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The Stax SRM-002 mod thread
Mar 8, 2013 at 11:00 PM Post #3 of 53
some more thoughts about the ebay 5000mAh power pack:
 
http://www.ebay.com/itm/5000mAh-2-USB-Output-Power-Bank-External-Battery-Pack-for-ipad-iphone-Mobile-New-/110996970095?pt=PDA_Accessories&hash=item19d7ef5a6f
 
The dimension (110x70mm) seems to be a near perfect match to the 002 amp (110x65mm).
 
I suppose there is a 3.7V lithium polymer battery in there with two voltage converter circuits (to step up to the 5V for USB). For better sound quality one can probably by-pass one of the internal voltage converter and pipe the 3.7V directly to one of the USB outputs.
 
Mar 9, 2013 at 10:14 AM Post #4 of 53
Hey A-Cats,
 
Thanks for starting a new thread (just seeing...); looks to be a fun mod project with some easier stuff and some hairier SMD soldering for the steady handed.
 
I'll help kick things off by documenting the op-amp being used (pulled the case this morning); it's the AD822A. I took a few macro-pics of the op-amp area and other locations; will try to get them posted soon.
 
For a first mod pass, I am mostly interested in a battery tray mod to accept larger capacity Lithium with a charging/protection circuit. The battery leads are easily accessible; got pics. I think Justin mentioned the idea of replacing the entire cell holder with a custom little black box of goodness.
 
I did also just order the external pack for which you provided a link; will try out for use at work instead of hauling the wall wart; plus, will provide extra power for all my other devices when off the grid (hammock camping, fishing trips etc.).
 
Cheers!
 
Mar 10, 2013 at 10:31 AM Post #6 of 53
Quote:
audiocats said:
Thanks for the opamp info. AD822 is a rather slow chip (slew rate at only 3v/uS), I suppose they picked it due to its low power consumption (1mA vs. the more common audio chip's 5~6mA) .
Click to expand...

 
Would you then suspect your 001 opamp roundup applies to the 002? I guess the real question is whether having two power transistors makes any diff in best opamp that can be used without changes in power handling (versus available capacity). Hope that made sense.....
 
Mar 10, 2013 at 10:44 AM Post #7 of 53
Internal pics: op-amp area, aux power area, available space around the input caps (with output board in place); battery tray connection area.
 
 

 

 

 

 

 

 

 

 

 
Mar 10, 2013 at 3:30 PM Post #8 of 53
Quote:
drp said:
 
Would you then suspect your 001 opamp roundup applies to the 002? I guess the real question is whether having two power transistors makes any diff in best opamp that can be used without changes in power handling (versus available capacity). Hope that made sense.....
Click to expand...

 
The transistors shouldn't change efficiency much. Probably don't want to change opamp yet if you want to keep the internal style power ( the 2xAA or internal lithium pack). The battery operating time is short as is, it will be cut down to half once you use upgraded opamps.
 
If you do want to change to higher performence chips.....Remember the two resistors in the 001 that have to be shorted (in order for the upgraded opamps to work)? In the 002 there seems to be four of them, R3/4/40/41. Along with C25/26/27/28 and C5/C6 they form some kind of pie filtering network for the +/- rail to power the opamps. Not sure what needs to be done to the resistors once you upgrade the opamps, R3/4 resistance probably need to be reduced and R40/41 reduced a lot (shorted, maybe). C5/6 are the +/- rail reservoirs, capacitance should be increase 10X.
 
Mar 30, 2013 at 2:43 PM Post #10 of 53
now the Ebay battery pack.....(Initially a major disappointment, but turned out to be a near perfect mod candidate for making your 002 power pack)
 
 
 
curiosity got the better of me and I went and bought the ebay battery pack. (it is nice have a backup charger for my phone anyway). It arrived in a few days and for $15 shipped it looks really really good.
 
I opened it up right away (inset a small screwdriver into the seam and pry.... the top cover comes right off ) and was shocked to see this shiny power pack, labeled as 5000mAH, contains only two 1350mAH lithium cells, so its real capacity is only 2700......And, they used two pieces of galvanized steel to add weight so it feels hefty, talk about attention to details
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........... Notice there are only two screws holding the board (cost saving?)
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..... $15 shipped, oh well.
 

 

 

 
 
 
However, all is not lost. The charging/discharging/fuel gauge functions all seem to work well. It charges up my phone nicely, the output voltage at the 1A socket measures 5.15v; Plugs it into my PC, the blue LED's start to chase indicating it is charging, at the end of charge the battery voltage is at 4.25V, so the charging circuit is good as well.
 
If you wonder if there is any difference between the 500mA socket and 1A socket.... they all feed from the same converter, except that the 500mA line has an extra 0.2 ohm resistor in series (to limit the current?).
 
 
 
 
 
 
Mar 30, 2013 at 3:34 PM Post #11 of 53
Now the mod plan: I will leave the original circuit alone so all the USB charge/convert functions remain the same, it will still function as a USB power pack. For the 001/002 power needs, I will tap the 3.7V power directly from the battery, go through a separate Li-ion protection board and pipe it out through a power pigtail. That way the power at the plug is pure DC directly from battery terminals.
 
A protection board is necessary as it protects the cell from over/under voltage and short. They are widely available, for about $2. They usually come with a technical sheet with installation info. 
 
Cell capacity upgrade: available space in the case is 75mm x 65mm x 12mm,  looks like a 5000mAH or two 2000mAH (4000mAH total) can fit in nicely. There will be enough space for the protection board. Or you can also add two more 1350AH cells and make it 5400mAH total (say, if you are modding two of these packs, then you can put all four 1350mah cells into one pack and put a 5000mAH cell into the other) 
 
Anyway, I have no urge to upgrade the cell capacity yet, so the mod shown below is still with the original 2x1350 cell.  
 
First,. disconnect the battery wires and dismounted one of the USB socket (there really is no need for two USB socket). I chose the 500mA one. Pulled out all the contacts, enlarge the holes in the white plastic block with a 1/8" bit.
 

 
Reinstall the now non-functional USB socket, feed a short power pigtail through it (feed through the slot in the case first.... guess how I know) and connect the wires according to the info sheet that comes with the little protection board. Leave one battery wire unconnected.
 

 
Mount the PCB back into the case, and solder the remaining battery wire. Cover the little protection board with heat shrink.
 

 
 
Reinstall the cover. The 2700mAH capacity is enough for me for now. Might upgrade to larger cells only if the need rises.
 

 
 
 
Final thoughts..... was this $15 USB pack a bad buy? Well, despite the shameless false advertisement (2700mah labeled as 5000mAH
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), the circuit itself functions properly, it comes with a nice assortment of tips, the plastic case is easy to open and it is the proper size for the 002. The circuit board is easy to solder to and there is plenty of room inside the case for cell upgrades. I'd say it is a great starting point for a 002 power pack, heck I can't even buy a USB-->Li-ion charger board for $15 shipped, let alone the case and battery cells.
 
Upgrade the cells for another $25 then you will have a true 5000mAH power pack for...Everything.    
 
Mar 31, 2013 at 2:01 AM Post #12 of 53
Is the any interference/noise with this battery mod?
Audiocats, thanks for the info. I am not sure I am ready to mod my amp, but would love to see and build a portable DIY electrostatic amp. 
 
Mar 31, 2013 at 7:09 PM Post #13 of 53
the power at the pigtail plug is directly from battery terminals (the protection board's function is to cut off power when something goes wrong, otherwise it is just straight through). Dead silent. As quiet as a .... er, battery.
 
In comparison, the USB power from my HP laptop has a low hssssss noise floor; the USB socket in the power pack is very noisy.
 
Mar 31, 2013 at 9:27 PM Post #14 of 53
update....
 
I was wrong about the "can't even find a USB charging circuit board for $15" statement. Just found out USB-> li-ion charger boards are not only available but also super cheap.
 
It should be easy to make custom power packs with build-in charger. The only hard part is to find an enclosure of proper size to go with the 002.
 
Apr 3, 2013 at 12:40 AM Post #15 of 53
Well, I am not having much luck finding a workable external power.
I purchased the Velleman I mentioned earlier, and it outputs 4.75 V but it has a ground hum that disappears  only if I touch the iPod.
Even with the iPod disconnected, the hum is present as soon as I connect the power adaptor.
 
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