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Best way to connect "Pro" interface to "Consumer" sub


Oct 7, 2022
I have Kali IN-8 and a DIY subwoofer with RCA inputs only. My only source is PC, and I do DSP on it, so it needs to output 3 or 4 channels (a pair for the speakers, and 1 or 2 for sub). From using gear I had on hand (eg Topping DX3 Pro+) I realised I need balanced connections to the speakers to stop interference from the PC monitor, and also there were ground loops if I connected straight to the PC's outputs. Something like MiniDSP Flex would have been great, but I didn't want to spend that much money. 2x Topping D10 would have worked, but I ideally wanted a volume knob.

So I decided to purchase a PreSonus Studio 68c for a good price during black Friday sales, and also got cables to suit (TRS for speakers, TS to RCA for sub). I unfortunately discovered during my initial testing that its "Main" volume knob is for the main outputs only (Line 1&2), and not the extra outputs (Line 3&4), and so those are "constant full power" at +18 dBu = 6.15 Vrms. Unfortunately my sub can only handle up to 2.3V stereo or 4.7V mono, which means the interface could overload the subs input. So I'm wondering how to proceed?

Using stereo connection;
1) The sub has a volume knob which could protect the power amp portion of the plate amp and the driver, but I presume it still wouldn't be happy receiving ~6v on its ~2v input circuitry. So this doesn't sound like a solution.
2) I am using VoiceMeeter Banana and EqualiserAPO. I could set the channels for the sub to about -9dB in either of those and hope for the best, but I feel like if something went wrong (eg EqAPO driver drops out, or accidentally crank main vol slider in VMB to +9dB), then it's still possible to send 0dBFS to the interface, and thus it would put out >6v. This setup doesn't limit the interface at all.
3) Perhaps I need to run the sub off the main outs, which have the volume knob (actually it is an attenuation knob from 0 to -80dB)? By setting the knob to about -9dB, it will only put out max ~2.2Vrms to the sub. The speakers will then be on Line 3&4 and thus "exposed" to a potential +18 dBu blast if something goes wrong on the software side, but that's not as troubling as 6v signal into a 2v input.

4) But maybe doing stereo>mono in EqAPO and connecting the sub via mono (1 TS to RCA cable only), then its putting ~6V into a ~4V hole. Maybe that's safer again?

Is there anything else I should be considering (besides buying more new gear :p)?
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