If you ever searched eBay trying to hunt down a desktop or portable headphone amp, chance are you already come across the brand ‘S.M.S.L.’ - a Chinese brand that has pumped out quite a few headphone amps over the years. Their Chinese name is actually quite interesting – “two wood three forest” (don’t ask me why as I don’t know either). The two models we are going to look at belong to their portable line-up.
Spec
sAp-4s
Output Power: 32ohm @ 1KHz (THD + N = 1% / Po = 100mW)
SNR: 93dB (32ohm Po = 10mW)
THD + N: 0.05% (32ohm Po = 10mW)
Frequency Response :20-40kHz
Recommended Headphone Impedance :16-300Ω
Charging: mini-USB DC5V 500mA (2.5 hours)
I/O: 3.5mm headphone jack
Battery: NOKIA BL-5C 3.7V / 1000mAH (60 hours)
MSRP: USD$60
sAp-7
Output Power: 150mW (16ohm) / 16mW (300ohm)
SNR: 100dB（A-Weighted）
THD+N: 0.01% (32ohm Po=10mW)
Channel Separation: -100dB
Frequency Response: 10-80kHz
Recommended Headphone Impedance: 16-300Ω
Charging: mini-USB DC5V 500mA (2.5 hours)
I/O: 3.5mm headphone jack
Battery：3.7V / 320mA (15 hours)
MSRP: USD$70
Accessories and Build Quality
Both amps come with fairly simple paper box packaging. With sAp-4s, you’ll get a USB cable for charging, a rubber band, a good looking interconnecting cable and a tight fitting pleather case. With sAp-7, there are also the USB cable, two rubber bands, interconnecting cable, but no case.
sAp-4s in its pleather case
Built quality is quite good. Nothing to praise or complain about as they are up to par with the competition. One last thing to note is that sAp-4s use Nokia BL-5C type of Li-ion battery so future replacement is possible.
Battery Life
I never took the time to fully drain the two amps for battery life testing, but based on normal use, the number quoted by the manufacturer seems to be right on the mark. sAp-4s literary can runs for weeks with my normal usage pattern (a few hours day at most) while sAp-7 can easily last over 10 hours for the least. Of course, it depends on how you use them as well. But I didn’t get any impression that they are running short of what they claimed to deliver.
Gain, Hiss and EMI
sAp-4s has a fixed gain around +10dB, which is quite a lot. sAp-7 has a fixed gain of +6.5dB, or drop to +1dB on EQ3. Neither amps is designed for line-level source but the EQ3 on sAp-7 allows it to be used with 2Vrms input without clipping. Hiss is extremely low on both amps and practically inaudible during use. EMI on the other hand isn’t good on both. While sAp-7 does have audible level of EMI when placing near a cellphone that is receiving call, it is the sAp-4s that gets way too loud and definitely not suitable to be used near any EMI source.
Sound Quality
Let starts with RMAA measurement first. Both sAp-4s and sAp-7 show clean result and nothing to complain about - Decently low noise and almost flat 20Hz to 20kHz frequency response (see the above graph), with very minor roll-off in the sub-bass region, but too mild to be any real concern. Output impedance are measured and calculated to be under 1ohm for both amps. Neither has a lot of current output, where sAp-7 is still quite decent while sAp-4s might not do that well with very demanding load. Otherwise they should good enough for your average headphone out there.
By the look of its internal circuitry, the sAp-4s is based on the reference design of Maxim MAX9722, which is a popular chip you can find many portable gears - from one of my all-time favorite thumb-drive sized DAP, the Cube C30, to the headphone amp stage of iFi Audio iDAC. The chip itself is known to be very clean and wide sounding, and that still holds true in the sAp-4s. It is indeed very clean and specious, if not actually a bit on the lean side. Some of the texture on the lower mid to bass region seems to be missing, given the impression of slight brightness. The dynamic range also seems to be slightly limited. MAX9722 is a chip that can really perform in an optimized circuit, but unfortunately it usually isn’t on its best when powered directly by Li-ion battery, such as in the case of sAp-4s. Overall, the performance is roughly on par with the old FiiO E5, though a bit on the opposite in presentation as E5 is more on the warm and slightly congested side of things.
For those who has owned a FiiO E6 before, you might be surprised by the similarity of features between the E6 and the sAp-7, despite of the difference in design. They both have a power switch, an EQ switch that doubles as hold (key lock function), three different EQ settings and digital volume control. I guess it doesn’t take far to guess that some of the components inside are very similar as well. Yes, they both use the same opamp+buffer combo (AD8692+TPA6130A), but the power section and micro controller are different in design. Since the basic amp section is based on the same chipset, the sound quality is very similar as well. Both are on the warmer and fuller side of the presentation, but sAp-7 edges out slightly for having a darker, deeper background that gives it a slightly cleaner image. Soundstage is about on par as well, neither very big nor small, but immersive and well rounded. Though sAp-7 sounds a little better than E6 in A/Bing, the actually difference is rather subtle. All and all, I’ll put it in the same class as E6, as it isn’t good enough to compete with those that are better than E6.
EQ
Besides having lower gain setting than E6 (+6.5dB vs +8dB) and having lesser hiss, the EQ setting on the sAp-7 closely resemble than of E6 as well (see graphs below). As you can see from the graph, EQ1 and EQ2 are both bass boost, where EQ3 is gain reduction that allows the amp to be used with line-level source.
Size comparison (from left and top): iBasso D-zero, Leckerton Audio UHA-4, iBasso T5, C and C BH2, sAp-7 and sAp-4s
Sum-up
I like the overall build quality of both amps, both externally and internally. They look every bit as good as any sub-$100 portable amps. But when it comes to sound quality, they are a bit short of their competition. That’s rather a shame actually – being that skilled on fabrication, I was hoping they do away with the generic design and opt for something more special. They certainly have the potential.
A thanks to S.M.S.L. for the review samples.
Spec
sAp-4s
Output Power: 32ohm @ 1KHz (THD + N = 1% / Po = 100mW)
SNR: 93dB (32ohm Po = 10mW)
THD + N: 0.05% (32ohm Po = 10mW)
Frequency Response :20-40kHz
Recommended Headphone Impedance :16-300Ω
Charging: mini-USB DC5V 500mA (2.5 hours)
I/O: 3.5mm headphone jack
Battery: NOKIA BL-5C 3.7V / 1000mAH (60 hours)
MSRP: USD$60
sAp-7
Output Power: 150mW (16ohm) / 16mW (300ohm)
SNR: 100dB（A-Weighted）
THD+N: 0.01% (32ohm Po=10mW)
Channel Separation: -100dB
Frequency Response: 10-80kHz
Recommended Headphone Impedance: 16-300Ω
Charging: mini-USB DC5V 500mA (2.5 hours)
I/O: 3.5mm headphone jack
Battery：3.7V / 320mA (15 hours)
MSRP: USD$70
Accessories and Build Quality
Both amps come with fairly simple paper box packaging. With sAp-4s, you’ll get a USB cable for charging, a rubber band, a good looking interconnecting cable and a tight fitting pleather case. With sAp-7, there are also the USB cable, two rubber bands, interconnecting cable, but no case.
sAp-4s in its pleather case
Built quality is quite good. Nothing to praise or complain about as they are up to par with the competition. One last thing to note is that sAp-4s use Nokia BL-5C type of Li-ion battery so future replacement is possible.
Battery Life
I never took the time to fully drain the two amps for battery life testing, but based on normal use, the number quoted by the manufacturer seems to be right on the mark. sAp-4s literary can runs for weeks with my normal usage pattern (a few hours day at most) while sAp-7 can easily last over 10 hours for the least. Of course, it depends on how you use them as well. But I didn’t get any impression that they are running short of what they claimed to deliver.
Gain, Hiss and EMI
sAp-4s has a fixed gain around +10dB, which is quite a lot. sAp-7 has a fixed gain of +6.5dB, or drop to +1dB on EQ3. Neither amps is designed for line-level source but the EQ3 on sAp-7 allows it to be used with 2Vrms input without clipping. Hiss is extremely low on both amps and practically inaudible during use. EMI on the other hand isn’t good on both. While sAp-7 does have audible level of EMI when placing near a cellphone that is receiving call, it is the sAp-4s that gets way too loud and definitely not suitable to be used near any EMI source.
Sound Quality
Let starts with RMAA measurement first. Both sAp-4s and sAp-7 show clean result and nothing to complain about - Decently low noise and almost flat 20Hz to 20kHz frequency response (see the above graph), with very minor roll-off in the sub-bass region, but too mild to be any real concern. Output impedance are measured and calculated to be under 1ohm for both amps. Neither has a lot of current output, where sAp-7 is still quite decent while sAp-4s might not do that well with very demanding load. Otherwise they should good enough for your average headphone out there.
By the look of its internal circuitry, the sAp-4s is based on the reference design of Maxim MAX9722, which is a popular chip you can find many portable gears - from one of my all-time favorite thumb-drive sized DAP, the Cube C30, to the headphone amp stage of iFi Audio iDAC. The chip itself is known to be very clean and wide sounding, and that still holds true in the sAp-4s. It is indeed very clean and specious, if not actually a bit on the lean side. Some of the texture on the lower mid to bass region seems to be missing, given the impression of slight brightness. The dynamic range also seems to be slightly limited. MAX9722 is a chip that can really perform in an optimized circuit, but unfortunately it usually isn’t on its best when powered directly by Li-ion battery, such as in the case of sAp-4s. Overall, the performance is roughly on par with the old FiiO E5, though a bit on the opposite in presentation as E5 is more on the warm and slightly congested side of things.
For those who has owned a FiiO E6 before, you might be surprised by the similarity of features between the E6 and the sAp-7, despite of the difference in design. They both have a power switch, an EQ switch that doubles as hold (key lock function), three different EQ settings and digital volume control. I guess it doesn’t take far to guess that some of the components inside are very similar as well. Yes, they both use the same opamp+buffer combo (AD8692+TPA6130A), but the power section and micro controller are different in design. Since the basic amp section is based on the same chipset, the sound quality is very similar as well. Both are on the warmer and fuller side of the presentation, but sAp-7 edges out slightly for having a darker, deeper background that gives it a slightly cleaner image. Soundstage is about on par as well, neither very big nor small, but immersive and well rounded. Though sAp-7 sounds a little better than E6 in A/Bing, the actually difference is rather subtle. All and all, I’ll put it in the same class as E6, as it isn’t good enough to compete with those that are better than E6.
EQ
Besides having lower gain setting than E6 (+6.5dB vs +8dB) and having lesser hiss, the EQ setting on the sAp-7 closely resemble than of E6 as well (see graphs below). As you can see from the graph, EQ1 and EQ2 are both bass boost, where EQ3 is gain reduction that allows the amp to be used with line-level source.
Size comparison (from left and top): iBasso D-zero, Leckerton Audio UHA-4, iBasso T5, C and C BH2, sAp-7 and sAp-4s
Sum-up
I like the overall build quality of both amps, both externally and internally. They look every bit as good as any sub-$100 portable amps. But when it comes to sound quality, they are a bit short of their competition. That’s rather a shame actually – being that skilled on fabrication, I was hoping they do away with the generic design and opt for something more special. They certainly have the potential.
A thanks to S.M.S.L. for the review samples.
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