View Full Version : Xiang sheng 728a schematic


KenP
01-29-2013, 06:05 AM
I've gone through every scrap of info I could find, including the one here that shows a schematic but came up empty. The schematic that's shown here is really tiny and if you enlarge it can't see any info. Does someone have a better copy or perhaps a full copy showing layout on the pcb.

Bought this thing used with a hum issue and it really sounds nasty and dirty, looked at the board and alot of solder splash and really poor components. If they are real then they probably didn't pass any type of QC approval, resistors look like they were made by a 5 here with playdow and so called mkp caps have such poor printing that I doubt if they are real.

Any help is appreciated,thanks.

gil41
01-29-2013, 12:53 PM
I have and been using this preamp for more than six months now and so far so good. For the schematic, I contacted the factory directly and was given an old schematic which they said is still applicable. I am attaching it here plus answer I got when I requested it. Contact info: xshn@xshn.com and Spencer was my contact. I've done only some tube rolling and I only hear some sort of hum when my speakers are so close (about a foot when testing some speakers) to it. I am a mechanical eng'r with very limited electrical/electronics background; thus, the schematic is only for a tech use in case I have an issue.

I had an initial look when received and the boards and soldering in mine (I've done basic soldering only) looked ok. Please let me know what upgrade you will make on the components and could serve as a guide for me when I am ready for the component changes (caps, etc.).

" Hi Gil,

For the latest 728A preamp, we adopts the 12AU7, 12AT7 tubes. The schematic diagram is the old one, but could use as the reference for the latest preamp. Because we do not make so many differences.

Have a nice day!

Spencer"

KenP
01-29-2013, 05:41 PM
I think mine is an older version, it doesn't have the large paper in oil russian caps instead it has the standard chinese mkp copies. I also noticed that it has a really big channel imballance. I got it cheap enough that I think I'll replace all the caps and all the resistors plus the volume pot, looks like an alps unit will fit right on the pcb.

newbee123
03-01-2013, 01:38 PM
any one doing mods already?

I just change the tube ....

newbee123
03-01-2013, 01:54 PM
Here what I got from xiang sheng

KenP
03-01-2013, 02:42 PM
I changed all the caps panasonic for the big ones some silmic on the inputs and mundorfs for the bypass on the tubes, changed all the large resistors, some of the originals a way out of spec. changed the volume for an alps blue velvet, changed the wiring for the outputs to teflon coated silver wire. Thats about it for now, still no time to really listen to it.

NOSValves
03-01-2013, 03:07 PM
I bet the changes you made will have a huge improvment if properly implemented...

KenP
03-01-2013, 08:11 PM
413985Here's a pic of the internals.

KenP
03-01-2013, 08:15 PM
Has anyone taken out the pcb for this pre? I need to know where the led for the power is connected on the bottom of it.

newbee123
03-02-2013, 12:21 AM
413985Here's a pic of the internals.

I think your and mine is little bit different

KenP
03-02-2013, 06:16 AM
Mine is an older model before they used those large russian paper in oil caps. Look on the net for pics of the 728 and you'll see it. Aside from those caps only real difference is the pcb is split just behind the tone/volume/switching section.

NOSValves
03-02-2013, 07:30 AM
I'd be upgrading every single part on that main circuit board carefully using parts for the Sonic benifits if it was mine. That parts on that board are like most Chinese made gear inexpensive crap. The nice thing about this piece is you have plenty of room to work with...

newbee123
03-02-2013, 11:20 PM
I'd be upgrading every single part on that main circuit board carefully using parts for the Sonic benifits if it was mine. That parts on that board are like most Chinese made gear inexpensive crap. The nice thing about this piece is you have plenty of room to work with...

let me know what is your upgrade and pls share it

tks

newbee123
03-06-2013, 06:53 PM
It seems that what on the schematic is not the same with actual product.
So many different after I trace out

KenP
03-06-2013, 07:01 PM
There are many differences on the schematic, don't use it as a reference.

newbee123
03-11-2013, 04:48 PM
So far changing all electrolit cap with ELNA 2 and Nichicon

KenP
03-11-2013, 06:20 PM
looks good so far, can you take a pic of the bottom of the board? I need to see where the led f.or the power connects

newbee123
03-28-2013, 06:39 PM
So far

Change all capacitor and the cable

KenP
03-28-2013, 07:47 PM
Looks good! you might consider changing some of the larger resistors too, I found the values sometimes differed greatly from what they should have been.

I haven't looked at mine in a while now, once I get back to it it'll be to finish it off and sell it since I bought a shuguang s200mk pre and s845mk mono blocks.

newbee123
04-02-2013, 04:41 AM
Looks good! you might consider changing some of the larger resistors too, I found the values sometimes differed greatly from what they should have been.


What value you are referring with? Schematic?

KenP
04-02-2013, 11:23 AM
The schematic that is posted here is almost useless. I took out the resistors one by one and checked them, then looked up the color code and made note of the location, type and wattage, then replaced them. Many of the colors are not clear or are discolored.

raindance
07-11-2013, 08:11 PM
The schematic that is posted here is almost useless. I took out the resistors one by one and checked them, then looked up the color code and made note of the location, type and wattage, then replaced them. Many of the colors are not clear or are discolored.

I had one. It worked OK out of the box, but had very bloated bass. Turns out it uses a center tapped volume control that drives a loudness circuit that gives about 18dB bass boost. In addition, the circuit is wired strangely; the schematic makes sense (although there are WAYYY too many gain stages and the tone circuitry is missing), but in the actual unit it is using a White cathode follower to drive a 100K pot before exiting the RCA's to your amp. So, even though the specs say "600 ohm" output impedance it is actually very high indeed (should not be able to drive any solid state amps without significant frequency response loss). It would probably meet the specs if the volume control was in front of the preamp rather than on the output, but I suspect they changed that because of hum. Too much gain.

Also, the smoothing caps are WAYYYYY too large for use with tube rectifiers (which, incidentally, are wired in hybrid mode - the tube does a half cycle and the diodes do the other half). You will find that the rectifier tubes arc when you switch the unit on sometimes and they won't last.

In true China tube preamp fashion, the last stage should have elevated heater voltage, but does not. It will eat 12AU7 tubes as the heater-cathode voltage is exceeded (plus this causes hum or buzz).

The tone in/out circuitry has no gain matching, so it is much louder out of circuit.

Lastly, the unit is wired for 110V. You will find that the tube heaters are running at about 7.6 volts and the HT is way higher than it should be. This means all the operating points are out of whack.

In short, this is a poorly designed preamp and no amount of boutique parts will rescue it from absolute mediocrity.

I tried rebuilding mine, but gave up and built something else in the case work.