After having owned the Xiang Sheng 708B for about a year and after having read this thread, I decided to have a go at modding it. I had already done the easy parts almost at the start, exchanging the power cord for a Lapp Ölflex, and rolling the signal tubes to military grade Russian ones. Until I did my final mods I was actually really happy with the amp, and at that time I couldn't imagine how it could possibly sound better (little did I know !
).
I must first say that I had not had a soldering iron in my hands for 25 years up until two months ago, when I slowly started to recable some of my headphones, just to practice and warm up for the amp project. I read this whole thread and made a plan of what I would want to change and how much money to spend. This took me a while, but then I placed my order at PartsConnection, where I ordered most parts, with the exception of the hookup wire and coupling caps, which I ordered at the Hong Kong-based DIY Hifi Supply.
I removed or replaced everything except all wiring connected to the transformer, the mains switch, headphone socket, two main filter caps to which I added a bleeder resistor. I removed the display tube with LEDs, the input caps and the complete pre-amp stage to make place for the new bigger caps.
This is what I used as replacements:
Black Gate VK 150uF output caps: I decided not to use film caps here, because they would either be too big and expensive or of too low value for my taste. I like my bass clean and I didn't want to introduce any phase shifts into the audible spectrum.
Solen 22uF MKP filter bypass caps: I have read
here that these are not that convincing in the signal path, but are excellent in a power supply. They barely fitted in the space vacated by the showcase tube.
Obbligato Premium Gold 0.22uF coupling caps: They should provide excellent value for money. See
this review and comparison. Can be had on DIY Hifi Supply website, but they also sell through the auction site.
I replaced the
potmeter with an
Audio Note that is supposedly even a little better than the Alps Blue. I replaced the diodes with Vishay's. I replaced almost all resistors with the exception of the two large ones at the main filter caps. I used the better ones for the crucial locations:
Caddock MK132 at the input,
Mills MRA-5 wirewounds at the output, with the rest a mixture of Kiwame, PRP and Takman. I replaced the RCA connectors, and used Wonder Solder to fix everything in place. For the signal wire I used solid high purity soft-annealed silver wire from Hong Kong.
While I was at it, I managed to auction a pair of Amperex Bugle Boys, which I used to replace the pair of Russian tubes. I thought, now that I was going to burn in for hundreds of hours for the Black Gates, I might as well get some better tubes as well. The only thing that's missing is some Bendix or Tung Sol 2C51, but that will come. I have auctioned a cheap RCA 5670 in the mean time, which should also be an improvement over the Russian one.
Here are the before and after pics:
I was a little afraid I would inadvertedly destroy my valuable amp, but it worked out fine in the end. I forgot to ground the pot, which created a hum, but besides that it was great fun and very rewarding. I started to burn her in, and couldn't resist listening to it after about 20 hours. If I wouldn't have had previous experience with burn-in, I might have wept at that point. It sounded awful. I did the same yesterday at 40 hours and it already sounds much better, not as smooth and coherent as it was, but with much more detail and musicality. I'm sure the rough edges will disappear during the rest of the burn in. I'll keep you posted.
