New setup, mono sub & potenatial crosstalk

[RandomEar]

I'm currently planning to upgrade my 2.1 setup. I've done some research and am pretty sure what to get, but I could use some input on how to connect the sub. It has a line level mono input.

Current setup:

• Rotel RA-12 (fed via S/PDIF from PC)
• ELAC BS 243 bookshelves
• ELAC SUB 2040 ESP (fed from RA-12 pre-out using self-build RCA stereo to mono adapter)

Planned upgrade:

• SMSL SU-9n (fed via USB from PC, with S/PDIF as fallback in case of a ground loop)
• Audiophonics MPA-S250NC XLR amp (fed from SU-9n XLR output)
• Same speakers
• Same sub (fed from SU-9n RCA output, using RCA stereo to mono adapter (?))

I'm mainly concerned about crosstalk due to the RCA stero to mono adapter. It's build according to this diagram, using two 4.7 kOhm resistors. I'm also not really sure this is the best way to generate the subwoofer mono downmix. I originally had it connected to just one stereo channel for years, but it just 'felt wrong' to leave it that way. Recently added the downmix adapter and it seems to work well, as far as I can tell. Do you have any experience with such adapters?

I also thought about doing the stereo to mono downmixing using a miniDSP 2x4, or some ADAU1701-based board (e.g. Wondom APM2). I would expect that this excludes the possibility of crosstalk to a reasonable degree and would allow me to properly configure the downmix. But it also adds an additional A/D->D/A conversion step to the LFE channel. I'm open for ideas on how to (better) implement this route.

To state the obvious: Yes, the sub has speaker level inputs. I don't want to use those because of excessive cable routing. It also has a build-in low-pass and a single freq room-eq, which I currently use. But they could just as well be replaced by DSP filters, in case that improves other things.


I've looked at the following threads (and a couple more I don't remember):

RCA Stereo to Mono converter where to buy ?

Was looking at merging Left,Right channels into Mono. I don't really want to build the adapter. Why are these so rare to buy ? Was looking at Amazon & Aliexpress but can't find much. for line out it would be this and with these adapters I could just ignore the soldering part. 'left + 1kohm'...

www.audiosciencereview.com

Resolved: Stereo to Mono Summing For Subwoofer

So I'm (mostly) retiring my Denon HEOS Link for a Topping E30. Unfortunately doing so has gotten me into a bit of a pickle. Currently, the Link has a Sub/LFE output that is summed in the digital domain. That LFE output is pumped into my single channel DSPeaker antimode and then finally to my...

www.audiosciencereview.com

Adding a subwoofer to a 2 channel DAC without adding an external crossover

I'm trying to add my subwoofer to my desk setup. I have an RME ADI-2 DAC. My subwoofer has only 1 XLR input available for use. Most hifi dacs/preamps only have stereo outputs. Only things like minidsp and some more expensive exotic units actually have dsp and multiple outputs. It's very rare to...

www.audiosciencereview.com

Those were helpful to build the adapter, but didn't really answer my additional question(s).

 

[RandomEar]

I got the SU-9n and an Audiophonics MPA-S125NC XLR. Summing up my experience for others who might be looking into a similar setup:

• I did not notice any audible crosstalk using my homemade stereo-mono adapter connected to the SU-9n RCA out and the amp connected via XLR. I tested this by turning the volume up to the highest listening level acceptable to me, setting the channel balance to 100% left in Windows and pulling out the left XLR connector on the SU-9n. Unless the SU-9n mutes one channel when detecting digital silence, this should result in only the crosstalk being audible on the right speaker, if I'm not mistaken. Could hear nothing at all. There might be something measurable, but what counts for me is, if I can hear it. Also, with the SU-9n turned up even higher (e.g. max level), it might become audible - but at that point, my ears would bleed due to the excessive SPL in my setup. Therefore, not of concern to me.
• The SU-9n had quite noticeable speaker pop when turning on/off, muting or changing filters. You could certainly avoid the on/off pop by turning on the amp last and turning if off first, but the pop when muting or changing filters made this a no-go for me. Measured between 1 and 2 mV of DC offset at the XLR output, which didn't seem much to me. But it looks like it was enough to cause the problem.
• The MPA-S125NC consumed around 8 W of power when in standby (switched off using the front button, but still connected to the wall outlet). That's absurdly high in my book - anything over 0.5 W isn't accebtable anymore nowadays.
• The MPA-S125NC had a noticeable hum when on or in standby. A spectrum app on my phone showed a small peak at 50 Hz and a triplet at approx. 540, 640 and 700 Hz. This was audible from ~2 m in a very quiet environment (at night) and still around 0.5 m during daytime. With the amp being situated close to me in my setup, this was another no-go for me.
• Speaker hiss was not a problem. Extremely low levels of hiss from the tweeter were audible up to 15, maybe 20 cm away. Not a concern, even in close range monitor setups.
• Sound quality was without issues.

Overall, the RCA stereo to mono adapter worked better than expected. The SU-9n may have been an outlier and went back and the MPA-S125NC had multiple issues and went back as well. Seeing the excessive standby consumption, I would not recommend any of the Audiophonics NCore amps. I'll probably get another "fresh" SU-9n at some point to see if a different unit has similar pop-issues. Also looking for a different NCore implementation, as I was very happy with the overall sound.