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Old 12th May 2009, 09:42 PM   #1
fotios is offline fotios  Greece
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Location: Δραμα - North Greece
Default Issues Of Asus Xonar Essence Stx

I bought this card beffore 2 weeks. I had already an EMU0404usb, but i turned in Essence STX for its top performance. I use the cards only as ADC-DAC converters for using with the Virtins MI 3.1 FFT analyzer.
Although the output section of STX is well designed and implemented, its input it is not so much. From the second day of use, i was faced with a failed input IC.
After this, i have examined the input circuitry of STX. I must to clear out, that the parts used are of top quality. The problem is found in circuitry.
The Line/Mic input jack socket, it is connected in a very good DPDT relay of NEC, from which in sequence is connected either in the DJ100 for Mic interface or in the 2 dual RC4580 single to differential converters for Line level interface. Lets to see the line in signal sequence:
The signal from the input jack, it is driven directly to the inputs of the two RC4580 one for the left and one for the right channel. Each half of 4580 is modified the one as voltage follower, and the other as unity gain inverter. Thus we get a ballanced (differential) signal from the outputs of each 4580. To my big surprise, there is not any protection for the inputs (such as clamping diodes) of 4580s! After the 4580s the signal it is driven in two dual 5532, which of the outputs are connected to the differential inputs of ADC CS5381. Because the inputs of 5532s presents a signifficant DC offset (i don't know how this happens, the only that i know it is that is used the suggested circuitry from the datasheet of CS5381... maybe from the Vq pin... is it normal this DC offset of some volts?) are AC coupled with the outputs of 4580s via 2 pairs of nice Nichicon gold electrolytic capacitors of 47F each one. I have an objection here. First, the value of these caps is very big. In datasheet of CS5381 is suggested the use of 10F instead the existing 47f on board. Second, the outputs of 4580s are directly connected in the negative pole of caps and the suggested 100K discharging resistors to gnd in the outputs of 4580 are omitted. Thus, the outputs of 4580 are continuously heavy capacitive loaded during each discharging cycle (or when a sinus get its maximum negative swing) of the 47F caps. To be sure, i checked this (with the sound card on my workbench powered from an external PC power supply) with my DSO, and there is a continuous ripple of 3,5mVrms on the outputs of 4580. I suspect that is the reason of the 4580 failure from the second day of use. Moreover, the only external device that i connected in Line input was a function generator with a 2Vpp sinus of 1KHz for calibrating the FFT analyzers, and one handy RMS voltmeter. The rest connections, was allways a stereo cable (2 RCA plugs to one TRS 6,5mm jack) for the hardware loopback test of FFT analyzers.
This moment, the sound card is on the way for the service departement of the shop of purchasing (ASUS does not offer its own service departement in Greece, it leaves this at the distributors). Tomorrow, i will get the reply from the technicians of shop. I fear that a hard debate it is expected, because also me i am for 25 years official service dealer of some very known proffesional audio brands and i know this proccess very well.
I will inform you.

Fotios
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