The original design came from Nelson Pass' design and is published in passdiy.com for the diy community.  You can reference the PDF document from the website.   You can also download the board Gerber files, if you plan to make one your own.

There are two modification done on top of original BZLS: the X-BZLS (or X-BSOZ) and the X-CCS-BZLS version.  When talking about the sound quality, the comment from Nelson Pass were: 

"Running balanced, they tend to be identical. Running with the input or output unbalanced, the CCS gives better performance."  (Quoted from diyaudio).  

As you know my plan is to modify based on my existing BZLS, therefore, I will pick the most convenient one to do.  If you are a new comer, then you can pick the best one to fit into your diy project plan.

Also, this is the comment of Ignatz on his X-BZLS in diyaudio: 

"Henrik:  I must compliment you again on the sound quality of your modification.  I have been auditioning your “X” modification to the bosoz for a few weeks now.  A few days ago, I changed back to my version of the original bosoz and after listening to it for about 10 minutes I immediately changed back to your X-bosoz configuration. The difference was so large that I do not have to make this test again. I will stay with the x-bosoz version. "

Are you tempted now to convert your's.  I do, hence this project....

• The original X-BZLS by Henrik and the schematic with PSU.  This is his result.
• The X-CCS-BZLS by Alan Dupont (Version 2)
• The X-CCS-BZLS by Henrik and his test report on CCS.

Other references are:

• X-BOSOZ first test thread in diyaudio

Why "X" it?
When audio DIYer says X, it means the Super Symmetry technology (Su-Sy) that are one of Nelson Pass' patents.  To get to know the benefit of Susy, the best place to read is from Nelson himself.  Nelson published an article about Super Symmetry Amplifications in his Pass Labs website.  Click the link here to see it.  Henrik also has a brief discussion of the benefit of "X" in his test report here.

How to calculate Gain now?
In the original BZLS, the gain is controlled by R15 and the formula is:

Gain = (R1+R2)/R15

I initially set my VR to 430 ohms, which will roughly gives me about 10 dB of gain (gain is 20*log[3.48]). 

In X-BZLS, the gain is now controlled by R120, R121, R122, R123.  Henrik's design has kept the original gain level, and this gave the best frequency response, according to Henrik.

 For your reference only, this is the parts list I used in the original BZLS.  I am posting here (in MS Excel format).   

• To be removed and replace w/ jumper wire: R9, R10 and R18, R19 (I did not install these two on my board),  R15 (see below) and the P5 (I use the jumper wire already).
• To be replaced: C1, C2 change into 33uF (or 47uF, if you purse is full!).  To save money, I only use a 10 uF in parallel to original to make it as 20 uF (soldered on the opposite side).

• New parts for X-BZLS: R120, R122 are 10K ohms and R121, R123 are 39K ohms.
• If possible, (re-)match the resistor R13 and R14 (R113, R114 on X-BZLS schematic).  I have 1% resistor used, so I did not re-match it!
• For R15, I use a 500 ohm VR (blue one in photo) for adjusting the gain.  To keep it simple, I simple turn the resistor down to minimum, instead of remove it. 

Since I have my Aleph P 1.7 done.  I can easily compare it to my newly mod X-BZLS.  My source is Sony CD changer using MSB DAC II with 96K upsampling.  My power amp is the wonderful Aleph 5.  Before I did the mod, BZLS I felt a little bit flat and less depth compare with AP 1.7.  Don't be confused by this, because I think BZLS is very good and lead me to sell my original Audio Illusions' L1 hybrid preamp.  With this mod, my initial impressions are (but by no means represent the real potential of both designs):

For string instruments: 
I played the Mozart Violin Concertos, No. 3 & 5 and the Four seasons by Vivaldi.  The X-BZLS has improves from the original a lot with more details on the backend instruments, but still lack (a little bit) of the 3D feelings of AP 1.7.  Also, the total feelings of AP 1.7 is more of you sitting in a good concert hall, with X-BZLS are the same musicians playing in the regular meeting hall.  For the violin solo, for example, AP 1.7 tends to let me feel a more roundness and fullness, even though, X-B is also very good and not edgy! 

For vocal songs:
I played one of my favorite singers that I hear very often.    As I hear along and compare the difference, I found out that AP 1.7 is more of pin point focus in vocal image and the instrument right behind the singer.  The other softer background instruments, such as Congo percussion drums and the Tambourine, the feeling is AP 1.7 is a little bit more clearer and I am sitting closer to the stage with the singer than X-B.  Also, with AP 1.7, I feel a wider angle of the music flow.  

For Orchestra:
For hearing experience in Orchestra or Symphony, X-B is better than the original BZLS in separation of the instruments and tighter image.  However, in comparing the AP, I still feel a more of a concert hall feeling than X-B.

For low bass percussions:
The best test is using Kodo (Taiko) drums.  The two are similar with AP is better in more focus on where the drum is coming from and wider angle in stage image.

(Photo left: From Top one are: Alchemy DTI Pro 32, next is X-BZLS, my AP 1.7, Sansui TU-717, Sony CD changer and MSB DAC II D/A.  Below the table is my beloved Aleph 5.  My speaker is Triangle Celius 202).

• Nelson Pass:  Many thanks for your wonderful design and generosity to share your intelligence!
• Thanks to Henrik, Alan Dupont for their modified design and input.
• My family's support in DIY hobby!