DAC output through interface to monitors - good idea?

Hello, everyone!

I have a small home studio driven by a Focusrite Scarlett 18i20. An SMSL SU-9 is arriving soon and will act as a dedicated DAC to listen to music. Currently I use the Scarlett and the studio monitors with Roon for this. I’d like to use the same pair of monitors without, at first, buying a splitter.

First of all, is there any objective argument in favor of the SMSL’s conversion in comparison to the Scarlett’s in this case or is the perception mostly subjective?

Second: I’m thinking of sending the digital signal through the SMSL via USB, sending the analog output back to the Scarlett using two of its inputs as a stereo pair and directly routing it (using its software) to the monitors. With the gain knobs zero’d, will the Scarlett interfere much on the sound using this or would the best option be using a balanced splitter after the SMSL/Scarlett and before the monitors, controlling which device is sending their analog signal? I think I probably can’t trust the “bypass” capabilities of the Scarlett but would like to discuss this more.


… would work, but is probably not optimal. You would be putting the Focusrite ADC and DAC in the signal path.

I’d suggest using a digital output from the Focusrite to feed the SMSL. That should allow you to listen to music if you set the Focusrite up as passthrough, and also give you the benefit of the SMSL DACs when mixing or mastering.

Check what the Focusrite routing allows though - I’ve not used one!

Hey, Andy!

I thought about that. USB to the Scarlett and routing it via optical to the SMSL. But there are two problems with that: the monitors are connected to the Scarlett so I’d either have to disconnect or add a balanced switch/splitter to the chain; I believe the Scarlett won’t be able to read and/or passthrough MQA.

I didn’t know the SMSL was an MQA renderer. That adds an additional level of potential complexity, but if the Focusrite will do a bit-perfect passthrough (ie you’d have to be sure the internal routing was not doing any resampling or level changes) I think it should work. You’d have to try it…

I don’t think you’d need to swap the speakers between SMSL and Focusrite - I’m suggesting you always use the digital output from the Focusrite, not the Focusrite ‘mains’.

I can only see that being an issue if you’re also using some of the other analogue outs for surround… are you?

Are you intending to drive both routes from your DAW?

What monitors are you using?

Oh, yeah, you’re right - since I’m only using stereo I can just connect the monitors to the SMSL and output everything through it using its DAC instead of the Scarlett’s. One less thing to worry but I lose MQA.

I can route anything before it hits the DAW using the Scarlett’s software. Supposedly it’s done at the hardware level. For example send outputs 1/2 (L/R) coming from USB directly to optical 1/2 to the SMSL.

This is also what I’d use in the USB>SMSL>Scarlett>Monitors chain I’d suggested before, and I’d still get MQA: connect the SMSL outputs to inputs, say, 7/8 of the Scarlett as a stereo pair, then route it at the hardware level to mains 1/2 (monitors). Like I said, considering I’m not adjusting the gain knobs I’m theoretically passing through an analog signal. I just don’t know if I can trust the Scarlett’s interference.

I’m using a pair of JBL LSR305s.

I’m not sure you would be. I think all routing is done in the digital domain, ie all inputs pass through the ADCs, all outputs pass through the DACs.

I think you’re going to have to try it. It might come down to whether you prefer decoded MQA, or the better DAC in the SMSL.

I think this means digital hardware in the Focusrite box, as opposed to passing a digitised analogue input to your DAW computer, processing it there, passing it back to the Focusrite, converting to analogue, and presenting it on an output - which can impose significant latency when tracking.

Edit - it occurs to me that you could also connect the SMSL to your computer by usb at the same time as S/PDIF or Toslink from the Focusrite. Use the SMSL as a source switch…

Yeah, I agree.

I think so, but from what I’ve read in the manual and experienced myself, when using this direct routing method with their software, inputs probably do pass through the DAC/ADC or at least through some kind of digital device (since you control the routing using a software) but it’s ultra low latency, one could say almost real time.

I never had a latency issue with a (now) fifteen year old M-Audio box - but some people are very sensitive to it!

Let us know when the SMSL arrives… :wink:

A bit late to the party but just came across this via google search.

I have a first gen 18i20 and have dealt with a similar issue. I use it as my DAW interface but also route my turntable, guitar, chromecast, you name it, through for playback on my monitors.

You can in fact direct monitor inputs without it going through the Scarlett’s converters. But if you’re monitoring through your DAW software, then it’s converted going in and out.

That said, I can’t say there’s an appreciable downside to the audio quality. Roon runs on that machine and uses the Scarlett DAC, so in that sense it’s not an issue. For anything else routed through it for direct monitoring, can’t say I hear any drawbacks. You should try it.

I’m in a similar position now myself btw (what brought me to this thread). The first gen’s DAC is absolutely obliterated by the DACs in the $200 to $300 range I have.

I’d love to just throw in an SMSL C200 or something and use it like you’re describing. I just assume it’s not suited for ultra low latency scenarios. So off to 18i20 replacement shopping I go.

@Hazen, thanks for sharing your experience and for replying so I could come back to update you all.

I would love to be proven wrong on this but I think even direct monitoring via the Scarlett’s software does route the signal through its converters. The manual describes this procedure as “ultra-low latency monitoring” so some sort of digital processing is happening. But I know what you mean: using the Scarlett’s software, like I’ve described on my previous replies, adds less latency and is more pratical than routing stuff using a DAW for example. One could easily say zero latency since it’s really, REALLY ultra-low.

For me, the SMSL arrived and I’m using it connected to the 18i20 via coaxial RCA (the only way to get PCM up to 192 kHz coming from the Scarlett since optical stops at 96 kHz) and also connected to the computer via USB, to get advantage of MQA as it only supports it through USB.

Yes, I have to switch inputs but this way I can get the best of both worlds and also use the SMSL as “master DAC” for what I’m monitoring/listening through the Scarlett. I have not tested this enough - and I should be getting some because of the additional ADC stage I’m assigning to the Scarlett - but I haven’t noticed any relevant latency for the studio stuff. One other advantage is that using the SMSL after the Scarlett (via coaxial) instead of before (via XLR) is that it would use two valuable mono inputs for recording (L and R).

I’ll update this further when I test the studio signal chain more but for now I’m really satisfied with what the SMSL offers via USB. I hadn’t tried MQA before and, in comparison to SOME tracks with same bit depth and sample rate on Qobuz FLAC, I was positively surprised. The 2021 remaster of CSNY’s Déjà Vu in MQA sounds fantastic to me: wider sound stage and more clarity on the lead vocals overall.

I had to do a sanity check to be sure, but yes, the direct monitoring routes the input signal directly to the amp and to the outputs without any conversion. For my purposes with most of these other sources - where the point is to simply hear and not feed into the DAW, for the most part - it’s basically acting as a mixer.

You are correct about the manual saying “ultra low latency” though. They say elsewhere, “The Scarlett 18i20, in conjunction with Focusrite Control, allows “zero latency monitoring”…You can route your input signals directly to the Scarlett 18i20’s headphone outputs.” (or any other output for that matter).

As for your SMSL, I’m curious what kind of latency you’ll get with it.

Still pretty sure the ADC/DAC are in the signal path…

The user guide describes it as a software mixer.

If it’s described elsewhere as 'zero latency ’ I suspect that’s the marketing department getting carried away…

Edit - I’ve just found it in the user guide… I think the " " are important - marketing translation for ‘low enough that we don’t think you’ll notice’.

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Glad it’s working for you! :slight_smile: