In the recent review of Topping NX1s Headphone Amp, question was raised regarding the reason there is a roll off in low frequencies. I speculated that it is likely caused by capacitor coupling. Capacitors are used to block DC output as they have infinite resistance to steady voltages (and become a short as you go to infinite frequency). A teardown was requested and the owner kindly agreed.
Opening the unit was very easy courtesy of smart assembly with just two very long screws that held the front and back together. Here is the front part of the PC Board with most of the components:
Starting bottom right, there is a switching power supply to create stable voltages are needed for the operation of the unit (likely positive and negative voltages). It also handles power management for charging of the Lithium cell on the other side (edit: there is an IC on the other side which may be performing this function). Nice to see an isolation ring all around that segment to keep its noise away from the amplifier section to the left and top.
Amplification and gain are provided through a pair of OPA1652 OpAmps (operational amplifiers). The OPA1652 is a common audio opamp with very low distortion ratings:
0.00005% results in distortion of -126 dB which is more or less what we saw in the measurements.
There are a pair of opamps in each package and each provides the gain/buffering for each stereo channel. I could not read the markings on the second (top) Opamps but I am assuming it is a higher current, BUF634 OpAmp as seen in the application note for OPA1652:
Edit: turns out the buffer is LMH66, not BUF634. See: https://www.audiosciencereview.com/...able-headphone-amp-teardown.10577/post-293270
Anyway, the interest was in the cause of bass roll off. There is none in the above schematic but let's look at the other side where the battery mostly lives:
We see two 1 microfarad capacitors directly after the volume control. They are film capacitors which are quite large for their value. They are rated at 63 volts and likely there to clock DC from input travelling all the way through the amplifier to the headphones. A larger value capacitor of a different type would not have caused the roll off that we see.
Overall, the value remains incredible for all the components and circuits in NX1s.
P.S. Yes, the wire came out of its socket when I tried to disconnect the battery. It uses a very flimsy connector that was a pain to disconnect.
Opening the unit was very easy courtesy of smart assembly with just two very long screws that held the front and back together. Here is the front part of the PC Board with most of the components:
Starting bottom right, there is a switching power supply to create stable voltages are needed for the operation of the unit (likely positive and negative voltages). It also handles power management for charging of the Lithium cell on the other side (edit: there is an IC on the other side which may be performing this function). Nice to see an isolation ring all around that segment to keep its noise away from the amplifier section to the left and top.
Amplification and gain are provided through a pair of OPA1652 OpAmps (operational amplifiers). The OPA1652 is a common audio opamp with very low distortion ratings:
0.00005% results in distortion of -126 dB which is more or less what we saw in the measurements.
There are a pair of opamps in each package and each provides the gain/buffering for each stereo channel. I could not read the markings on the second (top) Opamps but I am assuming it is a higher current, BUF634 OpAmp as seen in the application note for OPA1652:
Edit: turns out the buffer is LMH66, not BUF634. See: https://www.audiosciencereview.com/...able-headphone-amp-teardown.10577/post-293270
Anyway, the interest was in the cause of bass roll off. There is none in the above schematic but let's look at the other side where the battery mostly lives:
We see two 1 microfarad capacitors directly after the volume control. They are film capacitors which are quite large for their value. They are rated at 63 volts and likely there to clock DC from input travelling all the way through the amplifier to the headphones. A larger value capacitor of a different type would not have caused the roll off that we see.
Overall, the value remains incredible for all the components and circuits in NX1s.
P.S. Yes, the wire came out of its socket when I tried to disconnect the battery. It uses a very flimsy connector that was a pain to disconnect.
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