I found not many teardowns with this model, especially with the output transistors uncovered and with surrounding close-ups, so I decided to purchase one for myself (second hand) to pair it with my D90 MQA then write an article here on ASR. Looks like taking the PCB out from the case if somehow difficult, needing careful attention to the two ribbon cables that go to the rotary knob and to the LCD controller from front of the case, but also needing an unusual long and thin screwdriver that actually fits between the power supplies and side of the case. Assembling everything back is even worse, with a headlight on my head and a not-long-enough screwdriver, so I don't recommend anyone to tear apart the A90D, unless you're fine with possibly breaking it or losing its warranty.
The Topping A90 Discrete was reviewed by Amir here: https://www.audiosciencereview.com/...0-discrete-review-headphone-amp-preamp.35114/ and with it’s SINAD of over 119 dB, a max. output power of 6 Watts on low-impedance headphones and a perfect channels volume match, it was clearly measuring exceptional.
Topping A90 Discrete (A90D) - top view of the PCB
Right beside the power plug there is a Metal Oxide Varistor @ 400 V that provides surge protection by tripping the MCB from your electrical panel in case an electrical malfunction occurs upstream your home (e.g.: overvoltage, lightning etc.). Even if just a simple MOV will not make wonders in case of small power spikes on the mains, I'm happy to see there’s a MOV inside this headamp because it should offer a decent protection for the device when getting 400 V or more on the mains and should also minimize the risk of fire due to overvoltage.
Metal Oxide Varistor (MOV) used for power surge protection
The unit gets clean power from a couple of internal AC-DC encapsulated power supplies from a reputable manufacturer, Mean Well IRM-20-15, that deliver +/-15V @ max. 1.4 A. These are isolated power supplies with very good filtering inside and the measured ripple & noise below 50 mVp-p (per https://www.meanwell-web.com/content/files/pdfs/productPdfs/MW/IRM-20/IRM-20-15-rpt.pdf. The power supplies are protected from overvoltage (in case of malfunction internally) and short-circuit too. Isolation type is Class II, meaning that these power supplies can be safely used in electrical equipment with or without grounding, customers not being affected in case a malfunction inside the supply will occur. This is a great thing that alllowed Topping to easily implement in A90D the "ground lifting" feature that helps in cases where audio hum is audible due to grounding resistance difference between interconnected audio equipment, where electrical currents are flowing into signal wires due to this.
Mean Well AC-DC power supplies
We can also spot on the PCB three linear regulators ON-SEMI LM317D2T that power ON all the "logic" from the board that takes care of proper operation of the amplifier, relays and protections, but also the front panel lights and logic commands.
LM317 linear power regulators (used for logic part)
Volume control is fully digital, with a resolution of 100-steps, and is done around a rotary knob and an array of resistors and stepped relays, a choice rather difficult find in headamps on this price range. Relays are pretty low in noise, I can hear the noise made without headphones, but comparing it with let’s say Flux Volot the A90D is making like half of the noise when changing the volume up and down.
The relays used are 2-pole OMRON G6K-2P-Y @ 12 VDC with a contact resistance lower than 100 mΩ, ensuring a virtually zero voltage drop across the 2-poles of each contact. There are also fifteen small SOT-23 case SS8050 transistors that are taking care of providing power to the OMRON relays.
Small SMD transistors array takes care of powering the relays from the PCB board
The rotary knob does not only controls the output volume, but it is also used to select the right audio input (single press), to choose between PRE and/or HEADAMP operation (double press) and, of course, it's used for powering the unit ON (short press) or OFF (long press).
Between the bottom of the case and the backside of the PCB there is a protective isolation film that helps the soldering from the backside of the PCB to touch the bottom of the metallic case.
Protective isolation film
Copper grounding point between PCB and outter case
The front display is controlled by a TM1638 chip that is widely used for similar tasks in Arduino compatible devices, see https://www.instructables.com/Arduino-and-TM1638-LED-Display-Modules/ and https://www.handsontec.com/dataspecs/display/TM1638.pdf on how this chip is widely used for controlling LED displays in Arduino projects. There are libraries for ease of programming this chip and it’s nice and unexpected at the same time to see it used in a headphones amplifier.
Front LED controller
The output stage is created around four pairs of complementary PNP/NPN BJT BISS (Breakthrough In Small Signal) transistors: PBSS305PZ and PBSS305NZ. These are bipolar transistors with a very low Vce-saturation, meaning that heat dissipation will be lower due to a better efficiency. Transistors temperature is controlled by the PCB itself, but also by a small aluminum heatsink attached on top via double-sided self adhesive thermal pads secured with three screws by the PCB. These small and fast transistors are able to dissipate around 1.5 Watts each, max. amperage driven being up to 10 Amps (short term).
The eight output stage transistors (covered with thermal conductive tape)
and the corresponding goldish electrolytic decoupling capacitors
One of the four complementary pairs of the output stage transistors
All the logic and controls are takes care by a dedicated ARM Cortex microprocessor operating up to 48 MHz STM32F072C8. It has a CAN bus, but also an ADC and a DAC inside, so it can accept both analog and digital signal inputs at the same time. This way the microcontroller is able to read all the states about A90D’s operation like stand-by, volume level, gain, inputs & outputs, inside voltages, DC output etc. If a reading goes wrong then an error code is throwen on the LCD and device might enter in Stand-by.
There is also a UC25HQ40 that is a 4 Mbits ultra low power memory that communicates on SPI. This flash memory is probably used for storing the states for the front panel commands like In/Out, Pre/HPA, volume level, the two memories C1 & C2 etc., while A90D's firmware is stored inside the flash memory from the above STM32F072C8 microcontroller.
The STM microprocessor along with the memory flash chip
For DC-output protection there is a TL072C op-amp that detects any trace of DC voltage on headphones outputs and communicates this to the microprocessor that is instantly placing the amplifier in protection mode (-5) and cuts off the power.
TL072 op-amp responsable for triggering the DC-output protection
Preamplification and gain stages are built inside four NFCA (Nested Feedback Composite Amplifier) modules, each one containing several SMD transistors and passive components too Close-up pictures will be soon revealed once the new version of A90 Discrete amplifier will appear on the market, given that Topping decided to get rid of the outter case for its NFCA modules (probably due to this). Till then, below is an inside picture of these modules grabbed from manufacturer website.
The big blue adjustable multi-turn resistors from top of the NFCA modules are there for adjusting the output DC by the manufacturer. Those adjustable resistors are sealed on top and trying to manually adjust any of them will add DC voltage on the output of the amplifier and pops and click may occur, but this will also void the warranty of the device! I measured 0 mV (zero mV!) on the XLR 4-pin plug, on both channels, with no music playing, so my A90D headamp was perfectly adjusted by the manufacturer. However, if you hear any random pops and clicks on the A90D headamp, just email the manufacturer and wait for a resolution on this, but don’t try to handle this by yourself by adjusting those resistors!
Multi-turn resistors responsable for adjusting the DC-output by the manufacturer
The inside temperatures are quite good, hottest spot was the side with the outputs from the Mean Well power supplies at around 47-48 C, while the other faces of the power supplies were having temperatures between 35 and 41 C. The heatsink from the output transistors was oscillating between 40 and 42 C and the NFCA modules were pretty contant between 39 and 40 C. With case closed the above temps will definitely raise a little bit, probably with 5 C or maybe a bit more, but given that the top of the case stays below 35 C, I consider this headamp as being on the warm side rather than on the hot side like other headlamps I own (Matrix HPA-3B and Burson 3X GT 2023 are getting to the range of 45-50 C on the sides or top of the case, depending on the output volume).
Temperature measured directly on the heatsink
PROs:
CONs:
The Topping A90 Discrete was reviewed by Amir here: https://www.audiosciencereview.com/...0-discrete-review-headphone-amp-preamp.35114/ and with it’s SINAD of over 119 dB, a max. output power of 6 Watts on low-impedance headphones and a perfect channels volume match, it was clearly measuring exceptional.
Topping A90 Discrete (A90D) - top view of the PCB
Right beside the power plug there is a Metal Oxide Varistor @ 400 V that provides surge protection by tripping the MCB from your electrical panel in case an electrical malfunction occurs upstream your home (e.g.: overvoltage, lightning etc.). Even if just a simple MOV will not make wonders in case of small power spikes on the mains, I'm happy to see there’s a MOV inside this headamp because it should offer a decent protection for the device when getting 400 V or more on the mains and should also minimize the risk of fire due to overvoltage.
Metal Oxide Varistor (MOV) used for power surge protection
The unit gets clean power from a couple of internal AC-DC encapsulated power supplies from a reputable manufacturer, Mean Well IRM-20-15, that deliver +/-15V @ max. 1.4 A. These are isolated power supplies with very good filtering inside and the measured ripple & noise below 50 mVp-p (per https://www.meanwell-web.com/content/files/pdfs/productPdfs/MW/IRM-20/IRM-20-15-rpt.pdf. The power supplies are protected from overvoltage (in case of malfunction internally) and short-circuit too. Isolation type is Class II, meaning that these power supplies can be safely used in electrical equipment with or without grounding, customers not being affected in case a malfunction inside the supply will occur. This is a great thing that alllowed Topping to easily implement in A90D the "ground lifting" feature that helps in cases where audio hum is audible due to grounding resistance difference between interconnected audio equipment, where electrical currents are flowing into signal wires due to this.
Mean Well AC-DC power supplies
We can also spot on the PCB three linear regulators ON-SEMI LM317D2T that power ON all the "logic" from the board that takes care of proper operation of the amplifier, relays and protections, but also the front panel lights and logic commands.
LM317 linear power regulators (used for logic part)
Volume control is fully digital, with a resolution of 100-steps, and is done around a rotary knob and an array of resistors and stepped relays, a choice rather difficult find in headamps on this price range. Relays are pretty low in noise, I can hear the noise made without headphones, but comparing it with let’s say Flux Volot the A90D is making like half of the noise when changing the volume up and down.
The relays used are 2-pole OMRON G6K-2P-Y @ 12 VDC with a contact resistance lower than 100 mΩ, ensuring a virtually zero voltage drop across the 2-poles of each contact. There are also fifteen small SOT-23 case SS8050 transistors that are taking care of providing power to the OMRON relays.
Small SMD transistors array takes care of powering the relays from the PCB board
The rotary knob does not only controls the output volume, but it is also used to select the right audio input (single press), to choose between PRE and/or HEADAMP operation (double press) and, of course, it's used for powering the unit ON (short press) or OFF (long press).
Between the bottom of the case and the backside of the PCB there is a protective isolation film that helps the soldering from the backside of the PCB to touch the bottom of the metallic case.
Protective isolation film
Copper grounding point between PCB and outter case
The front display is controlled by a TM1638 chip that is widely used for similar tasks in Arduino compatible devices, see https://www.instructables.com/Arduino-and-TM1638-LED-Display-Modules/ and https://www.handsontec.com/dataspecs/display/TM1638.pdf on how this chip is widely used for controlling LED displays in Arduino projects. There are libraries for ease of programming this chip and it’s nice and unexpected at the same time to see it used in a headphones amplifier.
Front LED controller
The output stage is created around four pairs of complementary PNP/NPN BJT BISS (Breakthrough In Small Signal) transistors: PBSS305PZ and PBSS305NZ. These are bipolar transistors with a very low Vce-saturation, meaning that heat dissipation will be lower due to a better efficiency. Transistors temperature is controlled by the PCB itself, but also by a small aluminum heatsink attached on top via double-sided self adhesive thermal pads secured with three screws by the PCB. These small and fast transistors are able to dissipate around 1.5 Watts each, max. amperage driven being up to 10 Amps (short term).
The eight output stage transistors (covered with thermal conductive tape)
and the corresponding goldish electrolytic decoupling capacitors
One of the four complementary pairs of the output stage transistors
All the logic and controls are takes care by a dedicated ARM Cortex microprocessor operating up to 48 MHz STM32F072C8. It has a CAN bus, but also an ADC and a DAC inside, so it can accept both analog and digital signal inputs at the same time. This way the microcontroller is able to read all the states about A90D’s operation like stand-by, volume level, gain, inputs & outputs, inside voltages, DC output etc. If a reading goes wrong then an error code is throwen on the LCD and device might enter in Stand-by.
There is also a UC25HQ40 that is a 4 Mbits ultra low power memory that communicates on SPI. This flash memory is probably used for storing the states for the front panel commands like In/Out, Pre/HPA, volume level, the two memories C1 & C2 etc., while A90D's firmware is stored inside the flash memory from the above STM32F072C8 microcontroller.
The STM microprocessor along with the memory flash chip
For DC-output protection there is a TL072C op-amp that detects any trace of DC voltage on headphones outputs and communicates this to the microprocessor that is instantly placing the amplifier in protection mode (-5) and cuts off the power.
TL072 op-amp responsable for triggering the DC-output protection
Preamplification and gain stages are built inside four NFCA (Nested Feedback Composite Amplifier) modules, each one containing several SMD transistors and passive components too Close-up pictures will be soon revealed once the new version of A90 Discrete amplifier will appear on the market, given that Topping decided to get rid of the outter case for its NFCA modules (probably due to this). Till then, below is an inside picture of these modules grabbed from manufacturer website.
The big blue adjustable multi-turn resistors from top of the NFCA modules are there for adjusting the output DC by the manufacturer. Those adjustable resistors are sealed on top and trying to manually adjust any of them will add DC voltage on the output of the amplifier and pops and click may occur, but this will also void the warranty of the device! I measured 0 mV (zero mV!) on the XLR 4-pin plug, on both channels, with no music playing, so my A90D headamp was perfectly adjusted by the manufacturer. However, if you hear any random pops and clicks on the A90D headamp, just email the manufacturer and wait for a resolution on this, but don’t try to handle this by yourself by adjusting those resistors!
Multi-turn resistors responsable for adjusting the DC-output by the manufacturer
The inside temperatures are quite good, hottest spot was the side with the outputs from the Mean Well power supplies at around 47-48 C, while the other faces of the power supplies were having temperatures between 35 and 41 C. The heatsink from the output transistors was oscillating between 40 and 42 C and the NFCA modules were pretty contant between 39 and 40 C. With case closed the above temps will definitely raise a little bit, probably with 5 C or maybe a bit more, but given that the top of the case stays below 35 C, I consider this headamp as being on the warm side rather than on the hot side like other headlamps I own (Matrix HPA-3B and Burson 3X GT 2023 are getting to the range of 45-50 C on the sides or top of the case, depending on the output volume).
Temperature measured directly on the heatsink
Temperature of the output stage while simultaneously driving with high SPL
two planar headphones (70 Ohms LCD-2 Fazor and 25 Ohms Fostex TH-X00 ebony)
two planar headphones (70 Ohms LCD-2 Fazor and 25 Ohms Fostex TH-X00 ebony)
Temperature on the NFCA modules
Lowest temperature on the power supplies
Highest temperature on the power supplies
PROs:
- SOTA headphone amplifier with a SINAD of almost 120 dB is more than ears I would ever need.
- Matches all the planar and dynamic cans I have on the lowest gain, even my 600 Ohms balanced DT880 were driven at very high levels with lowest gain and volume set between 70...80 out of 99.
- Perfect channel matching while zapping between volume states, thanks to the relays array that takes care of adjusting the output volume in 100 output levels.
- The PREAMP function helps me drive active speakers and control the output level as well. I am more than happy with the PRE + HPA setting, because this gives me the option to connect the active speakers to the PRE-Out, while before that I needed to swap the XLR cables from the external headamp with the ones from the speakers to be able to listen to speakers or to use a passive In/Out XLR device. Now, since I got the A90D, all I need to do is to switch between active speakers and headphones (or have them both) from the remote control.
- The top of the case doesn’t get hot like I was used to with most headamps I had or I still have around. I’m talking about below 35 C after hours of continuous operation in a 21- 22 C bedroom, and having a D90 DAC on the bottom of the A90D that also gets some heat generated too.
CONs:
- The vertical angle of the remote control is just too narrow for my taste. I measured about 20 degrees, while my D90’s remote has more like 160 degrees on the vertical plane. Horizontally instead I get over 170 degrees for the A90D while the D90’s remote has almost 160.
- Also, I would have liked it more if the remote's volume control would be more fluent, more linear, instead of jumping so fast between volume states after a long press of about half a second. So, I would like to see a lower waiting time between when I firstly press on the + or - button and when the action becomes repetitive (maybe a quarter of a second instead of a half), but at the same time a lower speed in incrementing/decrementing the volume states would be lovely too. Hope a new firmware update might be able to help with my request, although I’m aware that most customers might be more than happy with how the remote control works right now.
- I don’t think I’m quite in love with the ease of switching to the Highest gain setting for the PREAMP, although I am aware that this is a feature implemented in many PREAMPs, but I'd prefer a harder way for passing over 4-5 V RMS on the XLR 3-pin output plugs (a long press on Gain or maybe a hard switch). I’m more...safe in this area and I’d prefer to stick with a max. output level of 4-5 V RMS for my active speakers, so I’d prefer to increase the gain not so easily for the PRE and I think for the HPA too.
