Why is the M Scaler a separate box instead of being built into the DAC?
Short answer
Watts's stated reason is electrical noise: the scaling FPGA draws large, signal-dependent current, and he has said he has not found a way to put it inside a DAC without hurting sound quality, so it is a separate, isolated unit Head-Fi, May 2017Head-Fi, Jul 2018Head-Fi, Mar 2023. Quartet also has its own separate power supply Hifi Pig TV 1:28article, Oct 2026. In an October 2026 interview he called one chassis a "nightmare" but did not rule it out article, Oct 2026article, Oct 2026.
Practical answer. Rob Watts's reason for a separate M Scaler is electrical noise. The scaling FPGA pulls a large current that changes with the music, and he has said this noise reaching the DAC's ground degrades the sound. His fix is a separate, isolated box. He gave this reason in statements dated from 2017 to 2026 Head-Fi, Jan 2017Head-Fi, May 2017Head-Fi, Mar 2023AVstore 1:38article, Oct 2026.
What he says the problem is
Blu Mk 2 and DAVE (2017). Writing on January 7, 2017, after the Blu Mk 2 launch, he said DAVE's depth perception depends on resolving small signals with no changes in amplitude, and that a chip next to the pulse array elements "injecting 10A peak of weakly signal correlated current would destroy that ability" Head-Fi, Jan 2017. On May 8, 2017 he said "the ground current from the FPGA on the M scaler is too signal dependent and large at 10 A" and that he did not know how to guarantee complete isolation "except for putting the M scaler in a separate unit". He added that the second prototype's galvanic isolation and the production unit's improved RF filtering each made an audible difference Head-Fi, May 2017.
In an audio clip published in October 2017 he gave the figures as about ten amps of ground current when processing and two when idle, and said that putting this in DAVE would truncate soundstage depth (check the recording) Hifistatement 0.
Hugo M Scaler (2018 to 2023). In a July 2018 post (date estimated from the post ID) he called the FPGA's RF noise "a nightmare" with "12A of correlated current with large amounts of 2GHz noise" Head-Fi, Jul 2018. On March 2, 2023 he wrote: "the noise that the FPGA generates must be kept away from the DAC part", adding that if the Hugo M Scaler's 12A were in the DAC ground plane "it would be a disaster sonically" Head-Fi, Mar 2023. In an interview published in August 2022 (recording date not stated) he said filtering that noise inside a DAC product is "really, really difficult" and that there will always be a benefit in an external scaler AVstore 29:50.
Quartet (2024 onward). At Munich in May 2024 he said the separation is needed to reduce the processing noise and isolate the unit from the rest of the system and from the DAC AVstore 1:38. In July 2024 he described Quartet as a two-box solution, a power supply plus the unit, because the FPGAs need "seventy five amps" (check the recording) Hifi Pig TV 1:28. On May 31, 2024, speaking of M scalers generally, he said "you need physical separation" and that common power supplies won't give it Head-Fi, May 2024. His June 2024 Hifistatement interview (a German translation of his answers) describes a multi-hundred-component RF filter in the separate power supply and in Quartet's input, to isolate Quartet from the rest of the system article, Jun 2024.
Could it ever be built in?
His answers vary by date and product, and none gives a date Head-Fi, May 2017Head-Fi, Oct 2017Head-Fi, Nov 2019Head-Fi, Apr 2021Head-Fi, Jan 2023article, Oct 2026:
- May 2017: perhaps in ten years, with FPGAs drawing a tenth of today's core current, but "can you wait ten years" Head-Fi, May 2017.
- October 2017: an economic portable M scaler was "many years away from that - perhaps decades" Head-Fi, Oct 2017.
- November 2019: a Hugo 2 with an integrated M scaler was still a hope, but "many years away from that, possibly decades" Head-Fi, Nov 2019.
- April 2021: he suspected "with desktop usuage we will always have a two box solution" but thought a portable M scaler/DAC might be possible given a decade and improved FPGAs Head-Fi, Apr 2021.
- January 2023: he was "even more pessimistic" about a portable solution at Hugo 2 prices by 2032 Head-Fi, Jan 2023.
- October 2026: asked about Quartet and a DAC in one chassis, he said "I think that would be a nightmare" and "I never say no" article, Oct 2026. He added that long term there might be a solution but he did not know how article, Oct 2026.
What it means for your setup
In December 2018 he said that with the Hugo M Scaler, where he improved the BNC isolation, clip-on ferrites will degrade the sound, whereas they work with the Blu 2, but that solid-core ferrites "do offer a small advantage" Head-Fi, Dec 2018. In August 2020 he said the isolation "is still not 100% effective" Head-Fi, Aug 2020. In November 2019 he said running the M scaler from a battery power bank almost completely isolates it from the DAC Head-Fi, Nov 2019. These statements concern the Hugo M Scaler and Blu 2, not Quartet.
What he has not said: in these items he does not say an integrated unit is impossible, does not give a date, and gives no cost or marketing reason for the two-box design article, Oct 2026Head-Fi, Apr 2021. When a 2022 interviewer suggested a separate unit also lets you use other DACs, he answered only "Yes" AVstore 30:57. His noise figures are for Blu 2, Hugo M Scaler and Quartet only.
Quotes
“the noise that the FPGA generates must be kept away from the DAC part - hence the isolation, and two units”2023-03-02 Head-Fi, Mar 2023
“I don't know how to solve these problems and guarantee complete isolation except for putting the M scaler in a separate unit”2017-05-08 Head-Fi, May 2017
“I have not been able to figure out how to do it without it compromising sound quality”2018-07-23 Head-Fi, Jul 2018
“I never say no, but the difficulty of preventing the noise of all the processing from upsetting the DAC was an enormous challenge”2026-10-17 article, Oct 2026
Dated positions
| When | Products | Position |
|---|---|---|
| 2017-01 to 2017-10 | Blu Mk 2, DAVE | The Blu Mk 2's FPGA draws large, signal-dependent ground current that he said would harm DAVE's depth if inside it, so the scaler went in a separate, galvanically isolated unit. Head-Fi, Jan 2017Head-Fi, May 2017Hifistatement 0 |
| 2018-07 to 2023-03 | Hugo M Scaler | He described 12A of FPGA noise current that must be kept off the DAC ground plane, hence isolation and two units, and said filtering it inside a DAC product is very difficult. Head-Fi, Jul 2018AVstore 29:50Head-Fi, Mar 2023 |
| 2024-05 to 2024-07 | Quartet | Quartet is a two-box design with a separate power supply and RF filtering, to isolate its processing noise from the DAC; in May 2024 he said, of M scalers generally, that common power supplies would not give the needed separation. AVstore 1:38Hifi Pig TV 1:28Head-Fi, May 2024article, Jun 2024 |
| 2017-05 to 2026-10 | Blu Mk 2, Hugo M Scaler, Hugo 2, Quartet | On integration he moved between 'ten years', 'perhaps decades', 'always two boxes for desktop' and, in 2026, 'a nightmare' but 'I never say no', giving no date. Head-Fi, May 2017Head-Fi, Oct 2017Head-Fi, Nov 2019Head-Fi, Apr 2021Head-Fi, Jan 2023article, Oct 2026article, Oct 2026 |
Caveats and open points
- The Hifistatement Quartet interview is a German translation; its content is paraphrased, not quoted.
- Head-Fi, Jul 2018's date is estimated from its post ID (between 2018-07-22 and 2018-07-24).
- His statements on isolation concern the Blu 2, Hugo M Scaler and Quartet; he has not said the same reasoning applies to other products.
- The Soundnews article is a published interview; wording is as printed.
Topics: Why upscale, and why a separate scaler, FPGA implementation and processing hardware, RF noise: mechanisms and audibility