55TXi/AX5i modification
55TXi (AX5i) has i-link, quite good clock as well as 49TXi (it is necessary to have such stable clock for PQLS support (Pioneer quartz lock system), as in other case the sync process between player and receiver can have problems. Being designed with lower MSRP then top level receivers, this model uses quite good, but not High-End level elements, the modification task is more difficult. We cannot access the level of modified Top receivers with this upgrade, but we may access the level of well known processors like Anthem AVM20 at the first level of modification, or the level of modified Anthem AVM20 at the second level of modification.
55TXi uses good enough (used for example in Anthem AVM20 processor with quite positive results in sound quality) multi-bit delta sigma DAC AK4383 (AVM20 uses AK4382, they are the same, the difference is that AK4383 supports DSD data input mode). This DAC has more integrated parts then used in 49TXi PCM1704, and different architecture, it means we may modify only outside components. It has an integrated digital filter (24-bit x8 FIR), Switching Capacitor Reconstruction Filter (SCF) with full differential outputs, removing the need for AC coupling capacitor. What can be done to improve the sound quality? We need to track the audio signal path, replace OPAs as the most important elements for analog sound quality and some coupling capacitors. Let's do it, taking in consideration that we will use i-link as the best possible audio interface (period) for connecting of DVD player to our receiver. We have 3 boards (2 channels are shown) to do it - DACs board (DSP board as DACs are located on it), the main board (volume adjustment circuits and pre-out amplification) and finally, the power amplifier.
Critical elements under scripted in red color. We need to use Black Gate capacitors or Elna Silmic to replace the existing ones in the analog signal path way, as well as OPAs. Used in 55TXi/AX5i-53TX/AX3 NJM4565 (PDF file) has only 4V/mks slew rate and is a real "bottleneck" in the sound quality. Another one, used in the main board (mPC4570), is also a budget OPA, and athough it's slew rate is 7V/mks, it still should be replaced due to the sound quality. We can use OPA627 (and they should be used for level 2 modification, which we intend to publish later), but they will require additional double-to-single PCBs and this upgrade will be quite pricey. Our recommendation is - to use pretty good Burr Brown OPA2134 (taking in consideration the aspect that the same DACs and the same DSP are used in AVM20, then OPAs are the main difference in sound quality between Anthem AVM20 and 55TXi, but as we have i-link in 55TXi/AX5i, it should sound even better after the modification). That will be a reasonable match to AK4383 in sound quality and price levels.
The same modification can be done for 53TX/AX3/45TX/2011, it will sound audibly better using SPDIF (coaxial or optical) input, but as it uses for DVD-A and SACD multi-channel analog inputs (the best way to listen to high resolution format), the improvement in sound quality will be only due to the coupling capacitor replacement in the power amplifier and OPAs replacement in the main board.
Now you have the main ideas, and we will post photos and measures as soon as we make this modification. It is necessary to send the whole receiver to make it, as it is not so easy to remove a few boards without special skills and knowledge. There is a risk to damage your receiver as well, not for professional personnel, correctly using the service manual though. The prices for it, taking in consideration that OPA2134 is more affordable then OPA627, are quite interesting, but they are not ready at the moment.
You can make the upgrade by yourself, but in this case ALEX HARDWARE IS NOT RESPONSIBLE FOR ANY DAMAGES TO YOUR RECEIVER (before doing anything, read the DISCLAIMER, please) and you will not be included in the database for the future upgrades. Our prices are so competitive, we doubt that you need to hold the risk.
All procedures have to be done WITH DISCONNECTED POWER OUTLET, it means YOU HAVE TO DISCONNECT the power cord from the AC power supply! YOU CANNOT DO ANYTHING BEFORE THAT.