Modifying the Xiang Sheng 708B [56K!!!!]

[dcheming]

Originally Posted by Chops

I'll get back to the sound in a day or so when I've had a little more time to listen to the amp with several different CDs.

So do you have any more impressions of the 6X4 Charles? How's that new Music Hall CDP working out for you now that you've had it for a while?

[Chops]

Originally Posted by dcheming

So do you have any more impressions of the 6X4 Charles? How's that new Music Hall CDP working out for you now that you've had it for a while?

Honestly, I haven't listened to the amp since I did that last mod. I've been busy and running about too much lately to be able to sit down and relax. Sucks really, but that's how it goes sometimes.

[dcheming]

My PC order came in today and I just finished installing all the new goodies. First I put in the new sockets for the audio tubes. I went with the two piece ceramic ones with gold plated cup style contacts. The solder tabs on these were quite a bit wider than the stock ones so I had to use a 5/64" drill to enlarge the holes slightly in the PCB. I also had to bend the tabs inward a bit to get the proper hole spacing. These new sockets grip the pins TIGHTLY!







Next I swapped out all of the resistors using mostly PRP metal films. I wanted to use a low noise NIWW type for the 6N3 anode load resistor, but I had trouble finding 33K in a 5W so I ended up using a pair of 12W MILLS instead.







I also got a pair of RIKEN 10R 2W for the output resistors to match the RIKEN input resistors.





Finally I added the Vishay-Telefunken SF4007 diodes and the PRP resistors in the power supply.

[caromire]

Hello, I can rolling the 6N1 for the 12AT7?

Thanks

[dcheming]

Originally Posted by caromire

Hello, I can rolling the 6N1 for the 12AT7?

Thanks

The 12AT7 has the same two pins for it's heaters as the 6N1, but it's designed to run off 12 volts instead of 6.3 volts like the 6N1. So in order to use a 12AT7 in the place of the 6N1 you'd have to change the heater supply wiring at the socket a bit. Since the 6N1 is just there to look pretty and is not even in the audio circuit I don't think it would be worth the effort to do the conversion though. Just stick with the cheapest 6N1 you can find for the window tube.

[dcheming]

I was intending just to simply swap the little jumper with a scrap lead from the 2W RIKEN resistors tonight, but due to boredom ended up three hours later with an auxiliary TRS jack with metallized output caps as well. I've always wanted to see how metallized would compare to electrolytic when used as output caps and now it will be easy to switch between the two. The biggest films caps I had laying around were some no-name 6.8uF 250V metallized mylar. I know they're pretty small value, but I have 300Ω impedance headphones so still get pretty good bass response. Depending on the sonic results I might go with some much bigger metallized caps later. I'll post my impressions in a few days.

[zer061zer0]

Derek, U are one serious dude in this modding. . . any impression of it.

hows the ck2III going? I got a frd to help me in the end as I was too busy with sch..

[dcheming]

Originally Posted by zer061zer0

Derek, U are one serious dude in this modding. . .

Thanks.

Two days ago I added a pair of 2.15uF mylar caps to each 6.8uF to bring it up to a total of 11.1uF per channel. According to calculation this should give my 300Ω impedance headphones a -3dB rolloff point of around 48Hz, compared to around 2.65Hz with the ELNA electrolytic caps.

Impressions: The 'lytics have deeper and punchier bass, but the mylar caps sound better everywhere else IMO. They still go pretty deep in the bass though and what is there is quite a bit tighter than with the 'lytics. The midrange is noticeably clearer with the mylar caps too. The high end is more crisp and the general presentation has more "air" to it. I'd really love to try a nice metallized polypropylene next, maybe something like a Mundorf 100uF M-Cap bypassed with a small 0.10uF M-Cap ZN, which would give a -3dB point of 5.3Hz. I don't think those bad-boys would fit in the XS though. Besides I'd much rather save the buying the good stuff for the upcoming Cavalli Lovell Futterman anyways.

Originally Posted by zer061zer0

hows the ck2III going? I got a frd to help me in the end as I was too busy with sch..

I haven't gotten around to ordering the parts kit yet due to truck related repair expenses. Hopefully in the next few weeks I'll be able to the ball rolling on it though.

[Chops]

I've been doing a little more tweaking to my amp lately, also with the output caps.

For now, I am replacing the Mundorf M-Lytic 100+100uF caps with a pair of Samwha 1500uF -385V Photo Flash caps. I also replaced the cloth-covered 18awg solid core wire with multi-strand 18awg tinned copper wire. As a result, I had to slightly enlarge the holes on the PCB to get the stranded wire to fit. Not a big deal at all.

I've had the amp powered up since 9pm last night. I have noticed that the left channel cap is reading 114v DC where the right channel cap is reading 118v DC. I'm hoping that this will equalize eventually once they have a good amount of hours on them. I plan on leaving the amp on straight until Wednesday morning which should give me just about 83 constant hours of burn-in.

As for the sound, I noticed right away (and to my surprise) that these new caps are much more open and airy sounding. The soundstage is simply huge! I am noticing a bunch of fine details in the music that I never heard before, such as faint shimmers of a cymbal or high-hat, the textured sound of human flesh tapping on hand drums and tamborines, the gritty, grainy sound of someone's throat as they sing, very fine and detailed sounds that were burried in the background, and a lot more ambience around the musicians and recording venues. There's just a bunch more detail in every part of the sound.

The only thing so far that is lacking a little (and this is probably because the caps are brand new) is the lowest bass. As I recall, this was also true with the Mundorfs when they were new. In all actuallity, these new 1500uF caps should be providing more bass output than the Mundies, and I'll just have to wait and see what happens after this first round of burn-in time.

Even with the slightly less bass output, the rest of the sound is so good at this point, I really don't mind the leaness that much. It's so good in fact, I'm not even sure if I'm going to put those Mundies back in.

I'm guessing the reason I'm getting all of this new-found detail and openness is due to the phase distortion being pushed right out of the audible range, well below 20Hz (probably around 15Hz or so at 62Ω). I'll definately keep you updated on how things turn out with burn-in.

Anyway, I just know all of you are dying for some pictures, so here they are!

Test fit...





Enlarging the holes with a very fine-tipped file...



The completed wire and cap upgrade...

[pwrusr]

Wow what a thread you have here dude!

I'm very interested in hearing your thoughs after you get done with the burn-in.