Help with Squeal/Feedback in different outputs.

  • Hi, I got a Kemper a week ago and I am struggling with a Feedback/Squeal issue.


    I have a Line 6 Amplifi that I am using as my FRFR and also to Jam along backing tracks as it has Bluetooth. It has really served me well but I wanted to step up my game with a Kemper. I do not need anything bigger that the Amplifi as I only play at home with the Bluetooth streaming at low volumes or Headphones.


    The issue I have is related with Ground loops and the Ground lifts in the Kemper. When I use the Kemper and the Amplifi together I have this weird uncontrollable squeal. It is not really a feedback since all of this happens over headphones too.


    I have tried this connections with the following results:


    Kemper Headphone Output -> Amplifi Aux In = Squal when I stop playing.

    Kemper Direct Out (Master mono) -> Amplifi Guitar In (Moddeling bypassed) = Squeal uncontrollably

    Kemper Main Out (Master mono) -> Amplifi Guitar In (Moddeling bypassed) = Squeal uncontrollably

    Kemper Monitor Out (Master mono) -> Amplifi Guitar In (Moddeling bypassed) = Squeal uncontrollably


    Kemper Direct Out (Master mono) [groud lifted] -> Amplifi Guitar In (Moddeling bypassed) = Works OKish

    Kemper Direct Out (Git Analog) [groud lifted] -> Amplifi Guitar In (Moddeling ON) = Squal when I stop playing.

    Kemper Main Out (Master mono) [groud lifted] -> Amplifi Guitar In (Moddeling bypassed) = Works OKish

    Kemper Main Out (Git+Processing)[groud lifted] -> Amplifi Guitar In (Moddeling bypassed) = Very bad squeal.

    Kemper Monitor Out (Master mono) [groud lifted] -> Amplifi Guitar In (Moddeling bypassed) = Works perfectly!

    Kemper Monitor Out (Git+Processing)[groud lifted] -> Amplifi Guitar In (Moddeling ON) = Works perfectly!


    All the test were done with different volumes in the Kemper and Amplifi. Profile is a high gain one as well as preset it the Amplifi when modelling is engaged.


    The issue does not happen if I connect the guitar directly to the Amplifi or if I use the Kemper by itself. I also tried a Boss GP-10 into the Amplifi with no issues (well the GP-10 does not have ground connection to the wall).


    So my questions are: how do the different outputs differ?

    From the tech specs they are basically the same (¼-inch TS unbalanced with ground lift, max output level: +16 dBu) and they are multipurpose based on the setting but they seem to behave differently. It is clear the Monitor Output works differently and the best performing. it is like the only one where the ground lift works as intended.


    Am I missing something?


    Any help is appreciated.

  • A weird uncontrollable squeal doesn't sound like a ground loop issue. ground loup would be either 50hz or 60hz depending on country. That's around bottom A on a Bass guitar. But would be audible in higher octaves too around 100hz and 200hz. I think there must be something else at play here but don't know the Amplifi so can't suggest what it might be.

  • Am I missing something?

    Try turning BT functionality on the Line 6 Amplifi off and removing your 'phone, laptop or whatever's transmitting from the equation.


    The high-pitched squeal could be the data stream's being amplified by the combination of the L6 and Kemper in much the same way that tube amps occasionally tune into local radio.


    Just a guess, of course...

  • A weird uncontrollable squeal doesn't sound like a ground loop issue. ground loup would be either 50hz or 60hz depending on country. That's around bottom A on a Bass guitar. But would be audible in higher octaves too around 100hz and 200hz. I think there must be something else at play here but don't know the Amplifi so can't suggest what it might be.

    Yeah it is very weird. But definitely it is related to the ground connection because by lifting the grounds the situation improves.

    I will keep making tests.

  • Try turning BT functionality on the Line 6 Amplifi off and removing your 'phone, laptop or whatever's transmitting from the equation.


    The high-pitched squeal could be the data stream's being amplified by the combination of the L6 and Kemper in much the same way that tube amps occasionally tune into local radio.


    Just a guess, of course...

    Yes, it makes sense and it is a fair test. Will test and see. The Kemper is sitting on top of the Amplifi so yeah, there could be some RF leakeage somewhere and perhaps that gets into the cable shields causing the ground loop to not be only 50/60hertz but also digital RF noise.

  • I think you might be on to something with the RF interference and possibly poor sheiling rather than ground loop. If you look at you examples there doesn’t appear to be consistency between when the ground button is in or out. If it was a ground loop you would expect it to be consistent for each connection and between connections. It seems like there may be sometuhappening in the Amplifi itself.


    So, is there anything sending signal back from the Amplifi to the Kemper (sending a signal to the Aux in for example). What you described sounds more like a feedback loop than a ground loop. I know it’s not a pleasnt sound but it might help to record a short snippet of it to let us hear as the type of sound might give away the secret.

  • I have one question .... When your using the Amplifi is it connected to a PC via USB?

    In certain circumstances if the Shield of a USB cord makes direct contact to a metal chassis this can cause a nasty digital ground loop !

    So say the Amplifi is connected to a PC via USB if the above condition happens since there will be some cord connection to the Kemper it very well maybe allowing USB packet chatter noise into the KPA. Try another known high quality USB cable ... Sometimes USB cables can have poor conductor isolation shielding

    NOTE:

    IF YOU ARE NOT USING A USB CABLE YOU MAY BYPASS WHATS WRITTEN BELOW.

    It does contain some information that may come in handy one day !


    THE ROOT OF ALL USB EVIL

    Or better yet ...

    Get the squeal sound to occur.

    Disconnect the USB cable if you are using one between the Amplifi and a PC at that moment.

    Did it go away ?

    Now I do not mean to be insulting but I have to ask the question.

    Are you sure the cable your using to connect the Kemper to the Amplifi is not only a shielded cable but a very well shielded cable.

    Sometimes its easy for people to make a mistake and find themselves using a speaker cable or a very badly shielded cable.

    Just turning over stones here !


    What to leave in and what to leave out !

    You cant trust China manufacturing to do things using proper engineering protocol !

    A product may work in a lot of ways although there might have been one tiny detail overlooked prior to and during production.

    FYI: Chinese manufacturing rarely performs any QC test procedures on production runs. A couple of units come off the line a production Engineer looks them over and then rolls the dice. Happens everyday all day long.

    The fact that the Roland device worked could be that Roland applied extreme babysitting measures to insure total ground isolation and RF filtering was performed on all inputs and outputs just in case some other China made device cut a few corners in their design !


    LETS CHECK THE USB GROUND ISOLATION of the LINE 6 AMPLIFI !


    1. Another test you can do is with a USB cable connected to the Amplifi only!


    2. Connect the AUDIO cable to be used with the Kemper only connected to the Amplifi.


    3. Use an Ohm TEST METER and set it to the CONTINUITY TEST [Beep] Function.

    Touch the two test probes together to verify the continuity function is working.


    4. Take one of the probes and touch it end of the USB cables outer shield while taking the other probe to touch the sleeve of the cord connected into the amplifi that would have been plugged into the Kemper. If you hear a sustained beep there is a USB ground issue with the Line 6 Amplifi !

    There is a work around that might work but lets see where this all leads to.


    Let me know what you find...

    I wish KPA would please change my descriptor to something other than Beginner? Thanks


    Cheers

    Edited 10 times, last by SgtPepper ().