QLS QA-550 SD card / WAV file player +DAC +AMP

I bought this player after reading some enthusiastic reviews online.


The main IC is http://ww1.microchip.com/downloads/e…Doc/70286C.pdf

QLS qa550

Unfortunately, the spdif signal from this device has lots of jitter and it sounded much worse than the Pioneer player I was using. I set about upgrading various parts like the oscillator, power, regulators, capacitors, screen, etc. Unfortunately, it seems the output is crippled by the jitter added by the CS8406 spdif transmitter and I gave up. In its original state it used 163mA during start up, then 154mA while playing, and a 140-something idle.

Here is a shot of the bigger screen :

QLS screen

MDL(S)=16168-1D Screen.

After 2 years gathering dust, I’ve now decided to try again. This is its current state :

I2s Output Only

I2s Output Only

The input diodes, 7805 regulator, CS8406 and associated spdif parts have been removed so input power must be 5V and this feeds the mosfet http://www.fairchildsemi.com/ds/FD/FDS4435.pdf. The diodes pads are connected through two 2A ferrites (not in the above photo).

The clock has been replaced with a better quality tcxo. It is is powered through a capacitance multiplier to give a low noise supply (5.6K and 22K resistors create a potential divider, the middle of which feeds the base of a BC550 and there is a 100uF cap to gnd to filter noise into the base) .

The 3.3V regulators for the Microchip IC and the CS8406 have been upgraded to ADP151.

The CS8406’s regulator powers a 74VHC04 I2S buffer which enables longer I2S lines. The I2S output is through an RJ45 LAN socket.

Initially, the sound was distorted and this was traced to a problem with the data line, and this was fixed with a 1nF film cap to ground on the input to the 74_04. The I2S output is significantly better than spdif.

Input / Output Sockets

Input / Output Sockets

In the space left by the cs8406, I fitted a basic DAC, a Wolfson WM8727. There are much better DACs: the ESS Es9023 is far better, but only the Wolfson and the CS4334 are as small and use so few external components. Also, the data is 16 bit so that is going to limit performance to 96dB SNR, DNR, etc, anyway. So the only real downside is THD+N @ 84dB.

I fitted a TPS793475 regulator to power the DAC. Again, not a great regulator but it has a very low dropout so it can regulate at 4.75v from as little as 4.9V and it is good enough for the DAC. It is connected before the mosfet so it should not affect the digital circuit. The DAC has a 2.2uF X7R cap across its power pins and a 100uF BC036 cap very close.

Output is through two of my last 1uF Blackgate N non-polar caps. It’s very simple and worked immediately, with no pops or noise, and pretty decent sound.

After that, I removed a section of the pcb between the I2S output and power input so I could fit a mini-amp, a class-D PAM8403 from Ebay. This broke the ground plane needed for the device to function  so the ground plane was re-created with a copper sheet. Class-D amps produce lots of switching noise which will affect the digital output so it’s important it can be turned off. The PAM8403 amp has an on/off switch just for this. The output is bridge-tied-load so that uses four lines through the mini-xlr output socket.


So now the QLS has become a usb-powered sd card player with digital I2S and amplified analogue outputs, and only needs a pair of speakers for a complete system.


Some fiddly DIY but it fits


Finished Player