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

[dcheming]

Well I figured it was about time to condense all the info on modifying this amp into a dedicated thread. If you're interested to see where the majority of this info came from please see the main Xiang Sheng thread: Xiansheng 708B headphone tube amp

Intro

While the overall build quality of this amplifier isn't that bad, the quality of most of the parts is pretty low, but that's exactly why there is so much room for sonic improvement by doing modifications. One of the great things about this amp is that there is quite a bit of room inside, unlike most of the other tube amp offerings in this price range, and this makes it quite easy to add big parts such as film caps, electrolytic caps, and volume pots.
So to start off here's an inside pic of a bone stock 708B courtesy of Nugget:





Here's the schematic for the audio section courtesy of zer061zer0:




Here's the parts list:

Capacitors

C1,2 330uF 450V electrolytic - Main filter.
C6,7 150uF 200V electrolytic - Headphone output.
C8,9 33uF 450V electrolytic - Main filter bypass.
C101,201 0.1uF 50V film - Input.
C104,204 0.22uF 400V film - Interstage coupling.
C105,205 0.22uF 400V film - Preamp output.


Resistors

R101,201 1M 1/2W
R102,202 33K 2W - Anode load for 6N3.
R103,203 1K 1W - Cathode bias for 6N3.
R104,204 200R 1/2W
R105,205 220R 1W - Cathode bias for 6922.
R106,206 1M 1/2W
R107,207 1k 1w - Input.
R1 1K 2W
R2 2K 2W
R3,4 100R 1W
R7,8 100R 1W
R9,10 330K 1/4W - The value was hard to read so I'm not 100% positive on this one.
R1,2 10R 1W - Headphone output.


Misc

Vol Pot - 100K stereo
D1,2 1N4007 1A 1000V diodes
Screws - M3 x 0.5mm



Safety

Before I get into the main mods, I would like to touch upon a few things concerning safety while working on this amp. First and foremost is the fairly HV (~220VDC) that is present while the amp is powered up. If you are ever taking measurements in the circuit with your DMM while the power is on, it is recommended to keep one of your hands behind your back. Doing this eliminates the possibility of your body forming a path for HV to travel through, which could be fatal. It's a good habit to get into while measuring tube amps or anything electrical for that matter.

The other main thing to keep in mind is that the big 330uF filter capacitors in the power supply will remain charged for quite a while. So even if you unplug the amp there is still a considerable shock hazard with these big caps. Many amps employ bleeder resistors on the filter caps to safely drain them as soon as the power is tuned off. Unfortunately the XS 708B doesn't have these, so I'd highly recommend adding these as your first mod, especially if you plan on taking this amp apart a lot to do extensive mods. Anything from about 50KΩ to 200KΩ will be okay for this purpose. I soldered a 2W resistor to both of the caps to divide the heat to be dissipated into two resistors. If you used a 100KΩ resistor on each cap, then they will dissipate about 1/2 watt each. If you used a 150KΩ resistor on each cap, then they will dissipate about 1/3 watt each. Adding these resistors will drain the filter caps within a few minutes of the power being turned off. In the mean time you could just use something like a 25KΩ resistor with a pair of alligator clip wires to manually drain these caps.

Bleeder resistors added:




Another simple power supply mod that I recommend is replacing R1 and R2 in the power supply with higher wattage resistors. This isn't really necessary, but in my amp these two resistors were getting pretty hot. For R2 (2KΩ) I used a 3W NTE with plenty of space around it for good air circulation. R1 (1KΩ) is situated in between the two filter caps so I wanted to do something different here since the larger size of a 3W would bring it pretty close to the caps themselves, which could potentially heat them up. So for this I crafted an equivalent 1KΩ 4W resistor using four 1KΩ 1W resistors in series/parallel. This is ideal here since it gives plenty of power handling, good clearance in between the caps, and much better air circulation.



1KΩ 4W combo resistor:




Crucial Mods

The mods in this group are the ones that I feel make the biggest improvement in the sound. If you want to know what will give the biggest bang for the buck, this is where to start. This crucial group includes the volume pot, the input resistors, the interstage coupling caps, the output caps, and the output resistors.


Volume Pot

The stock volume pot is a very cheap carbon type and it suffers from pretty severe channel imbalance. This is especially a problem in the 7 to 8 o'clock range, which is right in the range where you'll be the majority of the time due to the very high gain of the 708B. I measured how much difference there was between the two channels for this pot at a few different rotations and it averaged about 2000Ω, which is why it has such horrible channel balance. I upgraded this to a 100KΩ ALPS stereo pot and I was very surprised by how much this helped with the clarity. The size of the soundstage increased a bit as well, which was especially noticeable with instruments that are panned far to one side.

There's a few notes about doing this mod. The threaded barrel of the ALPS is slightly larger than the stock pot so you will have to enlarge the mounting hole a little bit. I did this by hand so I don't know what size drill bit is required. A new hole will also have to be drilled to locate the little tab on the ALPS that keeps it from rotating and I would recommend locating this hole to orient the pot with the pins facing up since this gives good access to them for later when upgrading the signal wire. The knob that I used is from Rat-Shack and since it's made for a 1/4" shaft I had to wrap a single layer of electrical tape around the shaft of the ALPS 6mm shaft for the knob to be centered. This particular knob almost fits into the circular recess milled into the faceplate and with a little bit of sanding to the knob's outer edge it'll fit inside.

Detail of new hole for tab and tape wrap:





Input & Output Resistors

Both pairs of these resistors are directly in the audio path and since low quality film resistors have the potential to be noisy I'd definitely change these four resistors. This is a very simple mod and there is a wide choice of options for these. You could just play it safe and use some nice metal films like PRP or Vishay/Dale, you could try something different like some RIKEN carbon films, or you could get crazy and go with some Audio Note tantalum film resistors here like some of us have.:allteeth: The input resistors are 1KΩ 1/2W (R107,207) and the headphone output resistors are 10Ω 2W (R1,2 *note that these two resistors share the same part number as the two resistors previously mentioned in the power supply so be careful).

RIKEN input resistors:


NTE output resistors on HP jack PCB:




Interstage Coupling Caps

The stock caps here are WIMA metallized mylar so there is definitely room for improvement. Changing these caps (C104,204) has a pretty big affect on the sound, right up there with the changing the volume pot. I went with Auricaps for these, but I would recommend the Mundorf M-Cap ZN as a better choice for around the same price. Like the input/output resistors this is a very easy mod. For those who also plan on using the preamp output of the 708B I would upgrade C105,205 as well.





Headphone Output Caps

The stock ones are just some cheap 150uF electrolytic caps and needless to say these can be significantly improved upon. If you are using lower impedance cans I would increase their uF value, which lowers the frequency at which the bass rolls off. Since I use 300Ω cans I just went with 200uF here in the form of two ELNA 100uf 250V caps paralleled per channel. Whatever you end up using just make sure it's rated for at least 250V to be safe and make sure you observe polarity when installing these.





Other Signal-Path Mods

This is all the other stuff in the signal path that could be upgraded after the crucial mods above. This includes the RCA jacks, signal wire, jumper, input caps, and HP jack. Upgrading these still helps, just not nearly as much as the above group.


RCA Jacks

The RCA jacks in my amp were of very low quality and one was undersized enough to let the IC easily fall off. The solder job on these was also horrible. I replaced these with some Teflon Calrad RCA jacks. The mounting holes are 3/8" so any standard RCA jack will work.

Stock jacks:


Calrad jacks:




Signal Wire

The signal wire used in this amp is of fairly low quality as is the solder job on them. I used some Belden 7882A CAT6 Teflon wire to replace this since it's easy to work with and very affordable. Even though the stock wire is shielded I haven't had the slightest problem with hum by using this wire. Of course there are many options for wire here, but I would use something smaller than 20ga to avoid having to enlarge the holes in the PCB with a small drill bit.

Stock vs. Belden CAT6 wire:


Amp with Belden CAT6 wire throughout:




Jumper

There is also a little jumper that is in the direct signal of the right channel. I believe the stock one is made of steel so it should definitely be replaced. I used a small piece of the CAT6, but a piece of lead-wire left over from a cap or resistor would work too.







Input Caps

The input caps (C101,201) can also be replaced with jumpers if you are sure that your source puts out an acceptable amount of DC. I just used jumpers made out of CAT6 for this.





Headphone Jack

The stock headphone jack is okay, but the way it's mounted kind of sucks. I replaced mine with a SwitchCraft open-frame type designed for thick panels and I soldered the 10Ω NTE output resistors directly to it.

[cotdt]

Also the CAT6 wires should be jacketed to lower the capacitance of the wiring. you get better bass response that way.

[dcheming]

Miscellaneous Mods

This group includes adding a toggle power switch, replacing the power wire, mounting the transformer on standoffs, rotating the power supply, adding a source selector switch, and reducing the heater voltage. These mods don't really improve the sound they just fix problem areas, add features, or just make things look nicer.


Toggle Power Switch

I'm not really a big fan of push button switches. The main problem with the one in the 708B is that both it and the HP jack are held up to the front panel by a very flimsy aluminum bracket. This bracket flexes back when you insert headphones and eventually the switch can become off center a bit. So I replaced the push button with a 15 amp medium duty toggle from ACE Hardware. I used some plastic washers to protect the finish of the front panel and to electrically isolate the switch from the front panel in case of internal failure. If you are leaving the stock HP jack but putting in a toggle switch, you'll have to cut the circuit board a bit to give some clearance for the switch.







Power Wire

This mod is completely unnecessary since the amp only draws about 20 watts while in use, but I was already adding the toggle switch so I did the power wire too just to make it look better. I used 12ga because that's all ACE had in stock at the time, but I would recommend 14ga as the biggest since I had to slightly modify the solder pins of the IEC jack to be able to fit the massive 12ga through. I also mounted the IEC jack with stainless fasteners that I counter-sunk to be flush on the outside.

Stock vs 12ga power wire :








Raising the Transformer

This was mostly done for better air circulation under the transformer, but I also think it looks a bit better having the transformer raised up like this. I used stainless steel fasteners and 1/2" aluminum standoffs.





Rotating the Power Supply

I did this for a few reasons, but mainly because the wiring works out better IMO. I also think it looks much better having the rectifier tube more on the left side of the amp since it becomes symmetrical with the audio tubes. The four mounting holes all line up so rotating the PCB 180° is straight forward. The yellow and blue twisted pairs are soldered to the bottom side of the PS PCB stock, so I moved these to the top like one would normally do and also shortened them for a cleaner look. For the DC out twisted pair you'll have to use some new wire since the stock one is too short now. Make sure you observe polarity with this twisted pair since it's DC. I used red and black to make it easier for future reference.





Source Selector Switch

This is by far the hardest mod I did due to all the metal cutting required, but as long as you have proper tools it's not that bad. First you'll need to figure out how you want to do the RCA jacks. You have two options here: if you don't ever plan on using the 708B as a preamp then you can just use the existing pre-out holes for the new input jacks. If you do want to keep the preamp function you can drill an additional pair of 3/8" holes to mount the new input RCA jacks wherever you like. For the switch I used a 3PDT mini toggle made by GC Switches. There are better mini toggles out there with silver contacts like those from TOCOS, but I didn't want to wait for shipping. A 2PDT will also work, but I wanted to switch the grounds as well to avoid any possible crosstalk or ground loops. You have two options for where to mount the switch on this amp. The easy way is to mount it on the back panel by the RCA jacks since there is so much room. The more functional way is to mount it on the front panel, but since the panel is almost 8mm thick this requires a threaded hole for mounting. The thread required is 1/4"-40 which is pretty hard to find a tap for. I got two of these special taps from ICS Cutting Tools for about $6 each.










Here's how I wired the switch:






Reducing the Heater Voltage

Unfortunately since the transformer in this amp is rated for 110V when we use it here in the US with 120V all of the voltages will be too high. With the tubes I use in my amp, the heater voltage measures around 7.42VAC for the audio tubes instead of the ideal 6.3V. The heaters in these three tubes draw right around an amp total so to drop about a volt requires a resistor of around 1Ω. The only place that sells electronic parts here is Rat-Shack unfortunately and all they had in this range for power resistors was a 1Ω 10W so I couldn't try any other values. Using this resistor in series with the heaters has brought the voltage down to about 6.63VAC which is still a bit high but much better than before. I'll have to go to a real electronics store to get some more power resistors to play with. I'm thinking a 1.2Ω 10W would be pretty close, but most likely I'll have to use two 5W in series to get it just right. I'll update this mod when I do more calculations and get the new resistors.





Here's the current state of my 708B as of 5-26-07:





Here's the bone stock pic again for comparison:

[Chops]

And now I will post about the tweaks and mods that I have performed to my 708B in the order that I did them in. Of course, I wouldn't have done any of this if it weren't for dcheming and his excellent research and testing through all of this. So here it goes.

First, a couple of tweaks...

Power Delivery:
First off was upgrading the nasty, tiny stock power cord. Luckily, I had a nice 3' long, 3 conductor, 14awg multi-strand power cord laying around unused, so it was a natural choice to go with.




Tube Rolling:
The stock Chinese tubes that come in this amp are not impressive at all. They have a rather thin midrange, are pretty bassless, have a brittle top end, and like to run out of steam early on and distort before the music even begins to get loud.

So for the first run of tube rolling, I first went with drarthurwells' recommendations of replacing the stock Chinese tubes with a NOS Russian Military 6N3P EB driver tube and a pair of matched NOS RCA 6DJ8 output tubes, but then switchd out the Russian 6N3P with a NOS GE JAN 5670.

These tubes really brought the stock 708B up to speed. No more distortion, a bit more bass, a fuller midrange and a tamed top end.






Upgrading and adding parts:
Next in line is my first batch of mods. This includes replacing the volume pot, coupling caps, output caps and adding a bleeder resistor to the PS filter caps.

1) I pulled the stock WIMA 0.22uF - 250V coupling caps in favor of a pair of Hovland 0.22uF - 400V Musicaps.

2) I removed the stock 150uF - 200V electrolytic output caps and upgraded to a pair of huge, monstrous Mundorf 100+100uF - 450V M-Lytic HV electrolytics. These are basically TWO 100uF capacitors in one body with a single (-) terminal and two (+) terminals. With the two (+) terminals wired in parallel, each one of these caps equals 200uF.

3) I replaced the stock cheap carbon volume pot with a quality ALPS 100k Blue Velvet. I won't go into the installation details and/or the "why" since dcheming has already done so in his first post.

4) I added a 68k - 2W resistor to just one of the PS 330uF - 450V filter caps. Since these two caps are wired in parallel along with the two 22uF - 400V filter bypass caps on the audio board, this single 68k resistor completely bleeds off the stored power in ALL four of these caps within a couple of minutes after shutting down the amp. This allows for a safe environment when working on the amp.

NOTE: THE OUTPUT CAPACITORS WILL NOT BE DISCHARGED FROM THE INSTALLED PS RESISTOR(S), AND THEY WILL PACK QUITE A PUNCH IF YOU SHORT THEM OUT, SO PLEASE BE VERY CAREFUL WHEN WORKING ON THIS AMP.

Refers to #'s 1 & 2


Refers to # 3


Refers to # 4


And one overall shot of the first batch of mods completed.

[Chops]

This second batch of mods was to clean things up a bit, but also to help in the sound quality as well. There are as follows...

A little tweaking and tidying up:
1) Replaced the 0.1uF - 50V input caps and steel jumper with 20awg OFC wire jumpers. Since my cdp doesn't put out any DC of any kind, I decided instead of upgrading these caps to better ones, to just completely ditch them and use wire jumpers instead, as well as replacing the nasty 24awg steel jumper in the right channel signal chain.

2) Placed PS transformer on nylon standoffs. The standoffs came from the LED brackets in the window display that I removed.

3) Omission of pre-out connections to RCA jacks, pre-out caps & garish window tube display. I don't ever plan on using this amp as a pre-amp, I removed these components to clean up the interior a little and free up some space. Plus, that window display just yelled out "CHEAP!" That was just taking up usefull room, probably adding a bit of noise and using up unneccissary power from the PS.

4) Moved Mundorf caps up front in window. This frees up room in the back of the unit. Plus, these caps are so large that they overlapped the audio board some, not allowing me to easily remove the mounting nut and/or board from the chassis. Now they are out of the way, and much closer to the headphone output jack, which will be perfectly convenient for a future upgrade.

5) Added a ground wire to the ALPS volume pot. I had to do this as there was a slight 60 cycle hum coming through when the volume was turned up anywhere between the 12 o'clock and 4 o'clock position. Not that I ever go up that high (the most I ever go is about the 11:30 position), but knowing that that noise was there bothered me, hence the ground wire. Now there's absolutely no noise anywhere throught the range.

Refers to #1: This...


And this...


Replaced with this...


And this.


Refers to #2 & 3




Refers to #4


Refers to #5


Here's a couple extra shots of the 2nd batch of completed mods and new "window treatments".





More Tube Rolling:
I decided to go out on my own this time and try some tubes that interested me. One of which others have tried on a "stock" 708B, but never on a modded one. This is the NOS GE JAN 5670. This one in particular is from 1968. The reason no one liked this tube is because it created too much hum or noise into the signal. Again, this was with all stock 708B amps. However, I put this 5670 tube in my amp and it is totally, completely and 100% dead silent, even at max volume and everywhere in between.

I also want to try some different output tubes as well. I've heard that the Amperex-Japan 6922/E88CC tubes were actually made by Matsu****a, which are very VERY good tubes.

Here they are...


As for the sound, I feel that they have a better "grip" on the music than the UK made RCA 6DJ8's and Russian 6N3P EB that drarthurwells recommended. They DO have a wider soundstage, the dynamics are more dynamic, bass seems to be a bit tighter and definately deeper, the midrange is full and natural, and the treble is still smooth, crisp and extended.

[Chops]

Now for the final mods that I made to the 708B. This mod put this amp well over the top, especially after a healthy, constant 92 hour burn-in time.

Resistor & Diode Upgrades:
I replaced every single resistor on the audio board that has to do with the sound getting from the input jacks to my ears, which means the ONLY stock resistors left on the audio board are the two very small, blue ones all the way in the back of the board going to the pre-amp outputs, one for left and one for right channel, which are disconnected anyway.

Almost all of the resistors used on the board are PRPs. There are three exceptions. For the anode load resistors, I used a pair of Kiwames. For the input resistors, I used a pair of Audio Note Tantalums. I also used another pair of Audio Note Tantalums for the outputs which is mounted directly on the headphone jack.

I also upgraded the stock cheap PS rectifier diodes with Vishay-Telefunken ultrafast soft recovery diodes.

Here's a few pics of the entire process.




















I'm not sure what kind of sound improvement the diodes made, if any. But as far as the resistors are concerned, WOW! This amp has totally transformed into something special, leaving my LDM+ and my brother's LD II++ lightyears back in the dark ages!

This is what I wrote dcheming in a couple of PM's...

This one was only a few hours after performing the resistor and diode upgrades.
"And as for an update on the amp... Man, this thing is blowing me away! Especially when you have a good quality recording spinning! This MH cdp is excellent also, but just to be safe, I reconnected the Sony cdp for a couple of hours tonight after work. These improvements I am hearing are definately coming from the amp itself. All I can say is "WOW"! And then when I put the MH back up to it, it's another "WOW" all over again! I'm telling ya, you need to upgrade all of those resistors when you get a chance. They really make this little amp come to life."

And then this morning.
"BTW, on a side note, I have had this amp and MH cdp playing constantly since Wed afternoon (4pm) when I finished the last batch of mods and installed the opamps. I now have roughly 89 hours on both the amp and cdp.

All I can say is WOW! Everything is working flawlessly, everything sounds excellent, and I am just loving this entire setup. For a couple hours last night, I had the Sony player on the amp again and I couldn't get over how excellent that little cd/sacd player sounds now. The amp was definately the weak link, but not anymore! However, once I reconnected the Music Hall cdp, I could certainly hear another huge improvement in sound.

So I can safely say that the mods to the amp are definately worth it, especially replacing all of those resistors. You'll see what I'm talking about whenever you get your resistors upgraded.

Imaging, soundstaging, air, space, depth, detail, bass... Now I can finally see (hear) what everyone has been raving about with these K701 cans!!"

[zer061zer0]

Hi guys,

I am also thinking of changing all my resistors on my XS. However, I do not have my multimeter with me now. Would it be possible for you to give me the values and voltages for the resistor values of R3 & R4 on the PS board and R102, R105, R202, R205 and R7, R8 on the audio board. thanks.

[laxx]

Wow, what a write up. I don't think I can do something like this, as I'm not very knowledgable in this area, but the documentation in this thread does inspire me since it seems so laid out. =T

[penguindude]

WOW that is one NASTY modded amp.

Note: "Nasty&q