SMSL SP400 vs Topping A90: Which Drives More Headphones?

SMSL SP400 vs Topping A90

Short answer

More power: the SMSL SP400, with 3.00 W into 32 Ω; the Topping A90 has 2.00 W (balanced: 6.00 W against 4.05 W). Both drive 567 of the 567 non-electrostatic headphones here to 100 dB, so on that test the difference changes nothing.
Quieter with sensitive IEMs: the Topping A90. 4 headphones stay below audible hiss on it but not on the SMSL SP400 (Shure SE846, Simgot EA500, Tanchjim EA FORCE, 1 more).
Price: the Topping A90 costs $30 less and gives up nothing on power, output impedance or hiss against the 567 headphones here. Whatever the extra money for the SMSL SP400 buys, it is not these three numbers.

Both are desktop amplifiers, $529 against $499.

On raw power they are level: each reaches 100 dB on 567 of the 567 non-electrostatic models here.

Specification	SMSL SP400	Topping A90
Power into 32Ω	3.0 W	2.0 W
Power into 150Ω	741 mW	427 mW
Power into 300Ω	440 mW	213 mW
Power into 600Ω	220 mW	107 mW
Output impedance	<0.1 Ω	0.1 Ω
Output noise	1.9 µV	0.2 µV
Price	$529	$499
Headphones it reaches 100 dB on out of 567	567	567

Specification

SMSL SP400

Topping A90

Power into 32Ω

3.0 W

2.0 W

Power into 150Ω

741 mW

427 mW

Power into 300Ω

440 mW

213 mW

Power into 600Ω

220 mW

107 mW

Output impedance

<0.1 Ω

0.1 Ω

Output noise

1.9 µV

0.2 µV

Price

$529

$499

Headphones it reaches 100 dB on
out of 567

567

The 4.4 mm or XLR socket, rated on its own

Balanced	SMSL SP400	Topping A90
Socket	XLR4	XLR4 + 4.4mm
Power into 32Ω	6.0 W	4.0 W derived
Power into 300Ω	880 mW	1.0 W
Output impedance	<0.1 Ω	0.1 Ω
Output noise	1.9 µV	0.2 µV
Headphones it reaches 100 dB on out of 567	567	567

Through the balanced sockets the two are level on the same test: each reaches 100 dB on 567 of the 567 models here. Derived figures are worked out from the loads the maker did publish.

Where each power figure comes from

Load	SMSL SP400	Topping A90
32 Ω	Published	Published
150 Ω	Derived	Derived
300 Ω	Published	Derived
600 Ω	Published	Derived

Both makers publish a power rating. SMSL SP400 publishes at 32 Ω, at 300 Ω and at 600 Ω; Topping A90 at 32 Ω. The other rows are worked out from those.

So the two columns are not equally firm row by row: at 300 Ω and at 600 Ω only the SMSL SP400 figure is the maker’s own. On those rows the derived number is the one to doubt first.

What the labels mean.

The headphones that tell them apart

On power they are interchangeable across this whole database: there is no headphone here that one reaches 100 dB on and the other does not.

4 headphones stay below the audible-hiss threshold on the Topping A90 but cross it on the SMSL SP400 (Shure SE846, Simgot EA500, Tanchjim EA FORCE, 1 more).

567 of the 567 models here reach 100 dB on both.

Headroom and hiss on 8 models, both sources

Headroom is at a 100 dB SPL peak.

The widest gap on the list is the Shure SE846: SMSL SP400 has +49 dB of headroom there against +42 dB. 8 dB is about 6 times the power, which matters for peaks rather than for how loud the music sits.

Hiss below about 20 dB SPL is inaudible in a quiet room.

Headphone	Impedance	Needs	SMSL SP400 headroom	Topping A90 headroom	SMSL SP400 hiss	Topping A90 hiss
Shure SE846	9 Ω	0.04 mW	+49 dB	+42 dB	20 dB	0 dB
Campfire Audio Andromeda 2020	9 Ω	4.0 mW	+29 dB	+22 dB	0 dB	-20 dB
Audeze LCD-2 Classic	70 Ω	0.79 mW	+33 dB	+31 dB	-2 dB	-21 dB
Beyerdynamic DT 1990 Pro (250Ω)	250 Ω	0.63 mW	+29 dB	+26 dB	-6 dB	-26 dB
HiFiMAN HE400se	25 Ω	7.9 mW	+27 dB	+23 dB	-7 dB	-27 dB
Audio-Technica ATH-R70X	470 Ω	1.6 mW	+22 dB	+19 dB	-13 dB	-33 dB
Sennheiser HD 600	300 Ω	6.6 mW	+18 dB	+15 dB	-17 dB	-37 dB
HiFiMAN Susvara	60 Ω	50 mW	+15 dB	+13 dB	-19 dB	-39 dB

The gap in dollars, against the gap in numbers

The SMSL SP400 costs $30 more than the Topping A90, and on these three axes the money buys nothing: among the 567 models here there is not one that the SMSL SP400 handles and the Topping A90 does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper Topping A90 keeps 4 models below the audible-hiss threshold that the SMSL SP400 does not.

Only published figures count here; blank axes are counted separately.