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Ultimate Fosi Audio Box X2 Phono Preamp Mod

I

IASTM

Member
Pedal Pimp Fosi X2 Mod

Ever since purchasing my Fosi Audio Box X2 Phono Preamp, I’ve become obsessed with making it the best it can possibly be. After plenty of trial and error and extensive A/B testing, here’s what I’ve settled on:

Outside Mods​

Feet:
The stock cheap adhesive plastic feet on the bottom of the X2 were replaced with premium Hudson Hi-Fi Platinum Silicone Isolation Dots. These absorb surface vibrations much better and suction to the table.

Tube Rolling:
I swapped out the stock tubes for a matched pair of Riverstone Audio JAN 5654W tubes. While there are other options available, these worked best for me. Just be sure to source them from a reputable seller and ensure they’re legitimate and matched. This is a very common upgrade and if you don't do anything else, at least change the tubes.

Tube Dampners:
Consider buying some silicone tube dampers to slip over the tubes and help eliminate filament vibration.

​

Inside Mods​

Warning: There’s a very thin wire behind the selector pot. Be careful when disassembling the X2 to avoid snapping it—otherwise, you’ll need to replace it.

Capacitor Upgrades:
The X2 contains 10 electrolytic and 10 film capacitors that are accessible. Everything else is embedded in the PCB. While replacing the capacitors is optional, I decided to upgrade both types.

  • Electrolytic Capacitors:
    The stock Rubycon capacitors are decent, but I replaced all the 470µF caps with 25V Nichicon UKW. If you can find and willing to spend on UFG or UKZ capacitors, they’d be an even better choice. Panasonic FC are also great. For the two 100µF caps, I used Nichicon UFG.
  • Film Capacitors:
    The stock film caps are all 1uf and were swapped for WIMA MKS 1µF 63V caps. If you can find and willing to spend on WIMA MKP10 or MKS2, I’d recommend using those instead.
Op-Amp Rolling:
The X2 comes with three dual op-amps (JRC 2068D) that aren’t soldered, making them easy to swap. Simply pop them out with a screwdriver. There are plenty of options depending on your budget and preferences, including NE5532, OPA2134, TL072, LM4562, and AD712. Be careful with the pinout orientation, as installing an op-amp incorrectly will fry it. A dot on the chip marks pin one.

After testing several options, I settled on the Nisshinbo Micro Devices MUSES01. It’s not the most expensive option, but the performance is excellent. Maybe one day I’ll spring for the Sparkos SS3602, which is likely the best option available.

Future Mods (TBD)​

The final step in my modding journey will be relocating the AUX jack to the back of the unit and installing a switch. This would allow the AUX jack to remain plugged in, with the ability to toggle between PHONO and AUX using the switch rather than constantly plugging and unplugging. I’ll update this post once I’ve tackled it. I’m cautious about adding excessive solder heat to the circuit, so I may explore cutting the jack loose as an alternative.

Final Thoughts​

After extensive A/B testing, I can confidently say these mods have significantly improved the performance of the X2.


Cheers,

The Pedal Pimp
www.pedalpimp.ca
 

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Tried to edit this but the edit button is gone for some reason.
Correction: Power electrolytic caps (6 of them?) should be low ESR and high ripple current caps, not audio caps.
You can leave the originals (Rubycon ZLH) in or use Panasonic FC, or something like that. Apologies for that.
 
doctor fuse said:
What did you A/B test it with?
Click to expand...
A/B the original Fossi X2 against the modified one. I think that the upgraded tubes and the upgraded op amps made the biggest difference. The warmth of the MUSES02 is very nice. Easy to pop out tubes and op amps to try different combinations.
 
IASTM said:
Pedal Pimp Fosi X2 Mod

Ever since purchasing my Fosi Audio Box X2 Phono Preamp, I’ve become obsessed with making it the best it can possibly be. After plenty of trial and error and extensive A/B testing, here’s what I’ve settled on:

Outside Mods​

Feet:
The stock cheap adhesive plastic feet on the bottom of the X2 were replaced with premium Hudson Hi-Fi Platinum Silicone Isolation Dots. These absorb surface vibrations much better and suction to the table.

Tube Rolling:
I swapped out the stock tubes for a matched pair of Riverstone Audio JAN 5654W tubes. While there are other options available, these worked best for me. Just be sure to source them from a reputable seller and ensure they’re legitimate and matched. This is a very common upgrade and if you don't do anything else, at least change the tubes.

Tube Dampners:
Consider buying some silicone tube dampers to slip over the tubes and help eliminate filament vibration.

​

Inside Mods​

Warning: There’s a very thin wire behind the selector pot. Be careful when disassembling the X2 to avoid snapping it—otherwise, you’ll need to replace it.

Capacitor Upgrades:
The X2 contains 10 electrolytic and 10 film capacitors that are accessible. Everything else is embedded in the PCB. While replacing the capacitors is optional, I decided to upgrade both types.

  • Electrolytic Capacitors:
    The stock Rubycon capacitors are decent, but I replaced all the 470µF caps with 25V Nichicon UKW. If you can find and willing to spend on UFG or UKZ capacitors, they’d be an even better choice. Panasonic FC are also great. For the two 100µF caps, I used Nichicon UFG.
  • Film Capacitors:
    The stock film caps are all 1uf and were swapped for WIMA MKS 1µF 63V caps. If you can find and willing to spend on WIMA MKP10 or MKS2, I’d recommend using those instead.
Op-Amp Rolling:
The X2 comes with three dual op-amps (JRC 2068D) that aren’t soldered, making them easy to swap. Simply pop them out with a screwdriver. There are plenty of options depending on your budget and preferences, including NE5532, OPA2134, TL072, LM4562, and AD712. Be careful with the pinout orientation, as installing an op-amp incorrectly will fry it. A dot on the chip marks pin one.

After testing several options, I settled on the Nisshinbo Micro Devices MUSES01. It’s not the most expensive option, but the performance is excellent. Maybe one day I’ll spring for the Sparkos SS3602, which is likely the best option available.

Future Mods (TBD)​

The final step in my modding journey will be relocating the AUX jack to the back of the unit and installing a switch. This would allow the AUX jack to remain plugged in, with the ability to toggle between PHONO and AUX using the switch rather than constantly plugging and unplugging. I’ll update this post once I’ve tackled it. I’m cautious about adding excessive solder heat to the circuit, so I may explore cutting the jack loose as an alternative.

Final Thoughts​

After extensive A/B testing, I can confidently say these mods have significantly improved the performance of the X2.


Cheers,

The Pedal Pimp
www.pedalpimp.ca
Click to expand...
Hi! Love what you did with the X2 and I am very eager to experiment with it myself. So can I ask you, is the thin wire just wound around the thread of the knob? And do you now if the SX52B op-amps are also compatible? Thank in advance for your answer. Regards!
 
Kaavanaa said:
Hi! Love what you did with the X2 and I am very eager to experiment with it myself. So can I ask you, is the thin wire just wound around the thread of the knob? And do you now if the SX52B op-amps are also compatible? Thank in advance for your answer. Regards!
Click to expand...
The thin ground wire is soldered to the PCB on one end and exits a hole in the front plate and wraps around the behind the washer for the gain switch/pot. I remember the opamp had to be a dual op amp instead of a single. I think that the SX52B IS a dual opamp, but the best is to check with the manufacturer to make sure it’s compatible to the JRC 2068D specs wise.
 
Just swapped the original op-amps for 3 Chinese, not to cheap, SX52B discrete ones. Must say, wow! Sounds really great. Have no one on one comparison but the sensation is there! Had already swapped the original tubes for a matched pair of Siemens NOS ones. Really happy!!
 
SX52B a discreet opamp like the SS3602:

• For a warm, vintage-style sound → SX52B

• For a highly detailed, modern, and transparent sound → SS3602

• For a good balance between detail and smoothness → MUSES01
 
IASTM said:
A/B the original Fossi X2 against the modified one. I think that the upgraded tubes and the upgraded op amps made the biggest difference. The warmth of the MUSES02 is very nice. Easy to pop out tubes and op amps to try different combinations.
Click to expand...
Alternatively you can try one channel modified and leave the other original and compare.
 
I just took delivery on the Fosi Box X2. I'm anxious to see what it brings to my listening experience. I ordered the upgrade Riverstone.

Ive enjoyed solid state audio since the early 70s, alas I've never heard audio that used tubes.

In the den I have a Sabsui Model Eight (since 1978)..with a Pioneer PL-A45D player with several very good carts and styli.

It would be greatly appreciated if those experienced with the tube side of audio to lend some recommendations.
 
Last edited:
OldTimer65+ said:
I just took delivery on the Fosi Box X2. I'm anxious to see what it brings to my listening experience. I ordered the upgrade Riverstone.

Ive enjoyed solid state audio since the early 70s, alas I've never heard audio that used tubes.

In the den I have a Sabsui Model Eight (since 1978)..with a Pioneer PL-A45D player with several very good carts and styli.

It would be greatly appreciated if those experienced with the tube side of audio to lend some recommendations.
Click to expand...
maybe i am not "normal", but recently i do some changes in my system...i have fosi x2 with ge jan tubes ( they are not american made, but that's another story), and pro-ject ds3b phono pre...by every standard pro-ject is much better pre, more features, sophisticated circuitry etc...but, something is missing, it sounds sterile to me, and i get back in the system fosi x2, and i can't stop changing records after records...it maybe lacks the ultimate resolution and details, but fosi sounds fuller and more analogue like, preserving vinyl warmth...i can do a longer listening sessions with fosi...i don't know is it tubes or what, or do they even doing anything, but i like it...

i suppose for many pro-ject ultimately sounds better, but it's not the end of the story for me....
 
Just bought one of these and modding it. Anyone try upgrading the power supply? I see multiple people touting improved performance with an Sbooster BOTW LPS ($450). I don't want to spend that, but thinking about trying this:

Amazon.com: SOLUPEAK 25W DC 12Volt Linear Power Supply Ultra Low Noise for HIFI Audiophile,Desktop Power Adapter Regulated converter 12V 1.5A : Electronics

Buy SOLUPEAK 25W DC 12Volt Linear Power Supply Ultra Low Noise for HIFI Audiophile,Desktop Power Adapter Regulated converter 12V 1.5A: Internal Power Supplies - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com

cheers
 
Did you eventually purchase a power supply? Using one of these phono pre amps though not too often as I have other gear and setups that have phono input but upgrading the X2 is obviously worthwhile to a point.Tubes and op ams first of course.
 
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