It has been quite some time since my last article on this blog. Quite a busy life (but that busy life pays this hobby, anyway). I tried to spend sometime few days ago to disassembly QLS QA-660 and planned some modification ideas. So here you go…

First, power supply simply my main target. QLS is using SMD diode from ON Semiconductor U1D. This fast recovery diode (rated <500 ns) and maximum at 1A/200V. Should be good stuff, but will see if we can have some room for improvement here. Soldering SMD will be a bit tricky, but as the solder pad is quite big, I think we can force to use better regular diode (if any).


The clock. This is definitely a very important part of the Digital Audio circuit. Based on discussion with Clark from QLS, this TCXO is using 3.3V supply. Two clocks are here: 22.5792 MHz and 24.576 MHz.


Each clock is powered with dedicated regulator and power supply section, and seems like dedicated ground plane.


SPDIF pulse transformer here. A basic one from Pulse, PE-65612NL. I’m quite sure some room for improvement by using a better pulse transformer.

4-SPDIF Transformer

Here is the pulse transformer from Scientific Conversion. The size is very small, around 1 cm x 0.5 cm in size and in 8-pins SMD package (a bit tough for soldering, probably gonna use PCB). For this one, I decided not to go with ‘audiophile brand’, but more likely to serious player in this digital industry.

4-SPDIF Transformer_2

Some filtering capacitors. Nothing really special, just some WIMA and (I think) Matsushita electrolytic.


Quite classic CS8406 Digital Audio Transmitter. I can see a very good and interesting PCB layout here.


I2S chip who gives I2S output via Ethernet-like connector (RG-45).


Some regulators. Nothing really special in here. I can see LM317, LM337, several LT1086, and even fix regulator like 7808 and 7806.


Definitely it can have benefit with better regulation and lower impedance design. But the tracing could be a bit pain.


The position of the regulator itself most likely quite crowded. Need to completely disassembly the PCB to get a complete view of the design. A LT1086 shown below.


If I’m not mistaken, this is sort of voltage drop resistor with a low value. Probably also will act as filter. Definitely worth to have it upgraded to a better one.


Custom power transformer from Primrose Audio. You can see the size different between the onboard toroidal and the new custom one. Oversize for sure, but I’m quite sure that’s not the idea of this upgraded power transformer 😉


Seeing the different from different angle. Definitely this is a massive upgrade in terms of the size and weight.


That’s all sort of basic idea of upgrading this QLS QA-660. More to come when the new chassis arrives. So I can start disassembly the PCB from the original chassis and trace more detail about which doing what and so on.