Apple Airport Express USB Audio Output Mod
The Airport Express is Stereophile 2014 recommended component [link]:
Apple AirPort Express: $99 $$$ ✩
While the Airport Express works only with iTunes v4.6 or later (running on both PCs and Macs), is limited to 16-bit data, and functions only at a 44.1kHz sample rate, the combination of iTunes and the Airport Express offered an easy way to pipe CD-quality music around the entire home. “The beauty of this unassuming component,” said JA, “is its S/PDIF data output, which allows the Airport Express to assume a respectable role in a true high-end audio system.” However, its lack of an internal clock can lead to the first couple of seconds of songs being missed with DACs that are slow to lift their mutes.
Its performance can be enhanced several notches if the I2S digital output can be tapped.
I have been contemplating modding the Airport Express since at least 2009 [link], but never got around doing it.
Now, adding an additional “Airplay” input to the audio system is a desirable option, especially for an “iOS household” like mine. With multiple built-in switchable inputs available in the Sabre-32 DACs which are easily implementable under software control [link], there is even no need for an external switching device.
Certainly no modding is necessary since the AE already has a Toslink output. But for diyers, there is always something to mod 🙂
For this model of AE, tapping into the I2S signals is not feasible since it is based on the PCM2705 which only supports SPDIF output. (for I2S output, you would need a different PCM chip (PCM2706 or PCM2707) or use a newer version of Airport Express that uses an I2S based DAC such as the second and third generation AE (the first generation “N” and second generation “N”. The original was 802.11 “G”).
Best Model for Audio?
Reports seem to indicate that the original AE is probably the best one to use for audio and for modding…
The original Apple Airport Express was introduced in 2004. According to Wikipedia,
The original version (M9470LL/A, model A1084) was introduced by Apple on June 7, 2004, and includes an analog–optical audio mini-jack output, a USB port for remote printing or charging the iPod (iPod Shuffle only), and a single Ethernet port. The USB port cannot be used to connect a hard disk or other storage device.
The original AE (“G” version as in 802.11g) seems to have the “best” audio performance of all the AE models.
1- Tests published at CA shows that the “First Gen N” version (the follow-on version to the “G” version) has better audio than the current “Second Gen N” version of AE [link]
…To make a long story short, yes, the AE First Gen will have a lower jitter and better sound with your outboard DAC, than the AE Second Gen. An outboard DAC can only suppress incoming jitter, but can never remove.
2- The “G” version in turn has better audio performance than the “First Gen N” version of AE:
I have both a wireless G model & a wireless N model. My DacMagic frequently lost lock when used with the N but is rock solid with the G. At the time I was struggling with this issue, forums and such indicated that the jitter in the N model was much higher than the G…
OPENING THE AIRPORT EXPRESS
For reference here is the “original” Airport Express take-apart post: [link]
The two halves of the Airport Express are bonded together with some kind of crazy glue. The only option is to cut it apart. But rather than brute force, I preferred an approach with some finesse…:
Several passes were required in order to gauge the appropriate depth and avoid cutting into any internal components. In particular the power supply wires are butting against the casing.
The case can still be used after cutting the the case apart. Apple is a master in industrial design. We’ll try to save the case.
Inside the Airport Express.
Certainly a cleaner approach than what iFixit did 🙂 [link]
The switching supply was made by Samsung in their China Dongguan factory (Dongguan is about an hour drive from ShenZhen). Nowadays, Samsung is likely not building “low-end” components anymore…
THE AUDIO BOARD
The AE is partitioned into two parts: the network board and the audio board. The audio board is a daughter card that plugs into the network board.
Removing the RF shield:
Removing the audio board:
The audio board in detail. Even for a simple DAC implementation such as this, the board is a 4-layer PCB.
The PCM2705 USB DAC
That is a lot of electronics in a $99 package!
COMPARING WITH OTHER AE
Here is the 2nd Generation (also knows as the first generation “N”) AE. This version connects to the on-board DAC chip with I2S
The DAC chip used in the second generation AE is the CS4344 (the 10-pin chip in the photo below)
Here is the 3rd generation AE (current generation) courtesy of Rogue Amoeba
Functionality without the audio board
I tried powered up the AE without the audio board in place. It turned out that the networking capabilities of the device work fine, but its Airplay capabilities were disabled. Seems the AE performs a power-on self-test and disables the Airplay capability if it finds the audio section not functioning.
According to iFixit, [link] the 3.3v and 5v supply wires are as follows:
- Black wires: ground. All three should be connected to the two ground wires from the power supplies.
- Red wire (middle): 5V, 0.7A power input.
- Orange wires (two on the right): 3.3V, 1.21A power input.
Functionality without the 5V supply
I removed the red wire from the connector to test whether the device would operate without 5V. (If this is the case, then replacing the power supply would be easier since we would only have to worry about a single supply -the newer AE only require 3.3v for operation)
The network part of AE works without the 5V supply. Now, lets see if putting back the audio board will enable Airplay… Airplay does not work without the 5V supply.
WHAT TO MOD?
Power supply replacement
The standard mod has been the power supply mod, replacing the switching dual supply with a linear dual supply. It is the most popular mod on the AE. In fact, there are commercial versions Apple Express with vastly enhanced power supplies such as La Rosita Alphas:
…and the Micromega Streamer products:
Notice the unique design of the Micromega: the antenna of the AE extends outside of the case for better reception (I suppose if you totally enclose the AE the WIFI reception would be very poor)
I found this project in the Vietnam AV Network. The user replaced the clock, power supplies, and output capacitors, plus a isolation transformer in the spdif output [link]
USB AUDIO OUTPUT MOD
I have not seen another reference to this mod. So I think it is the first 🙂
What if we can take the USB output, and feed it to an external USB interface? We can then try different devices such as the PCM2706 with both SPDIF and I2S output, or the TENOR TE7022 with similar capabilities. We can add isolation. Isolating the USB will greatly clean up the digital signal and isolate the more sensitive audio part from any digital noise in the AE, including noise generated by the network/wifi chips and related parts.
No need to replace the original PS?
Since we tapping into the USB Audio stream, a purely digital signal, there may be not much to gain in replacing the original PS, especially since we can later add a USB isolator for full isolation. In addition, being a communication device, we can be assured that the designers have done their work in ensuring no (at least unrecoverable) errors in the digital domain.
Undoubtedly, a PS mod would benefit the analog performance and even perhaps the conversion to digital audio, but for this mod, I think am going to leave the original PS in place.
Looking closely to the input power connector, we can see that the designers have taken good care in filtering out the noise from the switching supply. Aside from the EMI shield covering the components, we find that the power lines and ground are further isolated with ferrites:
Tapping to the USB lines
The USB data from the AE audio board is “readily available”. There are 4 vias where the the 4 USB signals can be tapped:
We can even leverage the built-in USB connector by first cutting the existing lines (which are only good for connecting to a printer) and tapping the USB signal from the Audio board. 5V and GND are already there, so only two wires are needed:
Here is the mod. I tried to move the wires away from any active circuitry. Zero cost and completely and easily reversible.
Cut the original signals to the USB connector (which we won’t use anymore since this is for printer sharing)
The metal shield covers everything including the USB wires
TESTING THE MOD
I used the iBasso D2 USB Portable DAC/AMP to test my mod. The USB interface is the PCM2706 and connects to the Wolfson WM8740 DAC. The PCM 2706 is part of the PCM 270x family, so it should work…
Airplay complains that there are no speaker connected to the AE. The device is just too smart. It senses if there is a plug in the output connector and prevents Airplay from connecting if there is nothing plugged in… No problem. Use any mini-plug into the output port.
Now Airport connects, I first try to to connect a portable speaker to ensure that everything is in working order. I get sound in the speaker…So far so good.
Now I connect the iBasso into the USB port of the AE… Nothing… The iBasso has a “link” LED and it is not lit. No connection to the iBasso…
OK, perhaps I need to reboot the AE with the iBasso connected… Reboot… Airplay does not show up in iTunes… This AE is just too smart. I figure that since there are two USB interfaces connected, the AE can’t figure out what is going on (there is no handshake/link to any of the two DAC chips) then the self test fails and the AE does export itself as a functional device…
Back to bench.
I theorize that the firmware in the AE is looking to one USB DAC to link up with. I would make sense to turn off the on-board DAC chip and allow a single device to hang off the “USB port”.
Pin 7 is the VDD (digital power) for the PCM2705. We can just lift the pin.
I didn’t even have to use the soldering iron. Just an exacto knife and a bit of pressure. There is also analog power, but I won’t bother lifting those.
Result: Yes! it works. After booting up the AE with the iBasso connected, immediately the link LED lights up. Airplay shows up in iTunes… Play a song… Ahhh… Music!
UPDATE (July, 2015)
Recently, low cost DACs and USB-I2S interfaces have been implementing with a new device: The SaviAudio SA9023/SA9027 [link]. No device driver is needed under Windows implying that it is compatible with USB 1.1 sound, the same as the PCM 2706 of the Airport Express.
The Teradak is a USB-I2S board using the SA9023. The chip is advertised to have jitter <100 psec. [link]
The HifimeDIY Sabre USB DAC has been updated with the SA9023 [link]. According to the specifications, no driver is required for Windows.
- Supports 32khz, 44.1khz, 48khz, 88.2khz, 96khz and 16 and 24 bits
- Works in USB Adaptive Isochronous mode (same as PCM2706)
The SA9023 USB interface chip can also be implemented as “Asynchronous mode” [link]
Notice a second 27MHz clock in addition to the 12 MHz clock. I am not sure if this configuration would work with the modded Airport Express.
The all digital amplifier SMSL Q5 uses the SA9027 for USB input. Since there is a single clock near the USB circuitry (upper right corner), it would be safe to assume that it is operating in Adaptive mode.
The SA9027 adds 32-bit support
I had a chance to try this device with the modded Airport Express. Sadly, it is not compatible. I does not recognize this device when connected to the USB port. I have not have a chance to try the SA9023 USB.