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.
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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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?
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 ).
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 ).
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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.
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.
An externally hosted image should be here but it was not working when we last tested it.
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.
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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?
Some one have any idea how to fix it?
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
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
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.
All in all this is a fun little amp, the cost is less than a raw case, knobs and pots.
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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^
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.
I searching datasheet for XR1075 for a long time, but find only this schematic^
An externally hosted image should be here but it was not working when we last tested it.
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.
I will post the pics within the next few days. It was a simple jump of the opamp. Now if you desire to use as a headphone amp, maybe try a higher value resistor as the increase of feedback lowers gain, but seems to push the noise floor a little higher (when using headphones). As a preamp, the sound is very transparent with the volume knob from low to mid level.
I do still hear the 1 khz, but I have not troubleshot with my scope yet. I honestly believe it is the PCM clock that is causing the noise.
I removed the XR1075 chip and the noise is completely gone, that is because it breaks the signal path , so I can only assume the origin is before the xr1075. I did some trace routing, but it is very difficult with black mask over the board.
I am sure with a little more troubleshooting, the noise can be illuminated, but my guess is the PCM/DAC must be disabled to do so.
Most of my Google searches for DAC hiss, suggested upgrading DAC's. There just has to be a way to illuminate it, either by upgrading to a much better opamp, or active filtering.
we will see.
I do still hear the 1 khz, but I have not troubleshot with my scope yet. I honestly believe it is the PCM clock that is causing the noise.
I removed the XR1075 chip and the noise is completely gone, that is because it breaks the signal path , so I can only assume the origin is before the xr1075. I did some trace routing, but it is very difficult with black mask over the board.
I am sure with a little more troubleshooting, the noise can be illuminated, but my guess is the PCM/DAC must be disabled to do so.
Most of my Google searches for DAC hiss, suggested upgrading DAC's. There just has to be a way to illuminate it, either by upgrading to a much better opamp, or active filtering.
we will see.
I got my FX98 yesterday and I had not much time.
I plugged it to a regulated 12V PS and to USB of my PC.
I tried it with Headphones (Beyerdynamic DT880 Edition 250 Ohm) only and the sound wasn't bad, and I heard no 1Khz Sound.
I downloaded the specifications of the BB2704 and I saw that the chip has SPDIF out at Pin 5 and 6.
http://www.ti.com/lit/ds/symlink/pcm2704.pdf
I will add a SPDIF out, that will be done fast.
My next test will be with line in and out.
Marc
I plugged it to a regulated 12V PS and to USB of my PC.
I tried it with Headphones (Beyerdynamic DT880 Edition 250 Ohm) only and the sound wasn't bad, and I heard no 1Khz Sound.
I downloaded the specifications of the BB2704 and I saw that the chip has SPDIF out at Pin 5 and 6.
http://www.ti.com/lit/ds/symlink/pcm2704.pdf
I will add a SPDIF out, that will be done fast.
My next test will be with line in and out.
Marc
Blast from the past! I joined just to see the pictures in the original post. I love this little unit. I have several of them. I have a question, though. All of my units are "FX98S PRO" and from FX-Audio's site they don't have the QS7779 QSound chip, but get their surround effects from an NJW1144. I don't have a 98S with the 7779. I do have a previous FX-Audio simpler DAC with the QS7779. As I recall, it is absolutely uncanny (the QSound effect) over speakers, but not so great over headphones. In the current 98S, obviously they are getting surround and BBE from one chip. I also really like the surround. I was thinking it was a more advanced QSound, but it seems to be what they say it is: SRS.
https://www.semiee.com/file/EOL2/JRC-NJW1144.pdf
http://www.szfxaudio.com/en/product/product-92-559.html
https://www.semiee.com/file/EOL2/JRC-NJW1144.pdf
http://www.szfxaudio.com/en/product/product-92-559.html
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