I admit I did not give ASUS the benefit of the doubt and seriously consider their Essence STX soundcard as a replacement for my Onkyo SE200-PCI. ASUS make nice motherboards, but unlike Onkyo have no previous expertise in high end audio.
I am happy to report - a bit late in the game, the card came out in 2009 - that they've done a really good job with it and the drivers for Windows 10, technically still in beta, work just fine.
Asus updated the design recently to the STX II. The PCB has been redone, but the only visible change is the PCM1792A DAC has been moved towards the top of the card closer to the IV conversion op amps. An second LDO regulator IC, U34, empty on the STX, is now populated. A "TXCO" clock source is added next to the ASUS audio controller IC. The four film caps next to the output IC are replaced with WIMA brand. It's basically identical, so it...
Current version is 5.60C, last update was 2012 to be compatible with Windows 8. Driver package 5.60C installs without issues on Windows 10 64 bit. The AudioDeck utility installs as well.
However, all is not well:
On installation, Immezio 3D effects are enabled. This locks the sample rate at 48 kHz. 44, 96, and 192 kHz cannot be selected. Deselecting the Immezio 3D effects prompts a reboot, but the Immezio 3D effects remain enabled after rebooting. The card is stuck at 48 kHz. 3D effects (which enables the DSP processing such as Qsounds, EAX, A3D) cannot be shut off.
I see three possible workarounds:
1. Find a command line switch, or edit the installation batch file to disable Immezio 3D on installation...
Posted 2nd October 2015 at 05:07 AM byrjm (RJM Audio Blog)
Updated 9th November 2015 at 01:44 AM byrjm
This is a headphone amplifier with digital inputs, not a DAC with a headphone jack. Though technically given equal board space, the headphone amp, with hot-running single-ended class-A output stage, is surely the centerpiece of the design. (The Asahi Kasei DAC, with MUSES01 for the I-V, is no slouch mind you.)
First impressions. It is large, solid, and very nicely made, but - after seeing the inside - rather simple, spartan even. From the DAC output to the headphone jack is just two op amps and two transistors, the op amps being shared between channels. A third op amp most likely just buffers the analog line output. Apart from the headliner MUSES01 op amp none of the parts are especially expensive, though many were clearly carefully chosen for sound quality - the 2SC5196 for example. The TE7022 USB receiver is a disappointment, as is, to be honest, the single set of power rails and the use of dual op amps shared between channels.
Posted 26th September 2015 at 11:49 AM byrjm (RJM Audio Blog)
Updated 9th October 2015 at 04:24 AM byrjm
My Onkyo soundcard drivers stopped working when I upgraded to Windows 10. Onkyo says they have no plans to release a patch, so I'm left with no high quality audio solution for my computer. Since I already have a good headphone amplifier, what I'm mainly looking for is a high quality line level analog output.
One options is another soundcard, the ASUS Xonar STX being the obvious choice. I dunno, it doesn't grab me.
I was thinking with going with an external box this time, connected via USB. As this opens up about a zillion options, I'm going to limit things to,
Respected audio brands with a solid reputation for digital audio.
Small enough to be placed on top of my computer case.
Posted 25th September 2015 at 01:56 PM byabraxalito Updated 26th September 2015 at 01:35 AM byabraxalito
Since I figured out the reason for needing all those caps in my earlier DAC designs was all brought on by using passive I/V, I'm now a total convert of active I/V in order to do away with the sheer bulk.
Having tried single transistor I/V and loved it, I found there was still some improvement to be gained by biassing the common-base transistor with additional current sources to reduce its input impedance. Since getting down to the region of 1ohm would require some 25mA of bias which isn't well suited to portable applications I decided to have a go at using feedback to obtain the impedance I'm seeking.
I'm not using an off-the-peg CFB amp because they still turn out to be fairly power supply quality susceptible (subjectively speaking) so here's a design I hope that greatly reduces the supply impedance requirements so that it can be used in a portable player.
The picture shows the second prototype I/V stage, coupled to a 6th order Chebyshev anti-imaging...
Right down at the bottom of the page the last filter he shows the schematic of is a 9th order Chebyshev, 1dB ripple, with a corner frequency of 1kHz. A textbook frequency response plot is obtained using LTC6241s. I latched on to this and tried changing the corner frequency to 18kHz, wondering if I could use such a design for an anti-imaging filter for my DACs. So I divided all the capacitor values by 18 and ran the sim. Disaster! The frequency response I obtained is below - a 7dB spike at 17kHz.
The problem seems to be inadequate Q - high order filters are composed of sections which increase in Q (more positive feedback) and the chosen opamps aren't fast enough (18MHz GBW). I went to a faster opamp for the highest Q stage which brought about some improvement...
Posted 14th September 2015 at 01:19 PM byabraxalito
I bought this amp because the case attracted me - no pics on the Taobao page were giving away anything about the insides, quite unusual. After receiving it I couldn't resist having a quick listen and it turns out its a hardwired tone control with bass and treble turned up to the max, about +16dB wrt 1.3kHz. What a surprise! - no matter as I was going to strip out the innards anyway to use as a test platform for my amp.
Posted 10th September 2015 at 01:28 AM byabraxalito Updated 21st September 2015 at 12:23 AM byabraxalito
In my search for opamps with better real-world PSRR behaviour, I came across this beauty. For now I'll just post up its simplified schematic and PSRR plot - if any of you know of it please put your deduction in the comments. I may add more clues later if nobody nails it early on.
The reason I find this part interesting is its cascoded output stage - I believe this is what leads to the 'hump' in the positive rail PSRR. I've never seen that behaviour on any other device.
Well over a hundred views now and not one single stab at the answer. Its the industrial-strength version of the now obsolete LM308, a Bob Widlar special with super-beta input stage. The output stage cascode I take it isn't primarily to improve the PSRR rather its due to the high maximum operating voltage (72V). TI does still have the DS on its website though it doesn't put in an appearance in parametric search as its been obsolete for a while. Even more interesting is its decompensated variant which...
On thinking over the sensitivity of the CS5381 to the drive and input filtering capacitor I decided to explore the capacitors further.
I was also interested to note that a number of manufacturers seem to recommend a range of different capacitor values here.
Purely because it was handy - i.e. right in front of me on the desk - I threw an extra 2nF capacitor across the differential input of the CS5381. Boom - the distortion dropped 6dB straight off.
I muttered a few choice profanities, which made me feel an awful lot better, then arbitrarily threw a 470pf NPO ceramic cap across that lot, just to be sure. Well given I was off doing such arbitrary things - why not?
On analysis, 5nF capacitance at 20kHz is about 1.5K Ohms (reactive) which is within the capability of the op amp to drive.
Which I found rather pleasing, as using the "default" 2n7,...