, I think I've found the cause and a workaround. Maybe this can be useful knowledge to others too:

I wrote that there was noise with nothing connected, while what I ment to say was that there is noise even when the connected wireless receiver is turned off.

I got hold of some probing gear and started measuring. What I found is that the input-signal to the KPA is completely flatline regardless of whether the wireless receiver is on or off. Nothing registers on the oscilloscope in any audible frequency that could cause significant noise. There's nothing until the wireless transmitter is turned on. If anything, the output from the receiver is exceptionally clean.

Then I opened the boxes and started measuring inside. What I found is that there's a small difference in voltage between ground as delivered by the KPA's power-supply and that on the wireless receiver, with both boxes fed from the same power-strip. The difference remains the same when the wireless-receiver is off. It's well within what audio-interfaces are designed to cope with so nothing will break. What I believe happens is that the slight difference in voltage creates a ripple in the KPA's AD-converter just as it comes live, similar to when you plug in a guitar. This explains why the problem gets so much worse with delay or reverb as those effects keeps repeating the noise. Add some feedback in the simulation and you've got a high-pitched squeal. With no effects it is really just a loud tick.

I've been checking the powersupply on 3 more rack-mounted units from different manufacturers today and found small variations between the ground-signal of all boxes so this seems quite common.

My workaround is simply to boot the KPA to tuner-mode with the tuner set to mute the signal. I have been used to boot directly to browse-mode so far. A proper solution is non-trivial and would require modifications to both the KPA and the wireless receiver to enable interconnect of DC-ground between the boxes. That's no DIY-project.