Feixiang FX-98s PCM2704 + XR1075 +QS7779 USB DAC - diyAudio
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Old 5th September 2013, 05:54 AM   #1
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Default Feixiang FX-98s PCM2704 + XR1075 +QS7779 USB DAC

This thread is to discuss everything about the Feixiang FX-98s PCM2704 USB DAC, including reviews, upgrades, mods, problems, troubleshooting, etc.

The FX98s is a budget USB DAC with a Burr-Brown PCM2704 DAC, XR1075 BBE, QS7779 Virtual Surround decoder, surround, bass, treble and volume controls, straight-through line amplifier (line in to line out) and headphone amplifier - all in a compact extruded aluminium chassis with good finish and production values.

It is upgrade friendly in a few areas - electrolytics and opamps, which are generally the only things that need upgrades, if at all. The pics below show the board and a few upgrades that I performed - the only upgrades that are really required are the opamps (2x NE5532 upgraded to LT1208 and LT1364) and C8, the analog regulated voltage bypass for the PCM2704 (upgraded from Rubycon YXA to Rubycon Black Gate PK). I upgraded some additional electrolytics in the signal path to Nichicon Muse SW and Black Gate PK, though not strictly required for a first-level upgrade.
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Old 10th September 2013, 01:50 PM   #2
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I joined just for this. I recently ordered a FX-98s, I expect it sometime next week. I have very little experience in the DIY Audio world (newb) however I have replaced capacitors on two of my TVs in the past, and modified a few gaming consoles close to a decade ago. Thank you for taking the time to report your upgrades... any chance you can post pics of the unit with the completed mods annotated?
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Old 12th September 2013, 06:03 AM   #3
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The full list of the mods performed so far:

1) C8: 47uF Rubycon YXA replaced by 47uF/25V Rubycon Black Gate PK. This decouples an internal regulated voltage for the DAC, so it might be worth putting a small SMD film cap (on the underside) to bypass it.
2) C28: 470uF/16V Sanyo WG replaced by 220uF/4V Rubycon Black Gate PK (this bypasses a regulated 3.3V rail for the PCM2704 DAC, so a 4V or 6.3V rated cap is fine).
3) C9, C10, C13, C14: 4x 10uF/16V Nichicon VR replaced by 2x 3.3uF/50V Wima MKS2XL. The original caps were back-to-back electrolytics in +--+ configuration. I tried 4x 10uF/25V Nichicon Muse SW initially, then 4x 4.7uF/50V Elma Silmic II. Both are very nice alternatives, but the Wima film cap is the most transparent. One Wima replaces C9-C13, and the other replaces C10-C14, with some lead bending to mount each lead in the +ve terminals of each location.
4) C64, C63: 2x 2.2uF/50V Panasonic M replaced by 2x 4.7uF/50V Black Gate PK. The Wima MKS2XL is another alternative, but will require lead(s) to be bent inward to fit the 2.5mm pitch, and it will be a tight squeeze.
5) C24, C25: 2x 10uF/16V Nichicon VR replaced by 2x 10uF/25V Nichicon Muse SW. This is a bypass for the QS7779 virtual surround chip, so this isn't important.
6) C30, C31: 2x 2.2uF/50V Panasonic M replaced by 2x 4.7uF/50V Elna Silmic II. This is in the signal path between the QS7779 and XR1075 BBE chip, so it's only relevant for the surround capability.
7) C52, C53: 2x 2.2uF/50V Panasonic M replaced by 2x 4.7uF/50V Black Gate PK. This is in the line-out to RCA signal path as the DC blocking cap, so it needs the best cap that will fit in the tight space - Wimas won't fit there.
8) C40, C41: 2x 2.2uF/50V Panasonic M replaced by 2x 10uF/25V Nichicon Muse SW. This is in series with 2x 1uF SMD ceramics that lead to the volume control RP3, so it's only relevant for the headphone-out section. That ceramic will dominate the sonics anyway, so it should probably by shorted out or replaced by a 1uF film cap, but there's no space and it's a 0805 footprint, so I left it alone for now.
9) U7: Dual NE5532 Opamp replaced by LT1364 (headphone driver)
10) U6: Dual NE5532 Opamp replaced by LT1208 (line-out amp). Also tried LME49860, LT1213 and LT1364 - and the LT1208 has the best soundstage and airiness here, but YMMV. A wide variety of opamps are possible candidates for this location, but they should have reasonable low-rail performance, since there's only +10.8V from rail-to-rail for the opamp. AD8022 and LM6172 (mounted on a SOIC8 to DIP8 adapter) are also worth considering, but the fit is tight because of C54 right next to U6.

(For use as a DAC alone, only the first three mods are really needed, apart from the opamp upgrades. In most locations where a Black Gate PK has been used, an Elna Silmic II, Nichicon Muse SW or Wima MKS2XL are easier-to-obtain alternatives, but may not be available in the exact same value or physical size/pitch ).

Last edited by linuxguru; 12th September 2013 at 06:13 AM.
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Old 12th September 2013, 08:23 PM   #4
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Wow. Very detailed, thank you. I look forward to getting this in, cracking it open, and determining how much of this I can apply on my own. Thank you again.
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Old 15th November 2013, 12:54 PM   #5
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Here's a face-transplant for the FX-98s - the original brushed/machined aluminium bezel was fine, but it looked too stark and sterile. So I decided to try a teakwood bezel just for fun. I'm a noob at woodworking, and managed to mess up the countersinking - several of the countersunk holes for the knobs are off-center and irregular ellipses, but finally it just survived being trashed.

Click the image to open in full size.

The thing that saves the day is the juxtapositioning of the post-modern industro-tech theme of machined aluminium knobs, stainless steel allen bolts, brushed black body, etc., against the decidedly old-world theme of a polished teak bezel with a slightly coarse finish and prominent dark grains.

Last edited by linuxguru; 15th November 2013 at 12:57 PM. Reason: fixed link to pic
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Old 29th December 2013, 10:31 PM   #6
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Finally it was my purchase. Soldered the power supply (on the toroidal transformer, with filters, in general, all as it should be). Turn the power switch on the headphones - all ok. But as soon as I connect cable USB - headphones comes high frequency noise (hiss). What does the noise only when the cable is connected. With the increase in volume on the controller - the noise level also rises.
Some one have any idea how to fix it?
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Old 4th January 2014, 03:35 AM   #7
roggom is offline roggom  United States
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Thanks for the great write-up. I recently purchased this amp as well. I did not realize that the RCA was straight thru, as I wanted to use it as a preamp with tone control.

I tried this unit as a DAC, but as Skromnjaga my amp has some severe hiss. So with line in it is acceptable.

I am using this unit as a preamp for a small DIY tubamp and its works ok for now.

Was wondering if you have a schematic? For the money it works nice as a headphone amp, but I really dont need that feature.

My goal is to reduce the gain of the headphone stage, for a more suitable output to another amp. I did roll a cheap JRC4556 into the headamp circuit and it smoothed the sound out. I also had a BB OPA2134, it was decent but not crisp.

Not sure of the approach, I can try to attenuate the incoming signal from the RCA through R47 and R48, but would like to find which resistor is the feedback and just parallel a resistor to allow more negative feedback

So far I changed C8 from a Nippon 47/25 to a Nichi FC 47/50 bypassed with a .1 on the bottom.

Any assistance is greatly appreciated.

Thanks
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Old 4th January 2014, 04:23 AM   #8
roggom is offline roggom  United States
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Ok, tackled my gain issue, I added 10k under the board, U7 pins 1 to 2 and 6 to 7 (negative feedback), this dropped the gain enough for me to get some travel through the knob without clipping. It also took some of the hiss away (well because of the db loss), but I will try to attack the hiss tomorrow. The hiss is most prevalent when the "SRS" is off or all the way on. In the middle its not as bad, but the whole time delay reverb is quite annoying.

All in all this is a fun little amp, the cost is less than a raw case, knobs and pots.

Last edited by roggom; 4th January 2014 at 04:25 AM.
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Old 6th January 2014, 09:55 PM   #9
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Quote:
Originally Posted by roggom View Post
Ok, tackled my gain issue, I added 10k under the board, U7 pins 1 to 2 and 6 to 7 (negative feedback), this dropped the gain enough for me to get some travel through the knob without clipping. It also took some of the hiss away (well because of the db loss), but I will try to attack the hiss tomorrow. The hiss is most prevalent when the "SRS" is off or all the way on. In the middle its not as bad, but the whole time delay reverb is quite annoying.

All in all this is a fun little amp, the cost is less than a raw case, knobs and pots.
Can you make photo of your modded PCB?
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Old 8th January 2014, 02:12 PM   #10
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(I don't find how to edit my previous post).
I searching datasheet for XR1075 for a long time, but find only this schematic^
Click the image to open in full size.
Then on the back side of the board i find soldered jumper pins. Unsolder of it turn off the XR1075. Hiss become lower, and so as to reduce the overall level of the signal. Now i can hear not only hiss. There is another signal: squeak somewhere at 1 kHz (i try to make record https://dl.dropboxusercontent.com/u/57533518/Record.mp3 )
I read some forums about PCM 2704 and noises: source of noise can be power subsystem and quartz resonator of DAC. The problem is that I do not have an oscilloscope to check it.
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