HELP: Analog output specifications?

  • Team Kemper,

    Happy Kemper user here, and I am investigating integrating my Kemper with other systems. I find the Kemper specs confusing and inadequate in my efforts.

    1) The Profiler specs say the analog output level is +16 dBu (or +15 dBu, depending on the model out output).  

    • Can somebody knowledgeable explain what this means?
    • In other words, when I set the master volume (and all other upstream volumes) to [0 dB], what is the output level? Is it +16 dBu? Is it +4 dBu (normal line level) but the max allowable (without clipping) is +16 dBu? What is the [0 dB] on the volume knob referenced to?!?!

    2) What is the output impedance for monitor and main outputs?

    3) Why doesn't Kemper give us good specs here?

  • I’m afraid I ‘m not an expert on this but there are plenty of audio engineers in the community so hopefully someone will give the full technical answer.

    However, in simple terms the Kemper can easily integrate with pretty much any other pro or consumer level gear.the output is signal is pretty strong so rarely needs to be run hard to supply sufficient level downstream.

    It is a +4db pro line level unit. The +16db figure relates to the maximum output gain not the measurement standard.

    The 0dB figure you mention is 0dBfs (full scale) which is the maximum internal digital level it isn’t directly comparable with the dBu analog output.

    Output impedance is low enough to successfully integrate with any other equipment.

  • It is a +4db pro line level unit. The +16db figure relates to the maximum output gain not the measurement standard. The 0dB figure you mention is 0dBfs (full scale) which is the maximum internal digital level it isn’t directly comparable with the dBu analog output.

    Output impedance is low enough to successfully integrate with any other equipment.

    If this is true, I wish the specs would say this:

    "Output: +4 dBu; Headroom: +16 dBu."

    Or in the very least, say in the manual that the [0 dB] setting on master volume is 0 dBFS normalized to +4 dBu; however, this may not be true. From what I understand, Kemper (and every other designer) is going to normalize dBFS to wherever they want and they probably are taking into consideration where clipping occurs.

    It's a good guess to say that [0 dB] = +4 dBu, but it's just a guess.

    Expressing my frustration here:

    Based on the amount of confusion between dBV, dBu, dB, line level, instrument level, unbalanced, balanced, input and output impedances etc., the "stupification" of the consumers starts with dumbing down specifications and user manuals. I don't understand the rationale of dumbing down specs, unless it is to obfuscate real details, or to hide a lack of rigorous electronics performance characterization.

  • I completely agree. Everything is awful nowadays.

    I like the specs in HiFi equipment, because they know their market is serious about measurements, even when they dont make an audible difference (like the crazy frequency range going from 5 Hz to 55,000 Hz, crazy good for dogs and bats, but useless for human ears). Its true that your average musician dont give a thing for THD, SPL, dBV or In/Out impedance, but I would be thankful to have all that data about the equipment I am purchasing.

  • Just this part of a sentence from your original post tells me that more detailed specs wouldn't help you.

    That’s a rude, disrespectful, and incorrect statement. Were you ever taught “if you don’t have anything nice to say don’t say anything at all?” Your knowledge is welcome, should you be willing to share it. By the sound of it, you know something I (or we) don’t know, so enlighten us.

    I’m assuming you’re making this statement because “upstream” isn’t a technician term. Although this is true, it is still relevant. dB must be referenced to something, and when Kemper sets dB = 0, they are setting a normalized dB to zero that was previously relative to a something (be it 1V, 0.775V, 1 mW, 20 uPa, etc). dB =0 by itself means nothing, and the fact that there is multiple gain stages in the Kemper makes it even more of an arbitrary number.

    When your master volume is set to 0 dB and your input voltage changes, like your guitar turned down to one, your output voltage will change, as evidenced by a lower voltage to your speaker. What does 0 dB mean given a change in “upstream voltage?” At 0 dB on master volume, is the voltage held constant on the outputs? If so, how does a speaker get quieter when you turn down York guitar? If not, does 0 dB mean anything at all?

    My whole point here is 0 dB is really arbitrary on its own, but I may be wrong. I’m happy to learn more from somebody who is willing to share some knowledge.

  • That’s a rude, disrespectful, and incorrect statement.

    My statement was neither rude, nor disrespectful or incorrect. It wasn't even an absolute statement but a relative observation from my standpoint.

    And this leads us to your problem ... you are confused by things that are meant to be relative as opposed to absolute. DeciBels aren't an absolute unit in itself (like meters, kilograms, liters, ...) but a relative describing the ratio between 2 absolute values.

    If (due to the design of a circuit or digital algorithm) an output value can't ever be higher than the input value then the ratio can't ever be bigger than 0dB. It can be smaller than 0dB though because you can turn the output level down, relative to the input level (negative ratio). That's why you have the option of negative deciBels.

    When you read the Technical Specifications of the Kemper Profiler closely, you will notice 2 things:

    1. They state "maximum output level"

    2. The unit for this maximum output level isn't dB but dBu which isn't the same.

    dBu (deciBel unloaded) is closer to an absolute value since it's the ratio based on a specific reference level. +4dBu equals 1.228 Volts RMS, if an audio output is rated as +16dBu maximum, then it has 12dB headroom over professional audio reference level. You can read the details up on the interwebz if you think you need to. You don't have to unless you're looking into a career as an audio engineer.

    Just one more word on your initial post:

    The fact that you asked for output impedance is yet another indication that you don't actually know (or should I say 'didn't know'?) what all these numbers mean. If we're talking about maximum output voltage, then the output impedance is irrelevant because the definition of dBu is already based on 600 Ohms impedance.

    The only facts that really matter are that the Profiler's (Head, Rack) maximum +16dBu on the TS output gives a headroom of 12dB over the nominal pro audio reference level of +4dBu. The XLR Main Outputs (maximum +22dBu) have even higher headroom of 18dB over the nominal pro audio reference level.

  • Thank you, Lighbox. I appreciate your response. And yes, if I knew everything, I wouldn't be asking questions in the first place. That's the point of forums: get opinions, get answers, and discuss. Asking a question or having a misunderstanding doesn't mean there isn't value in knowing the answer, or in teaching the answers, so thank you.

    I did not know dBu was measured for a 600 Ohm load, but glad I now know. Happy to learn.

    And yes, you reiterated what I said in my second post: When they say "maximum output +16 dBu" they are referring to "headroom," which to me is a more clear term to me. After all, Eurorack signals are often +15 dBu or higher, so, I prefer to ask a question than make an assumption.

    Follow on question:

    Since you reiterated dB being unitles, what is the master output [0 dB] relative to? 0 dB corresponds to +4 dBu? Is zero dB relative to the input voltage then normalized at 0 dB? Is it referenced to another unnamed voltage or to one volt and then normalized to 0? Do the specs or manual say it somewhere or is this another "assume" it's relative to the input or referencing +4 dBu?

  • what is the master output [0 dB] relative to?

    0dB on the Master Output setting means that the output of this "gain stage" is unchanged. Whatever signal goes into this "gain knob" goes out exactly the same. The purpose of the Master Output knob is to attenuate the incoming signal by a given ratio. -3dB equals 1/2, -6dB equals 1/4, -9dB equals 1/8, -12dB equals 1/16, -15dB equals 1/32, ...

    There is no "reference" and no "normalize" or anything like that. It's just the ratio, nothing else.

    There's nothing to be said in the Profiler's manual about that.