RME Fireface 400 Kemper SPDIF as DI for VST Sims / Impedance

  • Recently bought a used Fireface 400 due to RME's reputation for rock solid driver stability and low latency. Just as anticipated, the FF400 is indeed rock solid and very low RTL in Windows 10 using Cubase 9 Pro.


    I use my Kemper non-powered rack for just about everything guitar wise but have been experimenting with some NEURAL DSP stuff, over the past few weeks - quite good high gain simulations for sure.


    For connectivity to my DAW, I only use the SPDIF in and out of Kemper, no other I/O is being used or needed. I play through Dynaudio BM6A MKII monitors which are fed by the RME fireface.


    So my question for the readers here has to due with IMPEDANCE and DI SIGNAL and DAW interfaces in general. For some wacky reason, RME is using a 470k ohm INST input (either IN3 or IN4 on front) when set to INST via the RME software. This really SHOULD be 1M Ohm, it is baffling RME did 470k there. The Kmper of course has a 1M ohm impedance on the front input jack.


    So, with the RME instrument input being as it is, it just does not sound right and clips way too easily when hitting it with EMG. So I was going to buy a COUNTRYMAN 85 or even thought about getting a Grace m101 but then it occured to me to just use the Kemper input, and feed the VST AMP sim with the SPDIF out of the Kemper.


    How many of you are using the Kemper in this way, as a "DI" to feed a VST amp sim inside of your DAW vs. using a separate DI box?

  • Many of us use the DI out alongside the Main, Monitor (or otherwise) outs whilst recording; it's sort of the de-facto standard in case one wants to or might need to reamp later on. This applies to both the analogue and digital means of accomplishing this.


    So, nothing unusual about your plan, really, other than the fact that you won't be recording the KPA-processed signal as well / at the same time.

  • Do you know the spec of that DI signal coming out of the SPDIF and how the VST plugin would "see" that signal?


    Would be be completely unnecessary to even consider a DI BOX and using the RME INST input since the Kemper input and SPDIF out is pretty much solving the problem with the crappy FF400 INST input?

  • So, with the RME instrument input being as it is, it just does not sound right and clips way too easily when hitting it with EMG. So I was going to buy a COUNTRYMAN 85 or even thought about getting a Grace m101 but then it occured to me to just use the Kemper input, and feed the VST AMP sim with the SPDIF out of the Kemper.


    How many of you are using the Kemper in this way, as a "DI" to feed a VST amp sim inside of your DAW vs. using a separate DI box?

    I do, when tracking for a song I record the DI track and the Kemper track, or other times I just want to play thru an ampsim, I'll use the Kemper out. (all analog outs, don't care for SP/DIF) I have had the Countryman and the Boss DI, while they are ok, the Kemper DI sounds just as good or better. Also, If you're still having issues with interface input clipping and your input doesn't have a pad feature, then the solution you could try is to lower your pickup height.

  • Thanks, Lobsta. As it stands now I will continue use the front Kemper 1Mohm input and the SPDIF out "Git" to play through VST sims. I have been reading more and more about IMPEDANCE and how many interface Hi Z inputs are not really Hi Z at all. A true Hi Z input should be 1M Ohm or greater, not the 470k ohm that RME uses. The FF400 is killer, that is just the only flaw it has - sh*tty INST input impedance rating. I think I've gotten around it by using the Kemper as described.


    What is the ideal DI SIGNAL level supposed to be? As close to 0dB as possible?

  • Yah, I think I will hold off on any sort of DI box purchase and just use the KEMPER front input for the 1M ohm jack there and use the SPDIF out signal in Cubase to feed VST's and keep DI's for later use.


    Does anyone know what that SPDIF signal is, in terms of impedance or level? Just curious.

  • impedance isn’t relevant for SPDIF as you are working in the digital domain. Impedance is only an issue for analog signals.


    For recording guitar with an analog signal you shouldn’t need a 1meg ohm input either. I believe the generally accepted convention is that you need the input impedance to be at least 10 times the output. Most guitar pickups are less than 20k so 470k ohm is plenty for a guitar signal.

  • For recording guitar with an analog signal you shouldn’t need a 1meg ohm input either. I believe the generally accepted convention is that you need the input impedance to be at least 10 times the output. Most guitar pickups are less than 20k so 470k ohm is plenty for a guitar signal.

    That might be so, but I have had a multitude of interfaces over the years. I Mohm always sounded better for plugging guitar in direct. Less choked, more live sounding.

  • Thanks for the clarification on SPDIF vs ANALOG and levels.


    The RME has a setting for SPDIF of CONSUMER or PROFESSIONAL. The professional has a higher mv output vs the consumer. What is the Kemper expecting to see/what is it "rated" for in terms of SPDIF INPUT and OUTPUT milivolt levels?