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The Stax SRM-001 mod thread

post #1 of 403
Thread Starter 
My original posts were buried in the Stax thread. This is the summery of some of the technical info. I will add the output cap test result by the end of next January (have to get those boutique caps first).

If you have done mods to your SRM-001 or have mod related info please contribute, some DIY'er might want to mod his/her SR-001 system to the "king of the portable" status.....

My mods/test will be done in five steps, related info/result will come out after each step is completed:
--A) General info about SRM-001 mods. This gives some idea what can be done to your little portable amp. (done)
--B) Opamp rolling. Info and test results of replacing opamps in the SRM-001. (done)
--C) Output capacitor testing. Different output capacitors can give very different sound, and you will notice it. (coming next spring. caps on order).
--D) Bias voltage test. Believe it or not, the circuit board has traces for an easy bias level change. We will see if lower bias will yield a super smooth sound for vocal....---- will skip this one due to the experiment in post #13, I will instead go toward modifications for driving SR-X.
--E) A tutorial with step by step pictorial instructions of how a full version mod is done. Apparently by then my SRM-001 will have too many extra test features and will be unfit for demenstrating how a general mod is done. I will need a fresh amp to complete the tutorial. So I have one slot open for an "full version (i.e. SuperFatCat) ", mod (8599, BlackGate eletrolytics, sonic/kimber output caps, etc). If you wish to get your SRM-001 modded and don't want to do it yourself, please PM me. The full mod package will be $150, it will also includ a RCA/mini adaptor made with Neutrik connectors and solid silver/teflon wire. Only accepting one amp, preferablely from within the US (for easy shipping). I will perform the mods and take pictures for the tutorial. Here are the conditions: 1)you Must also have a SR-003 and a stax 3XX series amp. 2) you must have a high grade CDP and silver IC's. 3) you will promise to provide a 500+ word feedback/report on how the modded SRS-001 compares to a SR-003/SRM-3XX combo. I know the SRM3xx should win but want to know the details.
--- update 1-13-08: there won't be a pictorial instructions, instead I will provide three to-do/parts-to-use list. The mod slot is still open, PM me if you are interested.
--- update 1-29-08: the mod slot has been taken by Larry the HeadphoneAddict We still need reviewer with 003/3xx though, since Larry's amp is a SRM-1 which is too good for the SFC-001 to compete with. He will provide a report on the SFC vs. various $400 portable dynamic systems (phones+amp)

Where to find some of the info in this thread:

*Kevin Gilmore's info on 001's circuit layout (thanks a lot Kevin) -- post #8 (this page)

*info on battery power-- this post and post#71 (page 8)
*AC adaptor comparison-- post #23 (page 3)
*HeadphoneAddict's test of AA battery life on a stock 001 amp -- post #51-#55 (page 6)
*Heat dissipation of Q15 after SFC or UFC mod -- post #49 (page 5)

*opamp comparison test -- post #4 (this page)
*Interconnect to use -- post #40 (page 4)

*FC output cpacitor comparison test-- post #20 (page 2)
*FC input capacitor comparison test -- post #42 (page 5)
*FC to-do list -- post #47 (page 5)

*SuperFatCat input/output capacitor comparison test -- post #43 (page 5)
*SuperFatCat mod to-do list -- post #48 (page 5)

*FatCatUltra input/output capacitor comparison test -- post #44 (page 5)
*Ultra mod to-do list -- post #48 (page 5)

*sclee's modification to his amp -- post #29(page 3), #38 (page 4)
*Borisov's modification to his amp -- post #94 (page 10)
*SFC mod performed on HeadphoneAddict's 001-- post #74 (page 8).
*HeadphoneAddict's review on the modded 001 -- post #95 (page 10)
*catscratch's review on the 001 with SFC++RC mod (Rock/Classic)

some battery power info,for truely portable/on-the-go use:
The stock battery compartment configuration holds two AA Alkaline or NimH in series. There is quite a bit of sonic difference between using batteries and the 4.5V AC adaptor, the adaptor sound a lot sharper and a lot more detailed. If you want that clear sound while on the go (true portable, right?), you can convert the battery compartment from series to parallel configuration, and install two 14500 (AA size) Lithium-ion 3.7v rechargable in paralle. The voltage of lithium ion changes between 4.15V (full) to 3.3V (nearly empty), the 3,7V spec is kind of the average voltage. With two lithium ion in paralle you get 3.7V, 1500mAH~2200mAH (depends on the battery) power, which will give you at least a few hours of great sound (depends on whether you do the other mods and the kind of op-amp you use.).
Lithium batteries give the cleanest and most involving sound. This should be the first upgrade you want to consider, if you don't want to try anything thing else. Ultrafires 900mAH 14500's are less than $7 each and the charger is about $15. Get four batteries and one charger, convert the battery bay to parallel, then you will be all set
-- update 2-28-08: ****Caution!! DO NOT BUY the UltraFire protected 14500 li-ion rechargables, it will NOT fit into the 001's battery bay. After adding the protection circuit board, that battery is about 2mm longer than the standard AA, too long to use (strictly saying, the protected cells, being longer than 50mm, should not be called 14500 any more). The unprotected 14500, on the other hand, is the true AA size so can still be used. If you always use the dedicated charger for recharging and turn the amp off when the sound gets bad, the unprotected batteries should still be safe. Of course, the best and safest way to do it is find a open spot in the amp and add the protection board into the amp itself. ****
for more info, see post # 71
post #2 of 403
looks good, I might put my SRM-001 under the soldering iron in the future too
post #3 of 403
Thread Starter 

basic mod informations

Mods can be done to four areas:
1) input/output capacitor upgrade.
--The stock input caps are two 0.15uF poly(ester?) caps and two generic 10uf polarized electrolytics. There isn't much room in the little SRM-001 case to exploit, and the input stage does seem to benefit from large input caps (better bass response), so high grade electrolytic caps should be used. Replacing the stock polyester cap and aluminum electrolytic with polyprop caps and blackgate NX non-polar coupling electrolytic will probably bring 20-40% improvement over the original polyester (?) and aluminum electrolytic. Not sure about the % here since I (still) haven't even order the blackgates yet. There is indeed a difference between polyester and polyprop, with electrostatic transducers you Will hear it, the polyester has certain harshness in treble.
--Output capacitors (0.01uF/630VDC) testing is still in progress, I tested 4 different polyprop caps plus the stock poly(ester?) so far, and there is quite some difference in sound among the caps. The best sounding so far is the BC component 0.011uf/630V (MKP 420 series, digi key #BC2240-ND, $0.61 each), followed by Epcos 305vAC X2 series ($0.32 each, good mids, smooth highs, strong bass good texture), then the Panasonic ECWF series ($1.31 each, strong bass, warm mids but treble not impressing) I am ordering some boutique caps such as Auri and kimber and Sonic, will see if these high $$ ($4.5 ~$10 each) fancy caps can do better than the cheap X2 series.

2) high voltage power supply cap upgrade:
If you don't hate bass, you must install better/larger high voltage caps. The input cap upgrade will improve mids and treble greatly, but will also make the bass, though now much better defined, sound weaker than before. High voltage caps should be at least 0.3uF (the stock cap was a 0.01uF polyester). These new power caps MUST be as high grade as possible, polyprop is pretty much the only choice. (.47uf polyprop has better over-all sound than 1.5uF Mylar caps. Treble, mids, bass, everything ). Due to the size limit, the X2 series caps are about the only options. I use Epcos 305V X2 caps from digikey.
Since I like bass, I will say the high voltage upgrade can improve the SQ by up to 50%.

3) Opamp power supply upgrade, +/- , 9~12V:
R33, 34 and 35 need to be shorted. The original purpose of these resistors were probably for creating a Pie filter network so small filter capacitor can be used (stock caps were 10uF/16V). The new op-amps (next step) requires a lot more operating current than the stock JRC062L, if you leave R33/34/35 in the circuit they will drop voltage so much that the new opamps won't work. (most audio grade opamps require 3-5mA per amp, while the 062 only need 0.1mA), How are we gonna make sure the power filtering is still good? simple, you drop the R in the RC circuit, then you increase the C, intall larger capacity caps . High quality miniature 16V caps should be used, the minimum capacity requirement depends on the kind of opamp you use. AD823/NE5532/TL082 will need at least 100uf per side, AD8599 somewhere around 300uf per side, have no idea what the minimum will be for AD825's. The available room inside the case, again, is the limit here, so I suspect you can only pack enough uF inside the SRM-001 for the AD8599's.
---- update, 1-13-08: the uF # was if you are using a AC adaptor, without enough uf you will hear a hum from the adaptor supply. You can increase the capacitor size inside the AC adaptor, but since everybody is using a different wallward I will leave that to you. If you only use pure DC, ie. batteries or an AC wallwart with large reservoir caps inside (need to be at least 1000uF/10V), there will not be a requirement on how large the +/- caps have to be, just install the largest capacity that fits in. Again, quality matters, so whenever possible, use blackgates (the PK series are only $1 each anyway).
-- update 1-17-08: It turned out that my Panasonic 4.5v/0.8A wallwart has a lot of ripples, which caused the humm. There is no hum at all when using a Sony 4.5v/500mA wallwart, even when the total cap size for the +/- is only 47uf per side. But still, the +/- cap size affects bass quantity and quality a lot, you want to pack in as many uF as possible.
4) op-amp upgrade:
The SRM-001's design came out 10years ago (1997), so an opamp upgrade makes a lot of sense.....
--The standard upgrade chip recommandation is the AD8599, being a SOIC dual-opamp it can be soldered directly to the existing pads on the circuit board, very straight forward.
--If you have trouble getting the high capacity miniature caps for +/- supply, you can use available smaller capacity caps and low current opamps such as AD823 or NE5532 or TL082. I don't have AD823's to try so I have no idea how they will sound, but should be better than the 082 I supposed?
-- AD825 shows much better resolution/texturing than the 8599, but it requires more current hense larger filtering capacitor. IF you can somehow pack the required larger capacity +/- supply caps into the case it will be the best solution -- update: see the update above for new cap requirements when using pure DC power.

For DIY, you can probably get a mid-grade modification done with $35-$40 worth of parts ($3 for high voltage X2 polyprop caps, $8 input caps, $15 opamps, $6 for good miniature 16v caps, and $4 for output polyprop caps), then the parts shipping (from two different places) will cost you another $15-$20, so somewhere around $55 total for your new DIY project, not bad.
---- 1-13-08: this for the FC level mods (fat cat class).

below is the test set up, for performence testing when there is no size limit.
post #4 of 403
Thread Starter 

Opamp rolling

Each opamp gets at least 20 hours of burn-in before listening for sonic characteristic. They should be fairly stable enough by then, though the 825 did benefited a bit from 30 hours of burn-in.

some data about the stock amp:
-whole amplifier current draw when using 3.7V lithium batteries: with no op-amp plugged in=220mA; with original JRC062 plugged in = 220mA (wow, the 062 almost consume no power at all).
-SQ of stock configuration: If you haven't heard the upgraded version, the stock amp sounds pretty good......

Stock op-amp: JRC062L, very low current requirement (0.2mA per chip), low slew rate=3.5V/uS. The sound is clean, probably more due to the eletrostatic phones than the opamp itself. Sound is not airy, has an emphasize in mids, kind of "transistor radio" sounding when compared to the other better opamps. Now, by itself the 062L sound is not bad at all, it's just the newer/better opamps sound so much better......

now different chips

The ancient hot-rod (yes they were in the early 90's : NE5532
--current draw, whole amp: 400mA
--much more spacious than the 062L, the sound is very soft, not sharp enough for guitar tracks. It is also mids emphasized (or treble recessed), the mids are realistic, so that is good. Good bass, good ambience. Not very fast though.

LM4562 (the usual upgrade opamp for 5532):
--current draw, whole amp= 500mA
--The sound is clean, cleaner than the 8599. Not sure why I felt that way, it is either the 4562 is presenting highs better or it is not showing as much little micro details as the 8599. Not as much bass as the 8599, but bass definition is good. Anyway the 4562 is not as fully bodied as the 8599 or the 5532.

Stock opamps (062L's) are ultra low power yet lower performence version of 082. So it makes sense to see how the "uncompromised" version sounds like. These 082's I got here are the hottest 20V/uS edition.
-- whole amp current draw is 320mA.
-- sharper sounding than 5532, much better than the low-current 062L, but like the 062L, the 082 is still mids emphasized. Bass is kind of dry, simply not enough for my taste. The sound is pretty fast, ok for guitar tracks. Vocal not very realistic when comparing to other better chips. The overall impression is: sounds kind of dry and hard (?), maybe there is a mids spike somewhere.

AD8599 (supposedly very fine sounding and a great compromise of many worlds....)
--current draw, whole amp= 450mA
--This chip have received quite some hype/acclaim here, mainly from NelsonVendal. It is indeed very good. The sound is darker, more up-front, intimate, juicy and sensual. Warm and very full bodied sound. Great for acustic guitar, full rounded bass (it can even sound slightly too bassy if you make the "baby-stax-face" when playing bass heavy tracks). good bass texture, good details, good resolution.
--All in all a great general purpose replacement chip to use in the SRM-001, but especially good for playing vocal. Not as sharp as I want for playing Rock/electric guitar though. 8599 comes in dual amp SOIC package and will fit right onto the existing SOIC-8 pads on the circuit board, promissing a very very straight forward installation.

Modern hot-rods: AD825
--current draw, whole amp=500mA
-- when I firt install these chips, they sucked so much juice that the on-board power supply couldn't keep up at high volume. After about 30 hours of burn-in though, things were vastly improved, now I get full bass even at very high sound level (louder than I should be listening at). This is with those two external 640uF extra-low ESR caps though.......
-- Great sound stage, very spacious sound stage (for in-ear phones at least), much better resolution/finer detail than the 8599, but not as much body. Brighter and more "distant" sounding comparing to the 8599. Great bass, very clean and much better texture than any of the above, though not as much in bass quantity as the 8599's. This will be the chip to use for basically everything, it might be the best replacement chip you can install (there are better chips out there, but the +/- current limitation prevents the usual opamp king such as OPA627 from working in this amp).
-- Too bad the 825 are only single amps, fitting them onto the board involves quite a bit of work. But that is why DIY is fun, right?

The super fast budget op-amp: AD826
current draw, whole amp= 510mA
- wouldn't work, major clipping even at low volume, tried only installing one and the clipping was still there, though not as bad. Simply couldn't get it to work.

Opamp selection: You should not use any opamp that demands supply current more than 5mA/amp. The 5.5mA is the about the absolute cut point. AD8599 uses less than 5mA/amp and worked great, AD826 requires 7mA/amp and didn't work (major clipping), while the AD825 and LM4562 both still work ok (5-6mA/amp), though it took the 825's 30hours of burn-in to get there. I didn't get to use my OPA637's (7mA/amp)

The 825 is the overall winner in this test, but 8599's are still the #1 recommandtion for the opamp upgrade-- very easy to install and don't require super fancy miniature hi-cap capacitors for power supply. 5532 is actually not too bad, I will place it in the 3rd place. There is still AD823 that can work but I didn't buy any due to the bad rep it has received in this forum. AD823 is a low current requirement chip like the 5532 and TL082, and possibly sound better than either. It is also an easy-to-install dual opamp. Maybe somebody will put a pair into his SRM-001 and tell us how it works out.
post #5 of 403
Thread Starter 

The "baby-Stax-face"

* How to make your "baby-stax-face": put the earspeakers into your ears, then rotate the wire stem forward until it is almost horizontal pointing forward. you should now have a wider sound stage and the bass should get stronger (at least that is how it happens to me). Now open your mouth and drop your jaw as far as possible, as if you were startled, this will open up the connection between your inner ear and the throat and give you even better bass......Experiment different facial expression and find one that gives you the best sound effect from your earspeakers and that will be your "baby stax expression". Enjoy.
post #6 of 403

Thanks Audiocats

Thanks for all your excellent work, Audiocats. I look forward very much to your further results in January.

Thanks also for bringing it out as a seperate thread - makes it much easier to keep track and use the info to upgrade my SR-001 when you have finalised the best combo of mods.

Work like yours is well appreciated by the electrostatic HeadFi community.
post #7 of 403
Excellent work!
I look forward to the end result. Nicely placed inside the original case, and all.
post #8 of 403
For those that might want to see a schematic of what is going
on here, here is a little something less the power supply.
post #9 of 403
Thread Starter 
I partially traced the circuit, but didn't realize those are JFETs.....( thought they are BJT transistor to form darlington pairs).
Now is there a need to match the jfets? or are they supposedly already matched?
post #10 of 403
Originally Posted by AudioCats View Post
thought they are BJT transistor to form darlington pairs
nope, not darlington. Top transistor is common base drive.
Same as grounded grid if it was a tube amp. Only way to
get that much gain without miller effect killing the frequency
post #11 of 403
Thread Starter 

impression deleted

.... deleted the initial comparison impressions.
If you remember what it was, well after 4 days of burn-in, the initial comparison impressions still held true. Then why would I delete it, you might ask...... well, I will HAVE TO sell the 005 sometime soon, you see .

Have fun
post #12 of 403
Thread Starter 
Originally Posted by kevin gilmore View Post
nope, not darlington. Top transistor is common base drive.
Same as grounded grid if it was a tube amp. Only way to
get that much gain without miller effect killing the frequency
Thanks for the info

If it was a tube amp, replacing the three 390K resistor with a proper CCS will improve SQ and gain, right? can it be done here too?
post #13 of 403
Thread Starter 
Just a little experiment: SRM-001 test setup driving the ESP950........

result: can not play very loud (not as loud as I want). Sound is clean, very soft sounding, and since the volume had to remain low it was not showing some of details very well. But the sound was pleasant. Good bass when the output capacitors were increased to 0.1uF. (With baby stax 0.01uF or 0.1uf output had about the same bass, but this experiment showed that larger output caps can drive larger phones better).

The ESP950's driver are about the same size as the Lambda's, so the modded SRM-001 won't have enough juice to push the lambda's either. But the SR-XIII, on the other hand, have much smaller drivers and can be a very different story..... I have a feeling that the little amp will be able to drive the SRX reasonably well (just have to lower the bias to 230v). If we can locate a source of Stax special connectors, it won't be difficult to make adaptors to fit SR-X/3/5 with the little amp.

Not to mention all them electrect phones that have high sensitivity and don't even require a bias voltage, they should feel right at home teamming with the SRM-001
post #14 of 403
The smaller drivers should be fine with the SRM-001 as they have a lot in common with the SR-001/003. I think you need a Gamma Pro...
post #15 of 403
Thread Starter 
Originally Posted by spritzer View Post
The smaller drivers should be fine with the SRM-001 as they have a lot in common with the SR-001/003. I think you need a Gamma Pro...
no way dude the Gamma pro's cost a fortune nowadays, I'd much rather get a pair of SR-XIII and recable it to silver and convert the SRM-001 to 230V bias

by the way, I suspect the reason the 950's sounded so soft was due to the extra output demand dragged down the high voltage supply voltage, and this supply voltage also provides bias for the phones, so: increased output demand -> lower supply voltage -> lower bias than the normal 550~600V -> softer sound. That won't be a problem for low-bias version though, since the bias voltage will be dropped down to 230V anyway (replace R32 with a 10M resistor, install 0.01uF/630v cap at C19 location, and add two 110V/120V diodes at the "R" location, that should do it)......

update 1-3-08: just ordered caps (BlackGates, Auri, Sonic I/II) from SonicCrafts. Hopefully they will show up in a couple of weeks. Then I will let them burn in for 10 days straight, then the real comparison test..... the schedule is pretty tight, might not get it done by the end of the month.

update 1-12-08: Got the caps. SonicCraft shipped the caps out the very next day of my order (1-4-08), super! Highly recommanded vender.
Too bad the Auri 0.01uf/600v wouldn't fit, just so slightly too big Good thing I still have the Sonic I and II to use
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