Be aware of the digital sonic artifacts (aliasing) in your KPA!

    So far, after installing 1.5.2 I haven't heard any pops or clicks in the S/PDIF digital output, yet, but I will have to give it a little more time to be sure.

    Hey Miles, when you created that profile is there any chance the levels going back into the Kemper were a little hot? ?( I had this problem and had to add a trim between the mic pres and the Kemper. Just a thought.

    Quote from mikeb

    Hey Miles, when you created that profile is there any chance the levels going back into the Kemper were a little hot? ?( I had this problem and had to add a trim between the mic pres and the Kemper. Just a thought.

    No. The Great River Pre has output level control, and I experimented extensively with it and the KPA's Return Level control to ensure the lowest possible chance of distortion at that critical stage. The interesting thing about profiling a clean device is that it is easier to trouble shoot where clipping is coming from, so maybe the KPA can't yet create a super-clean profile. To me, that particular distortion sounds totally digital, like in, digital artifact.

    I mainly play jazz and blues, I have a ton of warm sounding clean tones without any artifacts. The culprit exist but somewhere else, not on clean sounds

    "Music is enough for a lifetime, but a lifetime is not enough for music" Serghei Rachmaninoff


    Quote from guitarnet70

    I mainly play jazz and blues, I have a ton of warm sounding clean tones without any artifacts. The culprit exist but somewhere else, not on clean sounds

    Possibly the fact that there is not a true cab associated with my profile, and the highlighting of a mild distortion that might otherwise be concealed in a warm, high frequency-limited jazz/blues amp/cab profile is why you haven't noticed it before--my example is clinical, and revealing like a DI, and I am curious to gauge what the KPA's current sonic limitation might be.

    Edited 2 times, last by miles ().

    Quote from guitarnet70

    I mainly play jazz and blues, I have a ton of warm sounding clean tones without any artifacts. The culprit exist but somewhere else, not on clean sounds

    I agree, I have a few very, very good sounding pristine clean profiles. i always lamented that the Line 6 POD HD had no JC-120 model as I often like a pure, pristine clean tone. A reason why I love Digitech's RP/GSP line. The Kemper can match the pristine, pure clean signal I got from the RP/GSP without issue when using a clean profile or even profiling the GSP itself. I think the OP should concentrate on profiling amps rather than a DI.


    Quote from miles

    Probably the fact that there is not a true cab associated with my profile highlights a mild distortion that might otherwise be concealed in a warm, high frequency-limited jazz/blues amp profile--my example is clinical, and revealing like a DI.

    Does your profile have a gain setting great than the minimum? In other words, did you choose clean when profiling the Grace? I'm not sure why I care at this point as this is an issue which I think is unique to you. The aliasing I can replicate though at a much quieter level than in your clip, and I have to dime the gain and definition (something I wouldn't ever do in a performance scenario). This I'm puzzled by (though maybe I should listen to your new clip to see exactly what your complaining about).


    EDIT: Hmmmm...the beginning has some very subtle breakup happening in the lower mids. The Kemper is slightly exaggerating this. This is really in an area of nit-picking subtlety IMHO. I think your energy could be better focused on profiling amps than on this exercise...

    Will, my first 'Grace' test was with my first KPA, and this new 'Great River' test was done on my second KPA--both had the same result several months apart. Kemper support asked me to do it. Can you not clearly hear this distortion? I thought that perhaps it might point to some other low-level software-related issue that might be in some way directly or indirectly connected to aliasing/processing.

    Edited once, last by miles ().

    I still get pops and crackles with the latest firmware and spdif. It mostly happens when there are profiles with multiple types of "drive" stomps enabled and the reverb.

    Quote from miles

    Will, my first 'Grace' test was with my first KPA, and this new 'Great River' test was done on my second KPA--both had the same result several months apart. Kemper support asked me to do it. Can you not clearly hear this distortion? I thought that perhaps it might point to some other low-level software-related issue that might be in some way directly or indirectly connected to aliasing/processing.

    On the new clip...in the first part I hear a subtle breakup in the lower midrange. On the second, that is exaggerated and there is some additional compression or distortion happening on the attack, but to my humble ears its not a night and day difference. Take that with a grain of salt though I'm not listening on the best playback system right now. My greater point is that I have some simply excellent clean profiles that folks have created somehow. I mean, what other modeling device can get as close as the Kemper to a target amp sitting in your studio? So maybe in reality the Kemper is only something like 97% accurate than 100% accurate, is anything really perfect in this world? Is there another competing product which does what the Kemper does better? Remember, this is bleeding edge stuff in the world of guitar gear. I think I've created pristine clean profiles of my GSP1101, but honestly have not put them to any clinical test as...well... the tones I'm getting are inspiring and that's what matters to me.


    Anyway, I have no idea what is the root cause of your issue. Maybe you've indeed uncovered a bug of the profiling process, maybe the Kemper institutes some type of limiting/compression for which you've hit the threshold, don't know...if the work your doing results in data to help the Kemper folks refine their product, then you deserve a beer! But for me, well perhaps I'm too entrenched in the honeymoon period to contribute any validity to this thread, or maybe I'm just not as demanding. Don't want to derail things further so I'll check out for now and lurk for updates.

    Quote from Will_Chen


    Anyway, I have no idea what is the root cause of your issue. Maybe you've indeed uncovered a bug of the profiling process, maybe the Kemper institutes some type of limiting/compression for which you've hit the threshold, don't know...if the work your doing results in data to help the Kemper folks refine their product, then you deserve a beer! But for me, well perhaps I'm too entrenched in the honeymoon period to contribute any validity to this thread, or maybe I'm just not as demanding. Don't want to derail things further so I'll check out for now and lurk for updates.

    I don't think Kemper has anything to worry about regarding the issue of subtle distortion on the cleanest of clean profiles--the KPA is certainly clean enough for the majority of peoples needs. I don't usually get quite this geeky about testing gear, but I love this indispensable piece so much, that I thought it was a valid observation worth sharing.

    Edited once, last by miles ().

    I did not read the entire thread, but regarding the great river...that amp is not a flat preamp, the transformer imparts quite some "Sound" on the Signal...especially in the lower mids
    Especially das the great river reacts to different impedances, so what you hear may result in the difference between guitar direct and buffered Signal from the kemper

    Quote from tenderboy

    I did not read the entire thread, but regarding the great river...that amp is not a flat preamp, the transformer imparts quite some "Sound" on the Signal...especially in the lower mids
    Especially das the great river reacts to different impedances, so what you hear may result in the difference between guitar direct and buffered Signal from the kemper

    I had the exact same result using a transformerless preamp, and though you are right that the Great River is not a flat sounding pre, that had nothing to do with the fizzy distortion artifact that is evident in the sound file.
    Great River-KPA Clean Test 1

    Quote from Guitartone

    Miles


    Have you tried making a clean Profile without the using your GR pre-amp...instead go straight into the Kempers pre-amp?

    Do you mean to use no preamp--to loop the direct out to the return in and profile nothing? I haven't tried that.

    Quote from miles

    Do you mean to use no preamp--to loop the direct out to the return in and profile nothing? I haven't tried that.

    I think Lance means if you've tried to make a profile without using the preamp and plugging the mic directly in to the Kemper (like a simple sm57)

    "Music is enough for a lifetime, but a lifetime is not enough for music" Serghei Rachmaninoff


    Quote from guitarnet70

    I think Lance means if you've tried to make a profile without using the preamp and plugging the mic directly in to the Kemper (like a simple sm57)

    I have profiled some dirty amps with the standard method, but I have not profiled my Fender Twin yet, I was going to, until I analyzed some of the current clean profiles that are available, and noticed that every one I experienced had that subtle veil of distortion that I could not eliminate, so I tried to create the cleanest signal that I could to try and isolate the distortion, and sure enough, it was present in all the clean profiles that I have heard. In my test there was no mic involved, it was KPA direct out - GR DI in - GR line out - KPA return in. Maybe the direct signal itself is one that the KPA can't capture without adding the artifact, and that a totally clean amp (JC120 or a Twin) could do it without the artifact, but I haven't had the chance yet to try that, but I plan to eventually.

    Quote from miles

    I have profiled some dirty amps with the standard method, but I have not profiled my Fender Twin yet, I was going to, until I analyzed some of the current clean profiles that are available, and noticed that every one I experienced had that subtle veil of distortion that I could not eliminate, so I tried to create the cleanest signal that I could to try and isolate the distortion, and sure enough, it was present in all the clean profiles that I have heard. In my test there was no mic involved, it was KPA direct out - GR DI in - GR line out - KPA return in. Maybe the direct signal itself is one that the KPA can't capture without adding the artifact, and that a totally clean amp (JC120 or a Twin) could do it without the artifact, but I haven't had the chance yet to try that, but I plan to eventually.


    Miles,


    Would you like to try a rig I posted two weeks ago, "Mysterious 1" (actually October 11), and tell me if you find in it this distortion artefact you are talking about. Of course, there's a reason why I ask that, but I'll tell you later, when you have given your advice :) .


    Best regards.


    Jean-Jacques G.