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4-Way Project: FIR & IIR with openDRC & nanoDIGI & more 5 months 2 weeks ago #65112

  • Hoffmann
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Hi folks,
following the freindly invitaion...

Post pics and description of your Hifi projects here. Share with us how you're using our products.

... I'll present the set-up of my 4-Way active speakersystem here. Maybe it inspires someone...

Basis of the project are old active speakers made by Backes & Mueller: BM-3 from 1984, BM-8 from 1994, Omega from 1986.
I'm reusing the slim livingroom friendly Omega cabinets, the 200mm Double-Bass chassis of the BM-3 plus the 120mm midrange and 38mm tweeter of the BM-8. The reason I chose these is that all of these chassis work with motional feedback (using inductive or capacitive sensors) and I have all the electronics going with them (describing all this would be a topic of its own). The fourth way is an AMT driven by a current drive Amp. Let's keep it at that and just say: The speakers poweramps, cabinet and chassis are good and fixed.

Why am I using miniDSP?
So far I worked with DEQX PDC-2.6p successfully and with pleasure, liked e.g. the IIR/FIR solution, but it has aged, is limited e.g. to 3-Way, IIR PEQs cannot be L or R assigned (just to both). So, the Flex series inspired me to seriously look at miniDSP and I kicked around several concepts.

Which miniDSP for ME?
The FIR taps limitation of the Flex series and the lack of ADC in the Flex Eight had me consider cascading a Flex Digital and a Flex Eight. This solution would theretically double FIR Taps, but using them effectively is hard to achieve and the two units together would greatly drain my budget.
I had read about the nanoDIGI 2x8, but without any FIR and limited IIR capability it would "just" be a digital x-over. It's EOL, but probably available on the used market... So where/how would I get max./reasonable FIR Capability to at least get an overall phase linearisation capability? A friend uses openDRC since many years which made me look at that one closely: 2inx2out with 2x 6144 FIR Taps @48kHz.
One more suggestion was to not use ASRC UPsampling of 44.1kHz in miniDSP because of its impact on sound, but upsample externally with a dedicated device.

What to do?
Well, from all the thinking an unusually "different" concept emerged:
  • External 24/192 ADC
  • ASRC down to 24/48 in miniDSP
  • FIR and some IIR DSP with openDRC
  • x-over and crossover region chassis "tuning" with nanoDIGI
  • Place two Stereo DACs into the activated speakers and feed them with SPDIF instead of analog
Here's a picture of the complete digital active 4-way system:

You see a few "optional" additions aimed at maximising digital signal quality (in a second phase):
  • Mutec MC-6 does the upsampling of all source signals to 192kHz to feed into the openDRC.
  • Mutec MC-3+ reclocks the output signal of the openDRC and feeds nanoDIGI with it
  • Mutec MC-3+ feeds Mutec MC-6 with a synchronising Clocksignal (soundimprovement is proven)
What you don't see (yet):
  • DACs are all Khadas Tone1 boards
  • Tone1 have 24.576MHz clock on board which can be "substituted" by a more stable NewClassD Neutrino
  • nanoDIGI has a (simple?) 12.288MHz clock, so we'll see...
Not to forget Software:
  • REW for the IIR x-over development and chassis response optimisation with nanoDIGI
  • Acourate2 for overall FIR linearization and room adaptation with openDRC
Don't get a wrong impression! I'm just on my way building all this up step by step and do this write up because I'm excited to have found a decent solution and roadmap!


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Last edit: by Hoffmann.
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