A/B box generates noise on input of Kemper Profiler

  • I typically like to use a 2 way A+B or A or B splitter box directly after my guitar to feed to the Kemper and an AX8 ( - I am fortunate to have a KPA, an AX8 & a Helix and rate the KPA as top amp sim by a good way but there's stuff I like about the other 2 also)
    Anyway, with the simple passive A/B in front I get a load of digital noise, and unrelated to the actual signal, becoming audible particularly with hi gain presets that ordinarily (i.e. with a direct input and no A/B) remain much quieter.
    Yes sure I can utilise the noise gate to gate it out but that's not quite the same and I would rather not fix a problem that may be avoidable altogether.
    Same A/B box to the AX8 and the Helix does not (perceptively) increase the noise floor on either of these other devices.
    Is the front panel input highly sensitive?
    Any ideas how I could avoid this?


    Thanks.

  • Thanks for the reply.
    Er, no. Its a very simple passive dpdt footswitch. Simple - cheap.
    So something like the Lehle switch is going to have isolating transformers presumably? That would probably make the difference....
    Presumably something like that isn't going to affect the sound output in any way?


    Wonder why it is the front end to the KPA is quite so sensitive when an AX8 or Helix doesn't do this?

  • Yes a quality A/B with isolated transformers will make all the difference. When I tried doing A/B comparisons between my Boogie and Kemper I used the two isolated amp outs on the RJM Mastermind PBC without any hint of noise. Digital gear is notorious for cross talk from the power I used to get this with Strymon Pedals and TC HoF on my board without isolated power supplies. The other really bad problem with some digital gear is insufficient current which can create terrible noise problems. However, I would expect the power to AX and Helix to be good. I wonder if the AX and or Helix actually has some sort of noise gate/filter built in that can't be bypassed? Whatever the cause a decent isolated A/B switch should stop any noise issues.

  • I had problems not unlike yours with a simple stompbox I built with a 3PDT switch. No Kemper, just two amps.


    Turns out that with my genius design I was still able to create a ground loop and add some noise, so you may want to check that you don't have one.


    An isolating transformer based switch will definitely cure that.

  • Thanks for the very helpful answers. I did rather think that an isolating transformer was likely the only way out of this and the Radial Tonebone looks to be most cost effective option. Still have slight concern about so called "tonesuck" but I guess if I buy one from the Amazon monster I can send it back if I detect any which does seem a little unlikely based on the comments here. It is clearly designed for this purpose and hopefully impedance matching or any passive attenuation aren't going to be evident.


    Not at all sure why the KPA is more susceptible than the Fractal or Helix - as you say may be something built in to those devices.


    As an aside I am impressed with the KPA. Haven't dare launch yet another comparison thread (tho suspect here there is less chance of starting a forum riot - kinda easy on Fractals place...!) but the profiled amps feel and sound a good deal more real than the other 2 competitors and I rather like the way the fx have been implemented. The interface (simulated amp) is OK but still feel the Helix suits me better.


    Thanks again for the replies.

  • what I have just noticed was that even with KPA powered down but with a commoned input to the KPA and AX8 the noise is still present at the output of the AX8. This and vice-versa i.e. with KPA on and AX8 off noise at KPA outputs). So it certainly isn't developing on the KPA but is created when the AX8 is linked to the KPA. A rather typical digital noise loop.
    Am now wondering whether there is another way to block this but I guess isolating either input will work anyway.

  • Just out of curiosity could you try to unplug the powered down device (Kemper or AX8) from the wall and see if you still hear the noise?


    Even with the device is powered off, there is still a connection (and therefore reference) to ground. If you unplug it, that ground will be referenced - through the DPDT switch - to the other device.

  • As an aside I am impressed with the KPA. Haven't dare launch yet another comparison thread (tho suspect here there is less chance of starting a forum riot - kinda easy on Fractals place...!) but the profiled amps feel and sound a good deal more real than the other 2 competitors and I rather like the way the fx have been implemented. The interface (simulated amp) is OK but still feel the Helix suits me better.

    You won;t start a forum riot comparing the Fractal and Helix, I think most people realise they are very good units in their own right.


    The riots start if you mention a replacement Kemper or if an editor is needed :)

  • Just out of curiosity could you try to unplug the powered down device (Kemper or AX8) from the wall and see if you still hear the noise?


    Even with the device is powered off, there is still a connection (and therefore reference) to ground. If you unplug it, that ground will be referenced - through the DPDT switch - to the other device.

    Good plan - and as soon as I pull the mains cord out of either device the noise disappears. So its mains supply borne...
    Wonder if there's a better strategy to consider here?

  • Have you tried toggling the Ground-lift switches on the rear of the unit in and out?


    They relate directly to the outputs, not inputs, but hey, it couldn't hurt, right?

    I hadn't tried that - was thinking too much about this presenting at the inputs and surprise (well sort of...) ground lift on the outputs drastically improves the issue - can still hear it buts its barely audible to a point I can probably live with it.
    Good call - thank you.

  • Good plan - and as soon as I pull the mains cord out of either device the noise disappears. So its mains supply borne...

    It is then a ground loop that somehow gets converted? ;( into digital noise.


    I think the transformer would still help, if at least one of the connections has one, if will disconnect the ground (and the signal too).


    If the transformer-based A/B switch box is expensive, it may be worth getting a simple transformer based "hum destroyer". My buddy uses one of these Pyle ones, works perfectly for less than $20.


    https://www.amazon.com/gp/product/B00BARTW3I


    Inside it only has two transformers and a couple of resistors and capacitors - I know as we took my buddy's apart. Completely passive. You could send the output of the AB box through this going to the amps. Or technically only one of the two would have to go through the box to break the ground loop.

  • Its a ground loop for sure and its not the more typical hum as its fluctuating HF noise similar to that which gets picked up as you approach a PC, monitor etc.


    And that is an interesting approach - essentially breaking the ground loop after the simple splitter?


    Just to put this into context tho - I am using hi gain amps with stuff like the treble booster in front - really a worst case scenario to amplify any background noise and its not like the noise gate couldn't help on the KPA. Like to avoid gates - even a really good one has a rather pronounced effect on the sound. When I think back to how things were when I used an EHX treble boost into a Muff Pi there was more noise than signal!!!!