Blueman2

I am finally getting around to this build and had a couple questions about the BOM. - The 470R resistor in the BOM is 2W. Is 1/2W OK? 2W won't fit in holes - The 100V diode in the BOM is 5W. That also will not fit in the holes. Can I use a 1W diode? - The BOM lists 2 R2k Trim pots, but pair of boards requires 4. Is that right?

I treated myself to a new SR-007mkii and a Schiit Gumby for Christmas, and have been doing different tests/mods to see what I like the best. Because of the darker nature of the -007 and the need for lots of power, I was assuming I would like the Carbon amp better. But after a couple weeks of listening, I am finding the SRX-Plus to be a better match with the -007. I initially did the blue-tak and spring mod on the -007, but reversed them when it did not seem to help the bass much (or at all that I could tell). With the -007mkii in its original state again, I find the SRX provides a bit more bass and an overall more pleasing sound. I tend to listen for 2 instruments to evaluate sound: bass guitar and snare drum. For bass, it is about, well, the bass. How much punch and depth. I found the SRX better than the Carbon. For the snare drum, I listen to see if I can clearly hear the metallic sound of the wire mesh under the drum. This is my 'acid test' for good source material. It is the only way, for example, I can consistently win blind A/B test of 160kbps mp3 vs flac. The -007 really brings this out, much better than the SR-507 I was using. But I was happy to hear that the SRX was just as good as the Carbon in correctly presenting the snare sound on my new SR-007. Not better, but just as good to my ear. That really surprised me because the Carbon is just amazing at the higher frequencies in this regard. So saying the SRX is just as good is amazing praise. I still like the Carbon for electronic music, for example, but am finding myself using the SRX-Plus as the go-to amp for my new SR-007s. But just just so that Jim L does not get too big of a head, I going to start this weekend on my BH build. The SRX is about to get some more stiff competition.

Maybe is it just a shadow, But does the left bridge capacitor look burnt?? That is the + side, so probably not the issue anyway.

Version .44, which I think he is using, has all 3 bridges with key at bottom. Here is bare board I have:

I know this is an old topic, but I wanted to add some pictures of the latest sr-007 so that people can see if there are any differences. I did the blue-tac mod on the port and increased the spring to 105mm and flattened it as recommended. I inserted the spring as per the old -007 model as shown in the instructions (credit to spritzer for the instructions sheet picture). The difference in bass was actually quite moderate. It did help with bass response, but the change was subtle. Which is really all I wanted. No issues with the Stax fart at all so far. See attached. This was purchased Dec 15 2017 directly from Stax in Japan.

Thanks, Birgir. Is it acceptable to just add another 2.2M or even 4.7M resistor after the cap? It would be easiest to just insert that into the line going to the bias pin and then leave the PCB alone. .

I bought an SRD-7 Professional that I wanted to give to my brother so that he can experiment with Stax headphones. These already have pro bias, but I was still going to re-wire it so that it has 4.7M final resistor on the bias. Should I add 5.1K resistor on the signal outputs while I am at it? I see most amps have these yet these SRD units do not.

To be clear, the SERIAL NUMBER is a lottery, but the model is the same as the current builds. Only difference is color (black for export, silver for japan). There are a lot of articles by otherwise knowledgeable people saying SZ2 and SZ3 indicate different models. This appears to be wrong.

Yes, exactly. But usually it is from 0 to around +300 then drift down. Going from -300 to +300 is more concerning. Especially since the membrane bias is at +590V on my setup. +590V right next to -300V is asking for trouble. Yes, especially given the high temperatures of the Carbon. The thermal cycling will stress anything less than a really good joint. No kidding, I have been thinking about this every time I turn the system off after use. I just did not think it would happen so early. I guess infant mortality for the poorest solder joints. EDIT: I accidentally left the amp on all night last night, so about 14 hours. Still working fine. Of course, it is not the heat that kills solder joints, but rather the cycling from hot to cold to hot to cold that does it. Only time will tell.

OK, I did some rework and am crossing my fingers I got the offending joint. I did find a few questionable joints, but nothing very obvious. It is working fine for now. I will just have to keep an eye on it. Unfortunately (or perhaps, VERY fortunately) the failure is not immediately obvious. I will have to watch out for a 'pop' in the earspeakers on startup or shutdown which means the problem is back. I am sure the instantaneous jump from -300 to +250 (which is what was happening on shutdown with the issue) is not good for my earspeakers! But at least it was both O+ and O- at the same time. Otherwise, boy, that would be nasty. Thanks again for the help!

Yes, this all makes sense now. I would feel better if I knew where to focus on reflowing the joints, though. Given it was only the offset, and it impacted both sides of the board, that should give me a clue. Just not sure....

Yes, I guess that is the next step. I did a compare of resistance readings at different test points comparing the working Right channel to the bad Left channel, and so far everything is exactly the same. I will continue with that route for another hour, and then just pull the board and check solder joints. I was also thinking of the A42s, but since this is impacting both + and - sides equally, it would seem odd that both would have equal solder issues. Any other thoughts of what would impact both sides at the same time?? Thanks again, sorenb, for the advice! EDIT: Bad solder joint, as you suspected sorenb!!! I have not found it yet, but I decided to use a probe to 'nudge' all the resistors and other components around a bit to stress the joints. Sure enough, on next power up, the offset was back down to essentially 0 volts. Of course, now I have to find that damn bad solder joint!!!!

I have a 100ohm tail now as I recall. I think the original spec was 182, right? But you are right in that this will directionally get me there. At 150 ohm across the trim pot + tail resistor (trim in middle at 50 ohms + 100 ohm tail), I am at -300V. When I adjust the trim pot to max (about 195 ohms total across tail and trim) I am down to -220V. If that is a linear relationship, it is about 1.6V per ohm. So I would need to increase tail by about 180 ohms, so need about ~280 ohms tail. BUT, the odd thing is that this amp ran just fine for several weeks with perfect offset using 100 ohm tail. I noticed about 2-3 days ago a bit of static intermittently, and an increased 'pop' when powering up or down. So rather than an incorrect resistor value, I am suspecting something else has caused this. Some root cause I have not figured out yet. Is the OP27A used when no servo is connected? What else might impact offset so severely yet allow the amp to still basically operate?

Great advice! I had not checked the output voltages. Sure enough, left channel shows offset on both sides of -300V (yes, negative). Right channel is perfect. I turned off the Servo, which make the LED normal brightness. Both the O+ and O- of the left channel show voltage of -300V. Trim pot does have an effect, but not that much. I can get it down to about -200V or so. I am still getting 20ma on the CCS current (1V on the test resistor). Also, still able to balance the O+ and O- perfectly. It is just the offset that is wrong. Oh, and it did still play music just fine from an audible standpoint (before I realized how off the offset was!). Recommendations on what points to test to narrow what might be wrong? Main clues I am going off of are: - Issue is impacting both the O+ and O- sides equally, it must be something that impacts both sides of the board. - Since it still makes music, I am guessing the silicon in the music path is still ok?

After installing a quad volume control and connecting the XLR inputs to prepare for a balanced output DAC, I noticed something odd. The LED on the Positive output side of the left channel dims quite a bit after startup. Initially, all 4 LEDs are about the same brightness, but then after about 1 - 2 seconds after power on, the Left channel positive LED (the one near the O+) dims. Happens just as the music starts coming from the earspeakers. Everything still sounds OK. Only things that I notice is a bit more of a 'pop' about 1-2 seconds after turning the amp off. The pop comes mostly from the left channel. Any idea what a dim LED might indicate? I did a quick check of voltages and they appear OK. And is it normal to hear a 'pop' when powering down the amp? It has always done this, but it seems louder now and more focused on the left channel.

Thanks guys. I have ordered a Schiit Gungnir Multibit, which is fully balanced by design (2 pairs of DACs to handle this, so 4 in all). As of now, I have not wired up the XLR outputs inputs in my Carbon since I only had SE devices in the past. I was wondering if it was worth the effort to go ahead and wire up the XLRs, install the quad volume control I have, install a DPDT switch to allow enabling SE grounding, and use balanced rather than SE? Sounds like it might be marginally worth it. I think I will do it and report back if I can tell any difference.

Since the Carbon (and I guess all Stax amp designs??) is inherently balanced, is it worth getting a DAC with balanced output? I know balanced helps for long cable runs, but is there any real benefit when the DAC is 1 foot away from the Carbon?

I plan to get used to mine for about 2 weeks, then will do port and spring mod as necessary to judge the difference. Will report back with pictures....

I too was worried. Just bought a brand new pair of SR-007A and was surprised to see the SZ2 serial number. But they look brand new, still in the box with plastic sealed around the box. BTW, johnwmclean, I am 230x, so ours were probably born on the same week!! Or, given the random serial numbers, years apart!?!

Yes, another point. Interesting that my 507s have a metal yolk, yet the L700 which is 2X the price has a plastic yolk. Or at least some have reported it being plastic. I have no complaints about the build of my 507s though. But rather than pollute this Carbon thread with a earspeaker discussion, I will stop here and say I just ordered a SR-007A that will hopefully arrive by Christmas. If I am unsatisfied and want to go to the 009, I will at some future date. By then, maybe the next gen -009 will be out! Thanks for the recommendations, everyone!

Yea, that has been the issue. I missed the last San Francisco meet up and there are no dealers in the SF Bay Area that I know of.

LOL. Yes, the cost difference is about $500. $1250 for L700 and $1800 for 007A. I cannot quite justify the 009 cost of $3300 (right?...right?)

For those that have the Carbon, what would you recommend as the better earspeaker? SR-L700 or SR-007A (newest version)? Price of the 2 are within about $500, so cost savings is only a minor consideration. I also want to be able to drive my earspeakers with the SRX-Plus since I find that amp gives a good tone for some music. Not sure if that changes the answer or not.

BTW, where does the Carbon name come from for this amp? Carbon makes me think resistors, but not sure what that has to do with it. Just out of curiosity.

Hmmm. Never a big fan of BTO, but worth a try I guess