availlyrics
Before to ask here my question, I completed everything posted about O2 on NvAvGuy blog. Yes, I saw excellent measurements. I more interesting in subjective point of view on comparison of O2 SOUND to SOUND of another popular amps like Shiit, Burson, iFi, TEAC, Violectric, QRV-08.
Video on youtube - just designer or/and manager failure. If anyone will start to omit safety or/and testing procedure - the result like on that video will not ask to wait for it.
Before to ask here my question, I completed everything posted about O2 on NvAvGuy blog. Yes, I saw excellent measurements. I more interesting in subjective point of view on comparison of O2 SOUND to SOUND of another popular amps like Shiit, Burson, iFi, TEAC, Violectric, QRV-08.
Video on youtube - just designer or/and manager failure. If anyone will start to omit safety or/and testing procedure - the result like on that video will not ask to wait for it.
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Nwavguy write
"ACTIVE CURRENT LIMITING: Say you’ve had far too much to drink some evening, you’re listening to your expensive low impedance headphones, and a favorite song starts playing. If you’re listening to one of those “over kill” amps, or even the FiiO E9 that can put out 1 watt, you may well be out several hundred dollars when in your drunken enthusiasm, you crank the volume too high. The O2 tries to prevent such accidents with intelligent current limiting. Instead of throwing a big resistor in series with headphone jack, as many amps do ruining the output impedance, current limiting makes a lot more sense. It allows maintaining a near-zero output impedance and still limits power into lower impedance loads. For those who think it compromises sound quality, the O2 measurements speak for themselves as do the blind listening tests. 166 mA was the target goal to drive nearly any headphone and the O2 doesn’t limit until around 200 mA. So in real world use you won’t get anywhere close to triggering the current limiting. It only comes into play to help protect the output stage and headphones from accidents."
But how he has realizied this active current limmiting? Dont find such a circuit on the O2 schematics.
"ACTIVE CURRENT LIMITING: Say you’ve had far too much to drink some evening, you’re listening to your expensive low impedance headphones, and a favorite song starts playing. If you’re listening to one of those “over kill” amps, or even the FiiO E9 that can put out 1 watt, you may well be out several hundred dollars when in your drunken enthusiasm, you crank the volume too high. The O2 tries to prevent such accidents with intelligent current limiting. Instead of throwing a big resistor in series with headphone jack, as many amps do ruining the output impedance, current limiting makes a lot more sense. It allows maintaining a near-zero output impedance and still limits power into lower impedance loads. For those who think it compromises sound quality, the O2 measurements speak for themselves as do the blind listening tests. 166 mA was the target goal to drive nearly any headphone and the O2 doesn’t limit until around 200 mA. So in real world use you won’t get anywhere close to triggering the current limiting. It only comes into play to help protect the output stage and headphones from accidents."
But how he has realizied this active current limmiting? Dont find such a circuit on the O2 schematics.
He infered it from the "Maximum Output Voltage Swing vs. Output Current" graph on page 3 of the NJM4556A data sheet:
http://www.njr.com/semicon/PDF/NJM4556A_E.pdf (opens PDF)
I had that discussion with him via PM once shortly after the O2 was released. The datasheet never explicitly says the chip has current limiting, but that graph does appear to be showing it. He also said he verified the chip's current limiting experimentally.
But even with the current limited that still doesn't protect against too much power dissipation in the NJM4556A output chips (rms output current plus quiescent current mulitplied by rms voltage drop across the chip). The output chip(s) have cracked in half before for folks under the extreme combined conditions of 16 ohm headphone load, low sensitivity headphones, high(er) volume levels and music with high sustained musical peaks.
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Hi AGDR,
I considering now to start to build an O2 from the kit. I have one question - what is the goal to use single half-wave rectifier ? Just to place the EMI source out of enclosure, far from sensitive components ?
I going to use the O2 on my desktop only. So portability not is an issue. Now the EMI. If I`ll making my enclosure from solid piece of aluminium, with separate chambers for the PCB and, AC-0-AC toroid transformer, is that will provide sufficient EMI isolation ? Or I missed something, and better to go with wall AC adaptor anyway ?
I considering now to start to build an O2 from the kit. I have one question - what is the goal to use single half-wave rectifier ? Just to place the EMI source out of enclosure, far from sensitive components ?
I going to use the O2 on my desktop only. So portability not is an issue. Now the EMI. If I`ll making my enclosure from solid piece of aluminium, with separate chambers for the PCB and, AC-0-AC toroid transformer, is that will provide sufficient EMI isolation ? Or I missed something, and better to go with wall AC adaptor anyway ?
Along with getting the transformer out of the box and avoiding any magnetic coupling into other wiring, here in the US it is mainly a legal thing. Any commercial device that has an AC wall cord going to it must have UL certification, or an equivalent like CSA, for safety. I've been through a few of those for commercial devices and they are costly and time consuming.
For DIY devices as far as I know it is up to the DIYer to make sure they don't burn their house down (!) but I know NwAvGuy was very careful on things like that from his blog and some email chats. The plug-in wall transformers are already UL certified. As long as 24V or less is being run to the device from the wall transformer the device is considered "low voltage" and doesn't need UL certification.
By using an AC transformer, rather than DC, and the half wave supply to create the two power rails he only had to run one pair of wires. There really are not any center tapped plug-in wall transformers out there on the market. If there were it would require 3 wires and therefore be more of a connector problem, requring a DIN connector or mini-XLR, etc.
BUT... having said all that... if you are using your own center tapped transformer to feed the O2 power rectifiers that is certainly OK. Putting that transformer - and ideally the rectifiers and filter caps - inside a metal shield (or in your case is sounds like a separate milled-out section of the case) is always a good idea. That will prevent EMI. Keep in mind that the magnetic field will go right through metal shielding (unless something like mu-metal), so in addition to the separate compartment you will need some spacing between the transformer and your amplifier parts to keep the magnetic field diminished and prevent 60Hz coupling into your amp circuits.
Also keep in mind that NwAvGuy measured everything with his dScope analyzer, including the residual power line harmonics, and they are too small to hear even with the original O2 setup with the wall transformer. I personally wouldn't worry about them and just use the O2 as it is designed. In his blog here:
http://nwavguy.blogspot.com/2011/07/o2-headphone-amp.html
search for the title "noise on AC power" which is about the 3rd dScope plot he shows. You can see that ALL noise, including the power line harmonics, are below -112dBV [referenced to 1 volt rms], which is very small.
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Anything is fine - even a grounded piece of thin tin foil would do the job. So if you are milling it just enough thickness for the wall to be good and solid mechanically. For EMI suppression at power line harmonic frequencies the only thing that matters is that the surface is conductive.
agdr,
on the thickness of a case such as you are just talking about wouldn't there be a major difference in whether the case is magnetic or, ferrous or non-ferrous metal and the way they would affect both magnetic and electrical problems.
I am just lurking on the thread reading it as I got my daughter a board to build. She is almost done but broke a pin in one hole and is having one hard time getting it out. I will have to fix that and let her move forward to hopefully a nice sounding little headphone amp.
on the thickness of a case such as you are just talking about wouldn't there be a major difference in whether the case is magnetic or, ferrous or non-ferrous metal and the way they would affect both magnetic and electrical problems.
I am just lurking on the thread reading it as I got my daughter a board to build. She is almost done but broke a pin in one hole and is having one hard time getting it out. I will have to fix that and let her move forward to hopefully a nice sounding little headphone amp.
A new ODAC revB is out!
I just learned about this. Apparently George Boudreau at his company Yoyodyne Consulting has produced a new RevB of the ODAC. Remember the story there from back in 2012 that George and NwAvGuy did the first ODAC together as a collaboration. George apparently wound up with the intellectual property rights in return for financing the project. NwAvGuy provided design input and did the dScope performance tests. It looks like Jon at JDS Labs did the dScoping this time.
The rev B cures several performance problems from the first ODAC and seems to improve on the specifications throughout. The write-up says the new board is the same size and mounting as the orginal ODAC, a drop-in replacement.
Yoyodyne Consulting Inc.
Releasing ODAC RevB | JDS Labs Blog
I would expect this one to be extremely good! George has had years now to think about the design and Jon certainly knows his stuff.
I just learned about this. Apparently George Boudreau at his company Yoyodyne Consulting has produced a new RevB of the ODAC. Remember the story there from back in 2012 that George and NwAvGuy did the first ODAC together as a collaboration. George apparently wound up with the intellectual property rights in return for financing the project. NwAvGuy provided design input and did the dScope performance tests. It looks like Jon at JDS Labs did the dScoping this time.
The rev B cures several performance problems from the first ODAC and seems to improve on the specifications throughout. The write-up says the new board is the same size and mounting as the orginal ODAC, a drop-in replacement.
Yoyodyne Consulting Inc.
Releasing ODAC RevB | JDS Labs Blog
I would expect this one to be extremely good! George has had years now to think about the design and Jon certainly knows his stuff.
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I'm sorry about the delayed reply, I haven't circled back to this thread in a while.
If the transformer is torroidal you are probably OK because the magnetic field should be largerly self-contained. But if a traditional EI tranformer, than yep having a ferrous surface at the right angle to the ends and close enough could be a bad thing, inducing eddy currents.
ODAC rev B probably elimnates the need for a galvanic isolator
In reading through Jon's blog notes (JDS) at the link above for the ODAC rev B, it sounds like he and George tracked down why some people had to use a galvanic isolator to make the original ODAC work correctly. I've seen enough postings about the problem over the years that something was clearly up there. For the last couple of years I've suggested to anyone who ask about the ODAC to always use a galvanic isolator with it to prevent any trouble.
Sounds like PC USB bus implementations that put a ceramic capacitor on the USB line resulted in an input ESR that was too low for the LDO voltage regulator chip on the ODAC, causing the regulator to oscillate. The blog says they've switched to a different AD part that doesn't have the problem.
So apparently the original ODAC didn't need galvanic isolation of the power and/or data lines per se, it just needed power lines that didn't have a ceramic capacitor on them. Presumably most galvanic isolator(s) didn't, which solved the problem as a side effect.
I know George is a member of the forum here. Good work on the new ODAC!
In reading through Jon's blog notes (JDS) at the link above for the ODAC rev B, it sounds like he and George tracked down why some people had to use a galvanic isolator to make the original ODAC work correctly. I've seen enough postings about the problem over the years that something was clearly up there. For the last couple of years I've suggested to anyone who ask about the ODAC to always use a galvanic isolator with it to prevent any trouble.
Sounds like PC USB bus implementations that put a ceramic capacitor on the USB line resulted in an input ESR that was too low for the LDO voltage regulator chip on the ODAC, causing the regulator to oscillate. The blog says they've switched to a different AD part that doesn't have the problem.
So apparently the original ODAC didn't need galvanic isolation of the power and/or data lines per se, it just needed power lines that didn't have a ceramic capacitor on them. Presumably most galvanic isolator(s) didn't, which solved the problem as a side effect.
I know George is a member of the forum here. Good work on the new ODAC!
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use a USB galvanic isolator with any dac(that don't have it built in) is good idea while PC is perfect source of RFI and EMI due to one, or even several reasons at same time :
a. Quite few PC SMPS stuffed with really good mains decoupling circuit.
b. The modern CPUs are able to feed from dedicated power stage (that not linear also) currents compatible to currents used in small arc welding apparatus.
c. The GPU - see above, just currents could to approach values of medium sized arc welding apparatus.
d. Are your cable/ADSL modem galvanic/RFI/EMI decoupled from network ? Modem's PSU stuffed with either input voltage conditioner ?
e. Even none of above, what about noise from: RAM, cooling fans, network controller ?
f. Even if DAC is insensitive to all of garbage from above, the preamp, power amp and so on, can, and probably DO sensitive.
a. Quite few PC SMPS stuffed with really good mains decoupling circuit.
b. The modern CPUs are able to feed from dedicated power stage (that not linear also) currents compatible to currents used in small arc welding apparatus.
c. The GPU - see above, just currents could to approach values of medium sized arc welding apparatus.
d. Are your cable/ADSL modem galvanic/RFI/EMI decoupled from network ? Modem's PSU stuffed with either input voltage conditioner ?
e. Even none of above, what about noise from: RAM, cooling fans, network controller ?
f. Even if DAC is insensitive to all of garbage from above, the preamp, power amp and so on, can, and probably DO sensitive.
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Hi ADGR.
The lack of consistancy made tracking the noise problem difficult. I tried testing the board with every computer I own. Antique WinXP, Linux boxes, netbooks, tablets, Mac Mini and could never reproduce the problems noted. It was like having a mechanic checking your car to diagnose a problem in your neighbours.
After a lot of effort John was finally able to duplicate the problem. There was no quick fix and the for those with the problem it was easier for them to use a hub or galvanic isolator.
For the ODAC-revB I switched to ADP151 low noise LDO which have no known problems with USB supply lines. The change was made to improve computer compatibility only. Any power supply noise reduction was an unintential bonus.
I have no argument against the use of galvanic isolation as it does put distance between your audio device and the computer. The ability to power the DAC with a clean, external supply can only be viewed as a positive.
Regards,
George Boudreau
Yoyodyne Consulting Inc.
The lack of consistancy made tracking the noise problem difficult. I tried testing the board with every computer I own. Antique WinXP, Linux boxes, netbooks, tablets, Mac Mini and could never reproduce the problems noted. It was like having a mechanic checking your car to diagnose a problem in your neighbours.
After a lot of effort John was finally able to duplicate the problem. There was no quick fix and the for those with the problem it was easier for them to use a hub or galvanic isolator.
For the ODAC-revB I switched to ADP151 low noise LDO which have no known problems with USB supply lines. The change was made to improve computer compatibility only. Any power supply noise reduction was an unintential bonus.
I have no argument against the use of galvanic isolation as it does put distance between your audio device and the computer. The ability to power the DAC with a clean, external supply can only be viewed as a positive.
Regards,
George Boudreau
Yoyodyne Consulting Inc.
Thank you for the background, George! Yeah a problem like that which only occurs rarely would be impossible to track down without having one of the systems that cause it to test. Even then it would require digging into the PC or laptop motherboard to figure out what was different from the rest to cause the LDO to oscillate.
Well the version B looks just fantastic from the specification. I bought one from Jon's site yesterday. It looks like you guys have managed to beat every single specification, some by small amounts and others like the jitter by a larger amount, and yet still kept the price low and form factor the same. I'm sure NwAvGuy would be impressed!
Well the version B looks just fantastic from the specification. I bought one from Jon's site yesterday. It looks like you guys have managed to beat every single specification, some by small amounts and others like the jitter by a larger amount, and yet still kept the price low and form factor the same. I'm sure NwAvGuy would be impressed!
As noted on my site and in JDS Labs write up I was not trying to improve the performance but only to deal with a few component issues. It took some effort to make sure I could meet the original ODAC specs and I was relieved when I was able to hit that target.
Any improvement in performance will be a numbers game and not noticable when listening. I expect there will be numerous contrary opinions as no device will satisfy everyone but that is life.
regards.
Any improvement in performance will be a numbers game and not noticable when listening. I expect there will be numerous contrary opinions as no device will satisfy everyone but that is life.
regards.
I'll disagree. There are quite not a few of "golden ears" around here, that "capable" to hear difference between mines cables and outlets. After biasing by reading the spec's of new DAC, they will find major improvements in it's sound. Now the objective guys, will be thankful to you because you solved the PSU problem, and still improved spec's.
So there is not to much place for pessimism.
Instead, you successful found and solved some hard to catch, unconventional bug. My respect and best regards !