As of right now, the ODA will also have 7 Vrms. There are some options under consideration that would allow for 15 volt rails and more output. But being realistic, 7V vs 9V is only about 2 dB difference. 1 dB is the smallest change most people can even detect. So it would hardly be an obvious difference between 108 dB peaks and 110 dB peaks.
I'm not a vintage AKG expert so I really don't know how many people have them who might be shopping for a new amp? I get the impression it's a fairly small number. I know they no longer make them and the newer AKGs are easier to drive and within the O2/ODA's capabilities.
I'm not a vintage AKG expert so I really don't know how many people have them who might be shopping for a new amp? I get the impression it's a fairly small number. I know they no longer make them and the newer AKGs are easier to drive and within the O2/ODA's capabilities.
OK.
I'm using almost exclusively my K701 these days anyway, so that was most by curiosity than a real request. 
Yes - everything is on the board.
Also, I just now read your post about the solders. Somehow I missed that when I posted about the solder diameter a few posts back. I would echo what nezbleu said - definitely go for eutectic alloys. All the components of the alloy solidify at the same time. Just scan down the "eutectic" column here in the wiki for a yes or no on the various alloys:
https://en.wikipedia.org/wiki/Solder
I would also agree with RocketScientist that no fan is needed. I fixed TVs and stereos in a large shop for years while putting myself though the BSEE. I breathed in a ton of fumes, as did the others and the owner of the shop who had been doing it for decades - and I'm still kicking.
Probably says that secondary x-ray emission off the back of CRT tubes isn't that great of an issue, either. I probably absorbed several chest x-rays worth every few months.
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Maybe just to double check, here's the measurements I recorded using the wau16-400 transformer:
Adapter output: 18.6 VAC
Battery terminals: 23.85 VDC plugged in with switch off and 23.42 VDC with switch on
C8 and C9: 11.75 VDC and 11.70 VDC
C2 and C3: Both 23.57 VDC
P2 DC: 0.004 VDC
J3 DC: 0.001 VDC
Input DC: 0.001 VDC
All ICs cool to the touch
I don't have batteries to test the MOSFETs and the current draw yet, but I did use a wrist strap and was very careful with handling them so I'm still unsure why my regulators are getting hot.
Adapter output: 18.6 VAC
Battery terminals: 23.85 VDC plugged in with switch off and 23.42 VDC with switch on
C8 and C9: 11.75 VDC and 11.70 VDC
C2 and C3: Both 23.57 VDC
P2 DC: 0.004 VDC
J3 DC: 0.001 VDC
Input DC: 0.001 VDC
All ICs cool to the touch
I don't have batteries to test the MOSFETs and the current draw yet, but I did use a wrist strap and was very careful with handling them so I'm still unsure why my regulators are getting hot.
If you're getting 18.6VAC out, I'm wondering if the 16V Adapters are too high for the un heat sinked 12V regulators. The extra voltage ends up as heat, so the greater the differential between the input and output voltages, the more heat that needs to be dissipated.
What AC wall transformer are you using? With 12 VAC the regulators should be warm but not hot. With 16+ VAC they get hotter, especially if the batteries are also installed and charging or the amp is playing loud into difficult headphones.
The actual case temp is around 35C - 55C (95F - 131F) depending on which transformer, if the batteries are installed, how hard the amp is working, etc. The 7812 on the inside runs a bit hotter than the 7912 on the outside as it doesn't have the aluminum case (or as much free air if the board is not enclosed) to radiate heat to.
Given all the talk about the regulators lately, it's worth sharing the actual calculations for anyone who's curious with the amp playing music at a typical "loud" listening level with my DT770-Pro 80 headphones. The regulators are rated for continuous operation with a junction temp of 125C:
12VAC = 39C
14VAC = 47C
16VAC = 62C
18VAC = 71C
20VAC = 79C
From the above you can see why I advised Jokener to aim for 14 VAC for the custom transformer for the European group buy. It's high enough to meet all the needs of the amp without getting the regulators very warm at all. But anything up to 20 VAC is still fine.
Here are the calculations for anyone who's curious. The average amplifier power consumption is under 40 mA, the batteries need around 10 mA, and the quiescent current of the regulators is (worst case) 5 mA, so that's 55 mA total current per regulator.
With the WAU12-200 12 VAC transformer the unregulated voltage is 16.1 VDC so Vdrop = 16.1 - 12 = 4.1V. 4.1 * 0.055 = 0.23 watts total dissipation. The worst case theta junction to ambient is 60 C/W. So 0.23 * 60 = 13.8C temp rise over ambient (some datasheets list a value of 40 which would mean considerably lower temps). With a 25C ambient, that gives a junction temp of 39C (102F) and a case temp well below that (under 35C).
With the WAU16-400 16 VAC transformer, the unregulated voltage is 23.1 VDC so Vdrop = 11.1V. That gives 0.61 watts total dissipation and a rise of 37C above ambient or 62C junction temp with a case temp around 50C (122F).
With a 18 VAC transformer Vdrop is 14V giving .77 watts and 46C rise or 71C junction temp.
With a WAU20-200 20 VAC transformer, Vdrop is 16.5V for 0.9 watts and 54C rise for a 79C junction temp. This is still well under the 125C continuous rating and the 150C auto shutdown temp. But it is hot to the touch.
If you work the amp really hard, it draws about 60 ma for both channels, which increases the total current to 0.075 amps. Even with a worst case 20 VAC transformer, you get 1.24 watts and a rise of 74C above ambient or around 100C junction temp. If the ambient is really warm you might get close to the 125C rating but you'd have to really work at it.
I was posting the above while dewasiuk and MrSlim posted their comments. With batteries installed, the voltage across C2 and C3 drops slightly so my 23.1 vs dewasiuk's 23.8 is about right.
With the amp idling, and no batteries installed, the current should be around 30 mA total (25 mA for the amp 5 ma for the regs). So Vdrop = 23.8 - 12 = 11.8 volts * 0.03 amps = 0.35 watts per regulator. That's temp rise of 21C over ambient or a junction temp around 46C. The case temp should be comfortably under 40C (104F). That should not be too hot to keep your finger on. If you can find any 9 volt batteries, even cheap disposable ones, you can measure the current as in the documentation. Or you can lift up (desolder) one end of D1 or D5 and insert your DMM in series using the current jack and range to measure the current on AC power. Do that without headphones connected just in case the power management circuit isn't working right and you end up with only one rail working.
With the amp idling, and no batteries installed, the current should be around 30 mA total (25 mA for the amp 5 ma for the regs). So Vdrop = 23.8 - 12 = 11.8 volts * 0.03 amps = 0.35 watts per regulator. That's temp rise of 21C over ambient or a junction temp around 46C. The case temp should be comfortably under 40C (104F). That should not be too hot to keep your finger on. If you can find any 9 volt batteries, even cheap disposable ones, you can measure the current as in the documentation. Or you can lift up (desolder) one end of D1 or D5 and insert your DMM in series using the current jack and range to measure the current on AC power. Do that without headphones connected just in case the power management circuit isn't working right and you end up with only one rail working.
Good point; if some people are having issues that would point to excess current draw, the obvious next step would be to measure the current draw! Do that with no opamps installed. If the number is even slightly high you have a short somewhere. If not, install opamps one at a time, measure again, until you find a jump in current draw.
This really isn't rocket science
, if a regulator is overheating then, as a DIYer, your job is to find the fault. Start by checking voltages, if that doesn't pan out start measuring actual current.
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I just wanted to add another data point to the discussion. I already posted here to say that I built two O2's, and they worked just fine and sounded superior to other HPA's I have heard.
However, I hadn't used them much, and I was powering one at a time via some batteries that I didn't have a lot of confidence in (they don't even have a mAH spec printed on them). Before I read RS's comments about high-mAH-rated batteries usually having high internal impedance, I ordered 4 from eBay rated at 450mAH. They arrived a couple of days ago; I checked their voltage and it was 9.05V, so I assumed they had arrived fully charged (more-or-less) and I inserted them into both amps and turned them on. One was left on but just idling (no input, no load), the other had input from an mp3 player on repeat and was driving a pair of 32-ohm iem's at slightly uncomfortably loud levels. Of course, I had neglected to check the battery level of the mp3 player, and it died long before the amp. The upshot is that about 16 hours later I checked them and both LEDs were glowing but neither amp was producing output. I did not observe any hysteresis effects, but perhaps if I had checked sooner...
Today I recharged one set of batteries in a charger; when they were done the measured voltage was 9.6V. During the same time the other set was in an O2 plugged into AC; after about the same time period the measured battery voltage was around 9.1V, so I assume the second set charging in the O2 was not fully charged.
Tomorrow, after the one set has run down I will plug the amp into the AC (I am using a 500mA 16V xformer), play music at unhealthy levels, and use the temperature probe of my DMM to measure temps of voltage regs, mosfets, opamps, etc.
To summarize: 2 amps, no observed low-batt hysteresis effects, no unusual (or even remarkable) observed temperatures, outstanding performance. If only the front panels would arrive (I understand they are on their way)!
However, I hadn't used them much, and I was powering one at a time via some batteries that I didn't have a lot of confidence in (they don't even have a mAH spec printed on them). Before I read RS's comments about high-mAH-rated batteries usually having high internal impedance, I ordered 4 from eBay rated at 450mAH. They arrived a couple of days ago; I checked their voltage and it was 9.05V, so I assumed they had arrived fully charged (more-or-less) and I inserted them into both amps and turned them on. One was left on but just idling (no input, no load), the other had input from an mp3 player on repeat and was driving a pair of 32-ohm iem's at slightly uncomfortably loud levels. Of course, I had neglected to check the battery level of the mp3 player, and it died long before the amp. The upshot is that about 16 hours later I checked them and both LEDs were glowing but neither amp was producing output. I did not observe any hysteresis effects, but perhaps if I had checked sooner...
Today I recharged one set of batteries in a charger; when they were done the measured voltage was 9.6V. During the same time the other set was in an O2 plugged into AC; after about the same time period the measured battery voltage was around 9.1V, so I assume the second set charging in the O2 was not fully charged.
Tomorrow, after the one set has run down I will plug the amp into the AC (I am using a 500mA 16V xformer), play music at unhealthy levels, and use the temperature probe of my DMM to measure temps of voltage regs, mosfets, opamps, etc.
To summarize: 2 amps, no observed low-batt hysteresis effects, no unusual (or even remarkable) observed temperatures, outstanding performance. If only the front panels would arrive (I understand they are on their way)!
Just to be complete, I will add the following: All the parts I used were either the preferred part specified in the BOM or one listed as an acceptable substitute in the BOM, with the following exceptions:
R1,R2: I used ERG-2SJ221A Panasonic Electronic Components Metal Oxide Resistors
They fit, they are rated at 2W instead of 1W, and they at least specify a temperature coefficient (although not impressive).
Electrolytics: I used Panasonic FR's, which have very low ESR and generally excellent specs, and I used 35V parts even for C8 and C9 (based on a general belief that caps rated for a higher voltage do a better job even at lower voltage -- like most capacitor-related opinions this is based on hearsay and is essentially religious in nature).
Oh, yeah: Mouser were out of the right-sized 2.7M resistors for R25, so I used 2.8M. I gather that should have made the hysteresis issue worse, but as I said I have not yet observed any problems of that nature.
Like I said, everything else is bog-standard.
R1,R2: I used ERG-2SJ221A Panasonic Electronic Components Metal Oxide Resistors
They fit, they are rated at 2W instead of 1W, and they at least specify a temperature coefficient (although not impressive).
Electrolytics: I used Panasonic FR's, which have very low ESR and generally excellent specs, and I used 35V parts even for C8 and C9 (based on a general belief that caps rated for a higher voltage do a better job even at lower voltage -- like most capacitor-related opinions this is based on hearsay and is essentially religious in nature).
Oh, yeah: Mouser were out of the right-sized 2.7M resistors for R25, so I used 2.8M. I gather that should have made the hysteresis issue worse, but as I said I have not yet observed any problems of that nature.
Like I said, everything else is bog-standard.
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Okay the power supply seems to be okay, without U2 and the other amps I measure +11.91 Vdc and -11.96 Vdc with switch "off".
Switch "on" changes the measures to +11.75Vdc and -11.8 Vdc.
Installing U2 changes nothing with this measurements. Seems okay to me.
I will do some more measurements around the LED later and report again. Thank you so far.
I have 23.6 Vdc there. Nothing is getting warm.
Okay we're probably on the right way to find a solution ... I have 23.2 Vdc across R6 and 0.4 Vdc on the LED. Should be less on R6 and more on the LED terminals, right?
The LED is bad, I finally soldered it out and replaced by one from another set I have here. Fine to see the new LED is doing the job!
There measuement are now 21.8 Vdc across R6 and 1.8 Vdc across the LED.
I will do the initial tests now.
I still have serious concerns what happened to make the opamp pop. Do you have any hints what I shall prove again before connecting headphones? I can't see any obvious faults on soldering or component disorder.
Testing just with AC power. I don't even have the battery connectors soldered yet.
Thanks a lot for your help!!
Thank you, how did you get that case temperature?
I have .615 watts dissipation based on my measured numbers. My unregulated voltage is over 24 volts.
The amp draws 35ma, and the regs have 15ma at the outputs.
Uncased, I measured the regs at 77C after an hour or so.
