Can I stack TT2 and an M Scaler, and how hot is too hot?
Short answer
Yes. In April 2022 Watts wrote that you can stack TT2 and the Hugo M Scaler "without any significant thermal or SQ problems, and certainly not any longevity problems", adding that with Wave or other solid BNC cables he would keep them apart for sound quality Head-Fi, Apr 2022. The M Scaler's thermal trip is set at a 65 deg C case temperature, to protect hands Head-Fi, May 2019. TT2's thermal trip is a risk mainly when driving low-impedance loads from its balanced outputs Head-Fi, Apr 2022Head-Fi, Mar 2025.
The practical answer
In April 2022, asked about heat and longevity, he wrote "You can stack TT2/M scaler without any significant thermal or SQ problems, and certainly not any longevity problems" Head-Fi, Apr 2022. The exception he gave is cabling: with Wave cables "(or other solid not clip-on ferrite BNC cables) then you would be better off keeping them separate from a SQ point of view" Head-Fi, Apr 2022. In another thread the same day he wrote "If TT2 thermally shuts down then you need to separate them - but if it's just running a bit warmer then I wouldn't worry about it"; the question that reply answers is not in the archive Head-Fi, Apr 2022.
TT2 and M Scaler stacked
In August 2018 he wrote "They were designed to be stacked together - it gets as warm as a Dave in operation"; the post does not name which units "they" and "it" are Head-Fi, Aug 2018.
Cables and spacing. In a follow-up April 2022 post he said "the Wave cables effectively isolate the two units - you don't want to spoil it by capacitive coupling when stacking" Head-Fi, Apr 2022. He added that one end of the cable touching or getting close to another end is something he knows "does damage SQ" Head-Fi, Apr 2022. Asked how far apart, he said "Leave a good gap - but I have never actually AB tested how much of a gap you need, I just keep them as far apart as the cables allow" Head-Fi, Apr 2022.
Sound. Traveling with both, he sometimes stacked them and sometimes did not, and in December 2023 said "I can't say I ever noticed a difference in SQ" Head-Fi, Dec 2023.
How warm TT2 should get
In that April 2022 follow-up, asked about TT2 running warm when left on all the time, he said "The TT2 will withstand a lot more heat and still be long term reliable, so I wouldn't worry about it running warm continuously" Head-Fi, Apr 2022. In July 2020 he said keeping it on "actually helps reliability, if you use it every day" Head-Fi, Jul 2020.
Balanced output. The TT2 heat problem he names is the balanced (XLR) output into low impedances. In 2019 he said the RCA and headphone outputs are fine, while balanced quadruples internal power loss and can trip "when driving 8 ohms or less" Head-Fi, Aug 2019. In 2020 he said TT2 restarts automatically once it cools after a trip Head-Fi, Nov 2020. In January 2021 he said his advice "has not changed": balanced trips into 8 ohms or less, and 16 ohms or greater is fine Head-Fi, Jan 2021. Other posts put the line at 10 ohms (March 2021) Head-Fi, Mar 2021, under 16 ohms if run loud (February 2022) Head-Fi, Feb 2022, and again at 8 ohms or less (April 2022) Head-Fi, Apr 2022. The most recent post on this in the archive, from 2025 or early 2026, says balanced into 8 ohm speakers "will eventually thermally trip" if you use more than 7W Head-Fi, Mar 2025.
How hot the M Scaler gets
In January 2019 he gave its power as 12W and said "in a stacked situation, the die is only 45 deg C", a temperature he said would not significantly affect lifetime Head-Fi, Jan 2019. In May 2019 he said the FPGA "is rated at 85 deg C - in that the timing tools are such that worst case it will still function perfectly well" Head-Fi, May 2019.
The thermal trip. It is set so the case will not burn anyone, "so that is 65 deg C", which he said would take ambient temperatures of 55 deg C Head-Fi, May 2019. In 2020: "The trip is actually to protect your hands" Head-Fi, Jun 2020. In June 2024 he said a trip turns the LEDs red and mutes it, "But the unit would need to be hot to touch for this to happen"; if hot, ventilate it and "Don't site it under or over something hot"; if it trips while cool, it needs service Head-Fi, Jun 2024.
In a bag. A talk at the Nintronics open day (November 2018, uploaded December 2018) has it getting "very hot. Probably about 45 degrees C" in a bag on battery (check the recording) Pursuit Perfect System 18:35. In 2019 it "gets warm but never hot" Head-Fi, May 2019; in 2020 it "got hot but nowhere near the trip temperature" Head-Fi, Jun 2020; in 2023 and 2024 he said the trip had never triggered Head-Fi, Mar 2023Head-Fi, Jun 2024.
Other devices
Hugo 2 on the M Scaler: asked in July 2018 whether permanently stacking Hugo 2 on the M Scaler was safe given heat, he answered "Yes it's fine on top of the HM scaler" Head-Fi, Jul 2018.
What he has not said. He gave no gap distance Head-Fi, Apr 2022 and no maximum room temperature for a stack beyond the 55 deg C ambient for the M Scaler trip Head-Fi, May 2019. The 45 deg C stacked figure is for the Hugo M Scaler Head-Fi, Jan 2019. On the Quartet M Scaler, the only stacking remark found (July 2024) is "I have only ever stacked the units", and it concerns sound, not heat Head-Fi, Jul 2024.
Quotes
“You can stack TT2/M scaler without any significant thermal or SQ problems, and certainly not any longevity problems.”2022-04-25 Head-Fi, Apr 2022
“Leave a good gap - but I have never actually AB tested how much of a gap you need, I just keep them as far apart as the cables allow.”2022-04-26 Head-Fi, Apr 2022
“The TT2 will withstand a lot more heat and still be long term reliable, so I wouldn't worry about it running warm continuously.”2022-04-26 Head-Fi, Apr 2022
“If it is hot, then make sure it's well ventilated. Don't site it under or over something hot.”2024-06-27 Head-Fi, Jun 2024
Dated positions
| When | Products | Position |
|---|---|---|
| 2018-07 | Hugo 2, Hugo M Scaler | Asked whether Hugo 2 can safely be stacked permanently on the M Scaler given heat, he said it is fine on top. Head-Fi, Jul 2018 |
| 2018-08 | Hugo TT2, Hugo M Scaler | 'They were designed to be stacked together - it gets as warm as a Dave in operation' (the post does not name the units). Head-Fi, Aug 2018 |
| 2019-01 | Hugo M Scaler | The M Scaler dissipates 12W and in a stacked situation the FPGA die is about 45 deg C, which he said will not significantly affect lifetime. Head-Fi, Jan 2019 |
| 2019-05 to 2024-06 | Hugo M Scaler | The thermal trip is set at about 65 deg C case temperature to protect hands, needing about 55 deg C ambient; he reported running it for hours in a bag without tripping; if it is hot, ventilate it and do not site it under or over something hot. Head-Fi, May 2019Head-Fi, Jun 2020Head-Fi, Mar 2023Head-Fi, Jun 2024 |
| 2022-04 | Hugo TT2, Hugo M Scaler | TT2 and M Scaler can be stacked without significant thermal, sound-quality or longevity problems; with Wave or other solid (non-clip-on-ferrite) BNC cables keep them apart for sound quality, with a good gap whose size he never A/B tested. Head-Fi, Apr 2022Head-Fi, Apr 2022 |
| 2023-12 | Hugo TT2, Hugo M Scaler | Traveling with both, he sometimes stacked, sometimes placed them side by side or separated, and noticed no sound difference. Head-Fi, Dec 2023 |
| 2019-08 to 2025 | Hugo TT2 | TT2's thermal trip is a risk on the balanced output into low-impedance loads (thresholds given as 8, 10 or 16 ohms in different posts); single-ended outputs are fine; in 2021 he said this advice had not changed. Head-Fi, Aug 2019Head-Fi, Nov 2020Head-Fi, Jan 2021Head-Fi, Mar 2021Head-Fi, Feb 2022Head-Fi, Apr 2022Head-Fi, Mar 2025 |
| 2024-07 | Quartet M Scaler | He had only ever stacked the units and expected it would make no difference, as he designed the screening and filtering with that in mind, but had PSU listening tests still to do; heat is not discussed. Head-Fi, Jul 2024 |
Caveats and open points
- Head-Fi, Apr 2022 ('If TT2 thermally shuts down then you need to separate them') is in the Watts Up thread and the question it answers is not in the archive; 'them' is not named in the reply.
- Head-Fi, Apr 2022's 'TT2 will withstand a lot more heat' answered a question about leaving TT2 on all the time, not about stacking.
- Head-Fi, Aug 2018 does not name which units 'they' and 'it' refer to.
- Head-Fi, Apr 2022 and Head-Fi, Apr 2022 carry dates estimated from post IDs (2022-04-25 to 2022-04-26).
- The 45 degrees C bag figure comes; check the recording.
- Head-Fi, Mar 2025 has no exact date; it is dated only as after 2025-03-27 and before the 2026-03-03 scrape.
- The load threshold for TT2's balanced output differs between posts (8, 10 and 16 ohms).
- The stacked-die figure of 45 deg C is for the original Hugo M Scaler; Head-Fi, Jan 2019 also gives a 125 deg C continuous die rating, while Head-Fi, May 2019 gives 85 deg C as the timing-closure rating; this archive does not discuss how they relate.