Configuring a miniDSP 2x4 for Room Correction using a Subwoofer

Posted by: Huge on 10 February 2016

I have now installed a miniDSP 2x4, replacing a system where the sub was wired directly to the low level sub out of the amp.

In the previous configuration, room correction was applied to the music files before they were streamed to the system. Now the files are left unaltered and the room correction is applied in the analogue feed to the sub.

The New Configuration:

NAS -> ND5 XS -> Nait XS -> Main Speakers

-> miniDSP -> Sub (low level input)

Advantages

Signal to the main speakers is unaltered (it isn’t degraded by the room correction system)
It’s easier to test the performance of this system (for reasons see installation notes below)
More extreme filters can be used without degrading overall sound quality

Disadvantages

Signal to the main speakers is unaltered (so can’t be corrected if they excite a resonance)
Correction of peaks can only be applied below the range of the main speakers
Cost (£100 for miniDSP, £90 for cables*, £10 for a USB PSU, £10 for additional ferrites)

* I chose to use 2 sets Chord C-Line as they maintain fairly good bass integrity

Room Correction Audio system

Pair Chord C-Line (Nait XS Sub Out -> miniDSP in) + ferrites
MiniDSP 2x4 + MiniDSP 2.1 Advanced plugin
1 Chord C-Line (miniDSP out no. 1 -> Sub in) + ferrites (mono connection)

Test Equipment:

miniDSP UMIK-1 calibrated USB instrumentation mike (£80)
Windows Laptop computer (ca £300+, but I already had one!)
USB soundcard (in my case a USB to S/PDif converted connected to the ND5 XS)
REW (donation-ware)

1^st attempt (REW automatic filter generation)

Initially I tried using the filter set generated by REW and downloading this to the miniDSP (using a similar procedure to the successful one below), but the results weren’t that satisfactory in my system.

2^nd Attempt (manual filter generation); Method:

1 Connect the miniDSP, UMIK-1 and Soundcard to the laptop.

2 With the miniDSP set to a flat response and the sub disconnected (or just not powered!), use REW’s <Measure> system to calibrate the main speaker in room response. This is the baseline: You shouldn’t significantly drop below this at any point (creating additional ‘nulls’ is bad).

3 Reconnect the sub (or turn it back on!), ensure the miniDSP plugin to Mono or Stereo mode as appropriate for the number and layout of subs. Use REW’s <Measure> system to calibrate the total room response. Increase the level so the minimum in the sub’s range is at the same level as the majority of the response of main speakers.

4 Set the MiniDSP crossover filters to a 48dB Butterworth response and set the frequency to the -3dB point where the response in step 2 starts to fall off significantly.

5 Using the frequency response curve from 3; take the miniDSP 2.1 plugin and set the channel 1 output parametric equaliser to approximately the inverse of the LF peaks seen in the frequency response. Upload the EQ curve to the miniDSP (i.e. click the synchronise button)

6 Use REW’s <Measure> system to calibrate the new (partially corrected) total room response. It’s unlikely you’ll get it spot on first time! Adjust the parametric equaliser to reduce unwanted peaks that remain (add extra EQ components up to a maximum of 6 bands as necessary) and re-test.

7 Unplug the computer and connect the miniDSP ‘s run time power supply.

Results

Before and After images.

Before, no sub

Before, +Sub
Before, +Sub not optimised

Using Room Correction in Files, optimised

Using miniDSP 2x4, optimised

Conclusions

When compared to modifying the signal for both the mains and the sub, the resultant sound with the miniDSP is not just smoother in the bass response (due to the greater strength of the filtration that’s possible), but also gives perceptibly sharper transient performance. The difference isn’t dramatic, but it is quite noticeable and is consistent. Due to the transient response advantage, there’s no loss of ‘PRaT’.

If the limitations of this approach fit within the characteristics of your system (particularly the limitation that the main speakers LF response is sufficiently curtailed so as to not excite the main room modes) then I believe this to be a good answer for room correction and sub integration.

Posted on: 11 February 2016 by Huge

Having had time to listen to the revised system for a little while, I'm appreciating that there are some rather more subtle improvements to the sound as well.

I think these improvements may all be linked, and I think the reason for the improvement is that the miniDSP is connected to the pre-amp with much shorter cables than were used to reach the sub (1m vs 5m), thus not capacitatively loading the output of the pre-amp to the same degree. I believe this capacitative loading may have been affecting the signal in the amp going through to the main speakers (in the case of the Nait XS-2 I don't believe that the sub out is buffered, but I may be wrong). The miniDSP can now act as a buffer / line driver to get the signal to the sub.

Anyway the improvements...

Readability: It's easier to distinguish the words in passages that are sung very quietly.

Detail: I can hear detail from quiet instruments with other louder parts present, particularly the harpsichord in a baroque orchestra. For this I use a standard test piece of mine a recording of "The entry of the Queen of Sheba", that I know particularly well and can easily pick up on small changes.

Timing: I find the cleaner, more consistent system transient behavior allows me to 'lock' on to the timing better. It preserves the tense feeling in Ari Mason's "The Curse"; this tension is easily lost (or changed to a mess) as it relies on the precise relationship between "uncomfortable" harmonic transitions and syncopated rhythms.

Musical involvement: Perhaps the most important feature. I've found myself enjoying tracks that previously I thought "meh".