SMSL DO400 Fully Balanced Audio Decoder & Headphone Amplifier

[nanook]

The RME ADI-2 DAC FS does raise a genuine question I’d put to the forum: in 2025, is it still a relevant product?

It has some features other DACs are still missing:
RME: "... full DSP processing with Bass, Treble, Loudness, 5 Band Parametric EQ, Crossfeed and much more."

5 bands PEQ is nice and will allow correction for the most important deviations in frequency response. When doing combined speaker + room correction, I was happy to have 10 bands in the WiiM.

 

[Delmont]

It has some features other DACs are still missing:
RME: "... full DSP processing with Bass, Treble, Loudness, 5 Band Parametric EQ, Crossfeed and much more."

5 bands PEQ is nice and will allow correction for the most important deviations in frequency response. When doing combined speaker + room correction, I was happy to have 10 bands in the WiiM.

Do you own one yourself? If so, could you describe its sound signature in detail? What key adjective would you use to characterize its sonic rendering? Regarding the headphone output: do you find it powerful enough for serious listening, or would you say an external headphone amplifier is required? If I have the opportunity to purchase a used unit, I’d be very interested. One of its advantages, in my view, is that it’s German-made—therefore European—which, for me, is a decisive and even melancholic criterion.

 

[Roland68]

Can someone confirm whether the RME ADI-2 DAC FS V2 can fully drive the Sennheiser HD 660S2? I noticed it only has an unbalanced headphone output—could that limit sound quality compared to a balanced output?

If so, then the symmetrical HP output of an HPA is the worse, not the other way around.
I've already written about this here #408.
My own tests with well over 50 HPAs in the price range between €150 and €10,000 over the last 10 years have revealed absolutely no sonic advantage of a balanced HPA output over a balanced HPA output. Rather, it's the other way around.

The RME ADI-2 DAC FS V2, with over 300 mW of power at 300 ohms, should be perfectly adequate, even with EQ. The RME is clearly the better device in terms of quality, and I would place it just ahead of the Loxjie D40 Pro / SMSL DO400, but these are inaudible nuances.
With separate devices, you'd achieve more for half the price.

 

[nanook]

Do you own one yourself?

No, and I never owned one.

A friend of mine has the the ADI2 Pro fs (DAC and ADC) , which very likely has the same HP Amp (additionally the option to connect a balanced HP) and he is very happy with it.
He uses the unit for measuring as well and is completely happy with the performance of the DAC and ADC.

 

[nanook]

I'm mainly using the SMSL D-6s with a pair of old Genelec speakers.
For listening with HP (Beyerdynamic DT880 250 Ohm), the Topping L30II is connected to the RCA outputs of the D-6s.
The L30II is single ended with 6.35mm TRS only and in high-gain setting of the L30II it's definitely loud enough.

I'm not sure about the sensitivity of the DT880 250 Ohm; the specification just says "96dB" without giving details like frequency or drive level.

Edit: seems sensitivity is given in dB(SPL) /mW.
This source says 101.5dB/V, so with e.g. 7.5V I should end up with ca. 108dB(SPL).

https://reference-audio-analyzer.pro/en/report/hp/beyerdynamic-dt-880-pro.php#gsc.tab=0

 

[nanook]

If someone is wondering why I don't use the DO400 for listening (but for measurements). It simply doesn't fit nicely on the window sill - my wife would complain.
;-)

 

[Delmont]

If so, then the symmetrical HP output of an HPA is the worse, not the other way around.
I've already written about this here #408.
My own tests with well over 50 HPAs in the price range between €150 and €10,000 over the last 10 years have revealed absolutely no sonic advantage of a balanced HPA output over a balanced HPA output. Rather, it's the other way around.

The RME ADI-2 DAC FS V2, with over 300 mW of power at 300 ohms, should be perfectly adequate, even with EQ. The RME is clearly the better device in terms of quality, and I would place it just ahead of the Loxjie D40 Pro / SMSL DO400, but these are inaudible nuances.
With separate devices, you'd achieve more for half the price.

I’m genuinely curious to hear what the RME ADI-2 DAC FS has to offer. Through the headphone output, the DO400 struck me as remarkably close to the MDA1 — so much so that any differences, if present, felt minimal.

By contrast, the JC Acoustics UDP-6 is on a completely different level. It’s a clear, immediate step up — no ambiguity there. That’s exactly why the RME intrigues me. I’d like to know whether it confirms this hierarchy, or if it somehow reshuffles the deck in its own way.

 

[Roland68]

I’m genuinely curious to hear what the RME ADI-2 DAC FS has to offer. Through the headphone output, the DO400 struck me as remarkably close to the MDA1 — so much so that any differences, if present, felt minimal.

By contrast, the JC Acoustics UDP-6 is on a completely different level. It’s a clear, immediate step up — no ambiguity there. That’s exactly why the RME intrigues me. I’d like to know whether it confirms this hierarchy, or if it somehow reshuffles the deck in its own way.

I don't want to destroy your dreams either, but the JC Acoustics UDP-6 only has a headphone amp based on 4 pieces of TPA6120a2.
Topping has already exhausted this story with the first A90, the L50, the L30 II and most recently with the L70. With the larger models, Topping is using discrete transistor technology for the HP power amplifiers again, as the potential of the TPA6120a2 has now been exhausted.
It's no coincidence that the output power of the L70 and the UDP-6 are as similar as two peas in a pod.

Just listen to an L70 or L30 II versus the UDP-6 in a truly blinded comparison. I would be extremely surprised if there were actually a difference or advantage for the UDP-6.

 

[nanook]

The small L30 II has just one pair of TPA6120a2 (single-ended output stage) and a 6.35mm TRS - and it fits nicely on the window sill. The transformer based wall-wart is bulky and heavy, but that's ok. if you have a chance to hide it.
I'm not sure which of the other Topping HPA units does have a differential output stage (4x TPA6120a2) and a 4-pin/ring connector.

The UDP-6 has everything you love to see as an audio - addict (including me ;-)). A large torodial transformer, lots of large reservoir caps, large heat-sinks and a machined aluminum cabinet. The attached fans have turned me off a bit I must admit.
BUT: switch-mode-power-supplies (SMPS) have gotten a lot better regarding leakage currents (that may cause hum when cheap RCA cables are used) and they do just need relatively small reservoir caps (due to the higher frequency). There are extremely low ESR caps around (polymer) so there's no need for huge reservoir caps.

Long story short: I agree with Roland - I would be surprised if the difference between the UDP-6 and the DO400 (both ES9039pro, both 4x TPA6120a2) is audible once the expectation bias is ruled out (blind listening with levels matched to within 0.1dB) .

When money is not a topic, why not, but I really doubt if ca. 2x the price compared to the DO400 / Loxjie D40 Pro is audible.

 

[nanook]

As far as I know the HPA of the RME ADI-2 DAC FS does not use the TPA6120a, but several high - performance OpAmps in parallel per channel.
See: https://forum.rme-audio.de/viewtopic.php?id=37437
I have not found measurements so far.

 

[Delmont]

I don't want to destroy your dreams either

Long story short: I agree with Roland - I would be surprised if the difference between the UDP-6 and the DO400 (both ES9039pro, both 4x TPA6120a2) is audible once the expectation bias is ruled out (blind listening with levels matched to within 0.1dB) .

The JC Acoustics UDP-6 and SMSL DO-400 share some similarities, but after extended listening with my HD800S, I noticed a distinct difference. The UDP-6 offers a more stable soundstage, greater harmonic richness, and finer dynamics, all without any harshness. As you have so accurately pointed out, I believe its linear power supply with a toroidal transformer, high-quality capacitors, and, I’m sure, the very careful placement of components, play a crucial role in this improvement. This overall coherence in design led me to sell my DO-400 and FiiO K9 Pro ESS, which had previously been my main DAC/headphone amp references. Interestingly, the DO-400 and MDA-1 sound quite similar through headphones, despite their different internal architectures, which highlights that final sound quality depends more on the overall implementation than on the specifications alone.

You’ve made me so curious about the RME ADI-2 that I wonder if it might perhaps surpass the UDP-6…

 

[nanook]

I agree, the implementation is extremely important. And as you pointed out, the actual layout is at least equally important. The most mistakes are being made when doing the artwork of the layout.
You have to delve deep into the circuit and understand where which currents flow and return in order to route the conductors in such a way that circuit parts do not interact. You can really do a lot wrong here.

I really appreciate good engineering. I have reverse engineered quite a few circuits and I am happy every time I see a "nice" circuit or a "real" layout.

 

[Roland68]

The JC Acoustics UDP-6 and SMSL DO-400 share some similarities, but after extended listening with my HD800S, I noticed a distinct difference. The UDP-6 offers a more stable soundstage, greater harmonic richness, and finer dynamics, all without any harshness. As you have so accurately pointed out, I believe its linear power supply with a toroidal transformer, high-quality capacitors, and, I’m sure, the very careful placement of components, play a crucial role in this improvement. This overall coherence in design led me to sell my DO-400 and FiiO K9 Pro ESS, which had previously been my main DAC/headphone amp references. Interestingly, the DO-400 and MDA-1 sound quite similar through headphones, despite their different internal architectures, which highlights that final sound quality depends more on the overall implementation than on the specifications alone.

You’ve made me so curious about the RME ADI-2 that I wonder if it might perhaps surpass the UDP-6…

The implementation of the four TPA6120a2s in the DO400 is definitely not on the level of, for example, the L30 II or L70.
That's also the reason why I recommended separate devices to @ArthurStein13.

I would also always prefer a well-matched switching power supply to a toroidal transformer. In the past, I wouldn't have touched a switching power supply in an audio device with a pair of pliers.

 

[Roland68]

I agree, the implementation is extremely important. And as you pointed out, the actual layout is at least equally important. The most mistakes are being made when doing the artwork of the layout.
You have to delve deep into the circuit and understand where which currents flow and return in order to route the conductors in such a way that circuit parts do not interact. You can really do a lot wrong here.

I really appreciate good engineering. I have reverse engineered quite a few circuits and I am happy every time I see a "nice" circuit or a "real" layout.

What you describe is, for me, exactly the difference between SMSL (+ Sabaj and Loxjie) devices and Topping.
SMSL uses standard implementations, mostly circuits published by the chip manufacturers, which have been optimized primarily for good measurement values. This is particularly noticeable with amplifiers and HPAs. The TPA6120a2-based HPAs, for example, also in the DO400/DL400/Loxjie D40 Pro/D60, are still at a level that is years old, such as the Sabaj A10h/A20h. The circuitry of the discrete HPA built into the RAW-MDA1 is also directly derived from the Sabaj A10d '2022. In fact, the entire RAW-MDA1 is a reincarnation of the A10d '2022 with the DAC chip successors.

But that's all whining at a very high level. All of these HP amps are very transparent and better than 95% of the HP amps on the market. We've just become very spoiled, especially with ASR.

Topping, on the other hand, puts much more of its own development effort and circuitry into both the power supplies and the amplifiers and HP amps.
But that's just my personal opinion.

 

[Delmont]

Topping, on the other hand, puts much more of its own development effort and circuitry into both the power supplies and the amplifiers and HP amps.

Your point about Topping is entirely valid. However, it remains surprising and somewhat frustrating, that aside from the DX9, whose price places it out of reach for many, the brand has yet to offer a well-engineered, integrated solution in the midrange segment that could directly compete with the DO400. This absence leaves a strategic gap, pending perhaps a more precisely targeted DX5 II.

 

[Roland68]

Your point about Topping is entirely valid. However, it remains surprising and somewhat frustrating, that aside from the DX9, whose price places it out of reach for many, the brand has yet to offer a well-engineered, integrated solution in the midrange segment that could directly compete with the DO400. This absence leaves a strategic gap, pending perhaps a more precisely targeted DX5 II.

Which is also very clever, because compared to a DO400, the effort in the DX9 is immense.
The DX9 is a very independent design and development.
- Completely separate and complex power supplies for the digital and analog sections, meaning three different and separate voltages.
- 4-channel AKM DAC chip.
- AK4118 transceiver chip.
- Analog and relay-based volume control.
- Completely separate headphone amplifier for SE and Balanced modes with 6 instead of 4 power amplifiers, with completely separate circuits for SE and Balanced modes. While this circumvents certain limitations in these areas, it requires significantly more space and more components.
- Large housing, which allows for a relatively separate design of the different sections.

Considering the significantly higher costs for the DX9—for example, components, a large circuit board, expensive assembly, packaging and shipping, development costs, etc.—which are spread over a much smaller number of units than with a DO400, I can't call the DX9 expensive.

Topping has at least 20 very good combinations of DACs and HPAs in its range, all of which feature a highly transparent HPA and are all priced in the DO400 range or below.
For me, the limitation of two separate devices only exists in the minds of users, as there is definitely no sonic limitation, and the spatial volume is in most cases no larger than that of the DO400.
But the advantages are clear.
- Two separate power supplies for the DAC and HPA.
- Two separate enclosures for the DAC and HPA.
- Easy replacement of a single device.
- Affordable price.
- Better measurement values.
The combination of, for example, the SU-1/C100/E30 II/Lite and the L30 II or the D50 III and the L70 is currently hard to beat by any other combination device, especially not for less than €1,000.

But Topping is supposed to release the DX5 II in the next few weeks. The board looks like it has a lot of effort in terms of both the power supply and the circuitry for signal conditioning and HPA. Added to that is an HPA power output that could exceed the performance of the DO400 at both 32 and 300 ohms. EQ on all inputs is also a big plus. The measurements published so far also point to an excellent device. If these measurements are confirmed in a test, it could become one of the best-selling devices on the market.

 

[yavormoskov]

I am joining the party. Just purchased SMSL DO400 used from eBay. I hope it's good, I really hope so.

 

[yavormoskov]

It's in the middle of the night, almost morning really and I am typing this. Received the SMSL DO400 yesterday and started testing it. It was in immaculate condition, very well taken care of by the first owner. The sound is simply stunning, just stunning. The preamp through both XLR and RCA is head and shoulders better than my SMSL M500 mkiii. And the gain of both the XLR and RCA's outs is definitely higher than on the M500 mkiii, by a significant margin. Had to lower the volume of my speaker amplifier and on the JDS labs Atom 2. The amplification stage can not be easily described, so much power on tap, but not just power, control. Really tight control on the drivers of my planars.
The music I used is: Colour to the moon by Allen Taylor, Hollywood Gala concert by the Danish national orchestra, Nessum Dorma by Andrea Bochelli, Stairway to heaven by Led Zeppelin, studio recording of Let it be performed by Joan Baez, the entire award winning album written for the Baldurs Gate 3 game, Preservation Hall's anniversary jazz album live from New Orleans, Mongolian heavy metal band The HU and heart breaking performance of the main theme of the movie Schindler's list performed by Itzhak Perlman.
And because of the form factor, the amp will stay under my enormous 43 inch gaming monitor. Great choice to purchase it, so exited right now.

 

[yavormoskov]

Firmware question. I followed everything but the amp would not connect to the app in order to be updated. Do I have to install the XMOS driver for greater compatibility?