KEMPER STAGE - LIVE - ACTIVE OR PASSIVE DI

  • While staying with Radial, there is another option in the ProD2 (link here). It doesn't have the premium Jensen transformer, but it is still a premium product and I think it is much better value. You'll feel less guilty about kicking it around the floor - I had a JDI that I subsequently sold to a studio owner I know, as I was too precious about it to take it to gigs! Also, I suspect no-one in the world is going to notice the difference between the JDI and the ProD2 at a live gig.

    thank you very much for everything.

    your answers helped me a lot.

  • Are you absolutely sure about that Edward? I realise you know a lot more about this that I do so I am asking to be educated not to be a smart ass.


    With my limited knowledge it appears that the Kemper performs exactly like an active DI Box for the following reasons:


    1 - Impedance - the reason a DI box can drive long cable runs without losing signal is due to low impedance. The XLR outs on the Kemper measue that same output impedance as the ART DI Box I have to hand (both are 220ohm on my multi meter across two pins. By comparison the bridge pickup on my PRS DGT measures 8.6k. Am I measuring wrong?


    2 - Balanced signal - the Kemper XLR Out is a balanced XLR signal which is suitable for long cable runs.


    3 - Level - while the KPA is capable of sending up to +16db (Head/Toaster or +15db Stage) this can be reduced to - infinity by using the Output Volume. A DI Box pads the output with a fixed attenuation (usually 20 - 40db). Why is simply reducing the volume being sent from the KPA different to engaging a pad switch on the DI Box?


    I agree with your point about the 1/4" TS not being balanced but according to my multimeter the output impedance is the same as the XLR so is in fact low impedance. This implies that it should be capable of driving long cable runs although it may introduce more noise than a balanced cable.


    Alan

  • Hi Alan,

    Well those are some interesting results, and I wouldn't say my mind can't be changed :D I think we're seeing that the Kemper performs a lot of the functions of an active DI, but not all.


    1 - Impedance - ...both are 220ohm on my multi meter across two pins.... Am I measuring wrong?

    I doubt it - it sounds like it is me that is plain wrong here. I'll take that one on the chin; anything under 600 ohms would be fine. I'll get the multimeter out for my Stage and Radial DI boxes later as I'm curious how they compare. I've got an Art dpdB lurking somewhere as well.


    2 - Balanced signal - the Kemper XLR Out is a balanced XLR signal which is suitable for long cable runs.

    Sure, the XLR out is balanced, and this is one of the primary functions a DI performs, but balanced line outputs on professional gear is par for course.


    3 - Level - while the KPA is capable of sending up to +16db (Head/Toaster or +15db Stage) this can be reduced to - infinity by using the Output Volume. A DI Box pads the output with a fixed attenuation (usually 20 - 40db). Why is simply reducing the volume being sent from the KPA different to engaging a pad switch on the DI Box?


    It's this last bit that, for me at least, that means the Kemper output does not replace a DI box. The output of the Toaster/Rack is only +16dBu on the unbalanced TS outs. The balanced XLR outs max out at +22dBu. The additional 6dBu equates to twice the signal intensity (peak to trough being measured by a differential amp rather than just peak to 0V reference).

    The Stage must have an attenuator for the XLR outs so that there is no signal level change between XLR and TS (I actually rather like this approach, to prevent 'blatting' during setup - I think this was the reason cited by Kemper for doing this).


    But mic level is down in the -60dBu to -40dBu range, which equates to a few millivolts, so even with the -12dB attenuation switch on the Kemper output, you've got a lot of gain reduction needed before it becomes suitable for a mic input that could well have a minimum gain value of 20dB to consider (most live engineer workflows I've witnessed don't start with all mic inputs padded or set to Line/unity gain, for example).

    I don't think using it in this way would be a sustainable workflow, as the potential and penalties for it going wrong are high. An analogue mic pot will be log, and if you're having to use it on minimum gain it will be difficult to adjust by a small amount if you subsequently need to, plus the ends of the pot travel are where the pot is noisiest, as that's where all the crud from the wipers gets pushed to.

    I guess you could argue that any low impedance line level output with a level control could be reduced enough to feed a mic input, but there is an inherent noise floor in any system, and trimming down the output gain this much will surely take you closer to it, with the noise floor being amplified up with the audio signal at the mic input with this reduced ratio. The Stage might be easier to manage, as it has the same -12dB output pad as far , but is already lower in signal level with its max +15dBu output. With the output pad, you'll still be at up to +3dBu, which is over 1V. I'll have to have a play with this to be sure- I'm interested to know what the noise floor performance is like at minimum output. Could do with finding someone with a D-Scope at home, as I'm unable to go to the office for the foreseeable future.


    By comparison, engaging a PAD switch on a DI would result in the noise floor also being attenuated, preserving the signal to noise ratio of the Kemper output. It's possible Kemper could be working some voodoo magic that means signal to noise is better than expected even when the gain value is at its lowest, but I will reserve judgement until someone can scope it out. I'm also not sure if the Kemper output PAD is an analogue function or not. If so, it will indeed be switching in/out circuit components so that the gain range remains optimised. It might be that this is just moving the digital trim range before the DAC. If it was a -30dB or even -40dB analogue PAD function, it would serve much better as a DI output.


    Still, after all this, the functions of a passive DI that still remain are the transformer isolation that can be reassuring in certain live situations. I mentioned line isolating transformers in my first reply, so perhaps they are worth mentioning again, as one of these, with a PAD function, might be closer to what the OP actually wants than a DI box, given that the Kemper is partway there.


    There is a lot of context dependancy here, for sure; playing level, the rig levels, the output trim, the Kemper model, the input stage of the mixer. If someone can bend it to their will and is happy with the audio results and the workflow without additional hardware, then ok :thumbup:





    Ed / Audio Systems Engineer / Kemper Stage + Fender fan

  • ...The XLR outs on the Kemper measure that same output impedance as the ART DI Box I have to hand (both are 220ohm on my multi meter across two pins. By comparison the bridge pickup on my PRS DGT measures 8.6k. Am I measuring wrong?

    Thinking about this a little more, this isn't measuring impedance. Your multimeter will use a small DC voltage to measure resistance, which isn't the same as calculating/measuring the impedance of the op-amp used as the output driver. I don't know how much difference there is between the DC resistance measurement and the actual impedance - it might not be significant.
    As far as I know, you'd use a 1 kHz sine and load the output to do it properly. I'll have a think about how this can be done. The alternative is to lift the lid on the Kemper and see what op-amp device is being used, then look up the specs for that. I'll let you go first hah!

    Ed / Audio Systems Engineer / Kemper Stage + Fender fan

  • Can I ask a quick question? I use my Kemper Profiler Toaster in the studio. If that ever went down for whatever reason, how many dB do I need to boost the signal into my audio interface from one of my Stage units to exactly match what I have currently with the toaster?

    I asked this a while ago and no one replied.
    It’s also useful info to know if I take my Stage unit to a different studio to carry on working on one of our albums. I sometimes do the solos and cleans at our singers studio for instance.

  • ...how many dB do I need to boost the signal into my audio interface from one of my Stage units to exactly match what I have currently with the toaster?

    For the Balanced XLR outputs, the published specs suggest the Stage is 7dB lower than the others so that there isn’t a level difference between XLR and TS outputs. As you have access to both are you able to prove this for us? You would use a flat profile on each and a constant test tone as the source, looking at your DAW input meters.


    If you’re recording at another studio though, there are surely other factors that will change the input level, even if you take your toaster. Different A/D converters for example have different operating levels, so the same level of analogue signal lines up to different dBFS meter values if they are not using the same standard. What equipment are you using in each case so I can look up the specs?

    Ed / Audio Systems Engineer / Kemper Stage + Fender fan

  • Thanks so much for the reply.
    In my studio, it’s a MOTU 1248. In our singers it’s a Yamaha DM2000 desk (I think via light pipe to a Focusrite interface of some sort or other).
    I see you what you are saying, unless the gear is identical, it’s a can of worms.
    The info regarding the stage and toaster has been most helpful for in my studio . Thank you. ???

  • I’ll set up a test for my Stage and get my mate to run the same on his Rack. Might take a while to sort out, but I’ve got some other latency tests and bits to run so I’ll probably put it all in a new post. Watch this space.

    Ed / Audio Systems Engineer / Kemper Stage + Fender fan