FraPia-62
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- Jul 3, 2024
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A friend of mine gave me these speakers for repair if possible, or at least to understand what was happening. According to him, the speakers would operate for a couple of minutes, then the sound would fade away progressively to 0. After a few tries and a firmware update the speakers no longer worked and there was no sound at all. He asked the KEF service center in Switzerland for a repair and they proposed either the replacement of the master MAMP board for a total cost of 506.00 CHF (562.00 USD) or the newer KEF LSX II for a discounted price of 850.00 CHF (944.00 USD) instead of 1299.00 CHF (1442.00 USD). Well, he declined both, I would have declined too given the prices.
I started by connecting the speakers to the mains (there's no power switch), and they went into standby mode normally (the master speaker LED glows orange). Then I turned them on with the remote; indeed, there is no sound, and the LED of the master speaker started blinking red/orange. In the manual, there are a lot of color codes indicating various operation modes, but, guess what? Blinking red/orange is not shown anywhere in the manual. I then tried to reset the speakers, but nothing happened.
When I was about to send the LED code to Bletchley Park in the hope that they could break it with their bombe, I found on the net a possible meaning: general failure. I then opened the master speaker, and I took the occasion to take a few photos. In the photos below, you see the inside of the master speaker, showing the main board and the SMPS.
With no documentation and no schematic diagram, I started by checking the voltages. Bingo! A +12V from U20, a LM29150-12 voltage regulator, is missing. This voltage regulator receives voltage from a pre-regulator made with a transistor (Q211) and Zener diode (Z203), which in turn should receive the voltage from the power supply of the power amplifier, but no voltage was present. Looking upstream, I found capacitor C433 cracked and the RF choke L236 completely burned out. C433, a 1206 ceramic capacitor likely X5R, is almost shorted (it measures a few Ohms).
I then replaced C433 (I used a 1210 2.2uF 100V capacitor since I had it readily available) and L238, the latter of which required some work since the PCB was burned out and the solder pad evaporated and no longer existed. Here I used a 1206 RF choke such that it was long enough to reach the via close to where the pad was. After scratching away some solder mask, I could solder the RF choke in place. After the repair, the speakers turned on normally and seemed to work. Cost of the spare parts: a few cents. I really don't find that the offer from the KEF service center was justified; come on, it's two inexpensive components, there's no need to replace the whole MAMP board!
Now the most difficult part comes: install and use the app. Here, I had the exact same experience as @amirm. In the end, I managed to get it working, but hey, KEF/GP Acoustics, not everybody has a Ph.D. in computer science! Maybe you need to do some work here.
The next difficulty is feeding some signals to the speakers. I had available either analog audio (balanced +4dBu on XLR connector), AES3 (again on XLR connector), or AES67/RAVENNA. Guess what? The KEF LSX speakers don't accept any of them. There is an optical input that would accept S/PDIF, but I had no AES3-to-optical adapter. Since the LSX has an Ethernet connection, I was hoping that it would accept AES67, but nope. Finally, I found a nonprofessional USB sound interface with RCA outputs and an RCA to 3.5mm mini-jack cable, and everything worked very well. Awesome sound, well done, KEF!
Finally, I checked the cracked capacitor under a microscope. Although it is difficult to be sure with a simple inspection, it looks like a 16-volt device. There is good agreement that the operating voltage of ceramic capacitors should be derated to about 50% of the rated voltage to achieve both good reliability and avoid voltage coefficient effects. A 64V capacitor would have been a correct choice here, but the one that I found in the speaker was definitely not 64V. Well, given how many users are complaining on the Internet about this problem with these speakers, I guess that a batch of capacitors with the wrong voltage rating have been mounted, or maybe even specified (yes, it happens!). Given how expensive these LSX speakers are, I find that KEF should have provided a goodwill repair, even if the guarantee has lapsed.
Enclosure
As you can see from the photos above, the enclosure is made mainly of plastic, with the sides covered with thin wood wrapped with cloth. A brace, not visible in the photos, mounted just behind the magnet of the loudspeaker supports the sides and the top of the enclosure. Although the plastic is rather robust and the enclosure is well made, I would have expected more wood at this price point. The plastic itself is white and is painted to the desired color. No screws can be seen from the outside; they are hidden by the back cover, which is pressure-mounted, as you can see in the photo here. This gives the speakers an elegant and clean aspect.
Four antennas are used. The BT antenna is the one on the low right, while the PurePath antenna (for transmission to the slave speaker) is on the top of the enclosure. Both are attached with adhesive tape. The two WLAN antennas are mounted, one on the top left of the cabinet with two self-threading screws and the other on the top right of the brace with plastic pins.
Loudspeaker
The next three photos show the loudspeaker driver. It is a coaxial driver with 4 ohm impedance for both the woofer and tweeter. The ceramic magnet is rather large, with the top plate about 3.5mm thick. The woofer has a 38mm/1.5in voice coil with an overhang of about 3mm, which should lead to an Xmax of about the same. You can see the voice coil in the photo; it is visible through the pressure relief opening. The basket is made of stamped steel sheet, painted black, and looks robust and well made.
The bass-reflex tube, not visible in the photos, is flared on both sides. It is rather long, with two 90° bents to make it fit the small box. The length of the tube suggests that likely a 6th-order alignment was used. The wires to the speakers and LED plate are covered with foam, likely to prevent buzzing.
Electronics
The next photos show the complete MAMP board, a zoom in on the various areas, and the back of the MAMP board with the connectors.
The main components mounted on the MAMP board are these:
The following photo shows the WLAN module with its two antennas and the PurePath module.
The top and bottom of the power supply unit are shown in the next two photos. The PSU actually includes two power supplies, a low power stand-by supply (the small yellow transformer and 7-pin IC close to the main transformer) and the main power supply.
It uses Infineon IPD60R360P7 (600V 360mOhm) MOS transistors and lots of SAMXON capacitors.
To conclude, if you ask me if I would buy these loudspeakers, well, the answer is no. These speakers are actually an audio player with an overly complicated cellphone-based user interface. What will happen to them in a few years from now? Will they still be supported? Will the app be ported to newer cellphones with newer operating systems? I would definitely prefer more normal active loudspeakers with an analog balanced XLR3 input and either AES3 or AES67 digital input and that don't require any apps to be controlled.
I started by connecting the speakers to the mains (there's no power switch), and they went into standby mode normally (the master speaker LED glows orange). Then I turned them on with the remote; indeed, there is no sound, and the LED of the master speaker started blinking red/orange. In the manual, there are a lot of color codes indicating various operation modes, but, guess what? Blinking red/orange is not shown anywhere in the manual. I then tried to reset the speakers, but nothing happened.
When I was about to send the LED code to Bletchley Park in the hope that they could break it with their bombe, I found on the net a possible meaning: general failure. I then opened the master speaker, and I took the occasion to take a few photos. In the photos below, you see the inside of the master speaker, showing the main board and the SMPS.
With no documentation and no schematic diagram, I started by checking the voltages. Bingo! A +12V from U20, a LM29150-12 voltage regulator, is missing. This voltage regulator receives voltage from a pre-regulator made with a transistor (Q211) and Zener diode (Z203), which in turn should receive the voltage from the power supply of the power amplifier, but no voltage was present. Looking upstream, I found capacitor C433 cracked and the RF choke L236 completely burned out. C433, a 1206 ceramic capacitor likely X5R, is almost shorted (it measures a few Ohms).
I then replaced C433 (I used a 1210 2.2uF 100V capacitor since I had it readily available) and L238, the latter of which required some work since the PCB was burned out and the solder pad evaporated and no longer existed. Here I used a 1206 RF choke such that it was long enough to reach the via close to where the pad was. After scratching away some solder mask, I could solder the RF choke in place. After the repair, the speakers turned on normally and seemed to work. Cost of the spare parts: a few cents. I really don't find that the offer from the KEF service center was justified; come on, it's two inexpensive components, there's no need to replace the whole MAMP board!
Now the most difficult part comes: install and use the app. Here, I had the exact same experience as @amirm. In the end, I managed to get it working, but hey, KEF/GP Acoustics, not everybody has a Ph.D. in computer science! Maybe you need to do some work here.
The next difficulty is feeding some signals to the speakers. I had available either analog audio (balanced +4dBu on XLR connector), AES3 (again on XLR connector), or AES67/RAVENNA. Guess what? The KEF LSX speakers don't accept any of them. There is an optical input that would accept S/PDIF, but I had no AES3-to-optical adapter. Since the LSX has an Ethernet connection, I was hoping that it would accept AES67, but nope. Finally, I found a nonprofessional USB sound interface with RCA outputs and an RCA to 3.5mm mini-jack cable, and everything worked very well. Awesome sound, well done, KEF!
Finally, I checked the cracked capacitor under a microscope. Although it is difficult to be sure with a simple inspection, it looks like a 16-volt device. There is good agreement that the operating voltage of ceramic capacitors should be derated to about 50% of the rated voltage to achieve both good reliability and avoid voltage coefficient effects. A 64V capacitor would have been a correct choice here, but the one that I found in the speaker was definitely not 64V. Well, given how many users are complaining on the Internet about this problem with these speakers, I guess that a batch of capacitors with the wrong voltage rating have been mounted, or maybe even specified (yes, it happens!). Given how expensive these LSX speakers are, I find that KEF should have provided a goodwill repair, even if the guarantee has lapsed.
Enclosure
As you can see from the photos above, the enclosure is made mainly of plastic, with the sides covered with thin wood wrapped with cloth. A brace, not visible in the photos, mounted just behind the magnet of the loudspeaker supports the sides and the top of the enclosure. Although the plastic is rather robust and the enclosure is well made, I would have expected more wood at this price point. The plastic itself is white and is painted to the desired color. No screws can be seen from the outside; they are hidden by the back cover, which is pressure-mounted, as you can see in the photo here. This gives the speakers an elegant and clean aspect.
Four antennas are used. The BT antenna is the one on the low right, while the PurePath antenna (for transmission to the slave speaker) is on the top of the enclosure. Both are attached with adhesive tape. The two WLAN antennas are mounted, one on the top left of the cabinet with two self-threading screws and the other on the top right of the brace with plastic pins.
Loudspeaker
The next three photos show the loudspeaker driver. It is a coaxial driver with 4 ohm impedance for both the woofer and tweeter. The ceramic magnet is rather large, with the top plate about 3.5mm thick. The woofer has a 38mm/1.5in voice coil with an overhang of about 3mm, which should lead to an Xmax of about the same. You can see the voice coil in the photo; it is visible through the pressure relief opening. The basket is made of stamped steel sheet, painted black, and looks robust and well made.
The bass-reflex tube, not visible in the photos, is flared on both sides. It is rather long, with two 90° bents to make it fit the small box. The length of the tube suggests that likely a 6th-order alignment was used. The wires to the speakers and LED plate are covered with foam, likely to prevent buzzing.
Electronics
The next photos show the complete MAMP board, a zoom in on the various areas, and the back of the MAMP board with the connectors.
The main components mounted on the MAMP board are these:
- Analog Devices ADAU1451WBCPZ: 32-bit DSP, clock up to 294.912MHz
- STM STM32F030: 32-bit Microcontroller, ARM Cortex, clock up to 48MHz.
- CIRRUS LOGIC CS4272-CZZ: 24-bit 192kHz Stereo Codec, THD+N=100dB (ADC and DAC).
- TI/Burr Brown PCM1754: 24-bit 192kHz Stereo DAC, SNR=106dB.
- Cirrus Logic/Wolfson WM8805GEDS: Audio transceiver, S/PDIF, I2S, 8 channels.
- Nisshimbo (NJR/JRC) NJM2068: OpAmp, GBP=27MHz, SR=6V/us, EIN=1.4uV (RIAA, Rs=2.2kOhm) (4558 derivative).
- Microchip CY920-C: Wi-Fi and Bluetooth (optional) Network Media Module.
- Qualcomm/CSR CSRA64215: Bluetooth 4.2 BLE.
- TI CC8520: PurePath 2.4GHz, 2-channels wireless digital audio streaming (proprietary).
- Texas Instruments TPS5430: 3A, 500kHz step-down (buck) converter.
- Texas Instruments TPS54260: 2.5A, 2.5MHz step-down (buck) converter.
The following photo shows the WLAN module with its two antennas and the PurePath module.
The top and bottom of the power supply unit are shown in the next two photos. The PSU actually includes two power supplies, a low power stand-by supply (the small yellow transformer and 7-pin IC close to the main transformer) and the main power supply.
It uses Infineon IPD60R360P7 (600V 360mOhm) MOS transistors and lots of SAMXON capacitors.
To conclude, if you ask me if I would buy these loudspeakers, well, the answer is no. These speakers are actually an audio player with an overly complicated cellphone-based user interface. What will happen to them in a few years from now? Will they still be supported? Will the app be ported to newer cellphones with newer operating systems? I would definitely prefer more normal active loudspeakers with an analog balanced XLR3 input and either AES3 or AES67 digital input and that don't require any apps to be controlled.
