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RocketScientist 4th August 2011 05:02 AM

The Objective2 (O2) Headphone Amp DIY Project
 
This has been in the works a while. At the request of friends (and a few critics) I've designed my own portable/desktop amp--The Objective2 (O2 for short). I just published the third and final main article to my blog.

It's a very objective approach (what would you expect from an electrical engineer?) at a low cost "one size fits all" headphone amp. The goal was to show how much performance is possible on a limited budget. It's designed to outperform even far more expensive desktop amps using solid engineering rather than audiophile designer parts or mythical topologies.

For now it's a DIY amp but it's a free open source design and a few commercial companies have already expressed an interest in offering it in various forms. Some have called it the spiritual successor to the Cmoy but the performance is in an entirely different league (sorry for not being more modest!). Some highlights:

* Enough output even for difficult headphones like the full size HiFiMan Planars and 600 ohm Beyers (7 V RMS and lots of current). It will drive nearly any headphone from 16 - 600 ohms to "live" levels.
* Completely silent even with the most sensitive BA IEMs (zero hiss with 131 dB S/N )
* Near zero ohm output impedance (0.5 ohms)
* Very low distortion of any kind--some tests push the lower limits of my dScope audio analyzer.
* ~8 hours battery life for the normal version & ~ 30 hours for the low power version
* Indistinguishable from Benchmark DAC1 in blind listening tests
* Switchable gain for use with different source/headphones
* Unique battery rundown protection
* DIY friendly, self contained on a single PC board, no surface mount parts
* Designed for a rugged inexpensive all aluminum case with a pre-made customizable front panel available.

And, perhaps the best part: You can build the fully functional self contained PC board for around $30 in parts with everything but the bare board available from Mouser.

The performance of this amp, as measured on professional instrumentation, genuinely rivals the well regarded headphone amp in the Benchmark DAC1 and is significantly quieter. It also beats every other amp, overall, I've ever had on my bench--especially its closest competitor the AMB Mini3.

I welcome feedback on the project and hope it develops into something worthwhile. I'm not looking to make any money from it in any way. Here's the link to the final article which has links to the earlier information as well:



http://lh5.ggpht.com/-hbgg7SOYy70/Tj...5B1%25255D.jpg


:cop:
I have put together a fault finding guide on the O2 that gives step by step testing and checking procedures. Click HERE

RocketScientist 4th August 2011 06:04 AM

Hmmm... there's no link! Let's try that again:

O2 Open Source Hardware DIY Headphone Amp Project

The above article is the best place to start but there are two more articles, a Wiki and some Google Docs files as well.

qusp 4th August 2011 05:40 PM

hang on, it beats everything youve had on your bench, yet its closest rival is the mini3 an amp that you publicly rubbished for its poor performance?

RocketScientist 4th August 2011 05:52 PM

Quote:

Originally Posted by qusp (Post 2660711)
hang on, it beats everything youve had on your bench, yet its closest rival is the mini3 an amp that you publicly rubbished for its poor performance?

I believe both statements are true and not in conflict. I can't help it if the Mini3 doesn't perform as promised but why isn't it otherwise close competition?

Do you know of another reasonably priced DIY portable headphone amp that's an overall closer rival to the O2?

qusp 4th August 2011 06:07 PM

ahh ok, the way your amble is worded and the use of the dac 1 headphone amp as a reference, did not really leave me thinking you had a limit on budget, or size for this comparison or position of superiority. in that case i dont know one off the top of my head, but i'm pretty out of touch in that sector

RocketScientist 4th August 2011 06:13 PM

No worries. I just happen to own a DAC1 making it a handy, um, "benchmark" to compare things against. It's also fairly well known and has been widely reviewed professionally so most people know what you're talking about. I agree it's otherwise kind of an apples-and-oranges comparison as it's a high-end desktop headphone DAC.

firechief 4th August 2011 06:16 PM

RS - do you accept complements? Clearly this is good work. I would use the term professional.

RocketScientist 4th August 2011 06:20 PM

Thanks firechief! I'll take any and all kudos! :) And while you might say "professional" others fall asleep reading some of my articles or their eyes glaze over. I'm still trying to improve my writing and presentation skills and have a ways to go.

qusp 4th August 2011 06:21 PM

there was a pretty impressive and very compact Japanese, or Korean language only lion powered and predominantly smd project posted on head-fi about 6 months ago that looked pretty spiffy. it was posted in the general portable amp forum and not diy area, with no schematic and they didnt export kits due to the fact lion cannot be shipped by air easily, if at all. i'll see if i can search it out, but it might not be terribly accessible for you to test or build for comparison.

I must say of its type the power supply in your amp is pretty cool. i dont always agree with your method or agenda, but the science is sound. i do also think that mostly you dont intend things to come off quite the way they do, or in the spirit that they seem to; perhaps linked to the writing i dont know and then things seem to get out of hand due to ego on both sides. just an observation, not a judgment.

RocketScientist 4th August 2011 06:30 PM

I'd be hesitant to publish a Li-Ion project as the dangers are substantial (search for "LiPo Fire" on YouTube for some entertaining, and often sad and very expensive, pyrotechnics). There are even documented cases of cheap eBay cell phone batteries burning cars to the ground because they caught fire on the passenger seat.

The power supply is a huge part of the O2's low cost performance. A real ground always beats a virtual ground. And the half-wave AC supply might seem "cheap and dirty" but properly engineered it works as well as any other AC power supply in this application. The proof is 131 dB of S/N and 0.0009% THD on battery or AC power.

qusp 4th August 2011 06:33 PM

one thing, i havent looked up the part numbers for the opamps, because actually its not that easy to read them in the parts list linked to the google docs (pita to navigate too with no quick keys, even space bar doesnt work to create a hand to move the page when zoomed in) but just wondering, i assume the opamps you have used are limited in bandwidth? having a pretty cheap pot in the middle of the feedback loop for volume control doesnt cause any issues with stability?

qusp 4th August 2011 06:39 PM

yeah i know all about the battery issues and comedic videos, just check out the RC forums, ive just started finishing up a lifep04 powered psu for a personal project with charge monitor for low voltage shutdown etc,(not so bad and actually pretty safe, but still subject to the save airfreight limitations as it hasnt been classified on its own, just comes under lithium chemistry)

lion is the worst (but its just so compact with high energy density its appealing), followed by lipoly, but its still manageable with the correct systems in place for protection. possibly still risky for diyers of varying levels though i guess

RocketScientist 4th August 2011 06:41 PM

The volume pot is NOT in the feedback loop. Each stage uses local feedback.

There should be a "-" and "+" button on the left side of the Google Docs viewer. Click the "+" button to zoom in as much as you want. Unless you're using some really weird browser it should work fine.

You can also just download the PDF and use whatever PDF viewer you want.

The gain stage op amp has GBW of 27 Mhz. The output op amps have a GBW of 8 Mhz. They're almost eerily close to the ideal of "fast enough" without having excess bandwidth to create stability issues.

In the first article I show the 10 Khz square response into capacitive loads--there's zero ringing or overshoot. None. I've tried to invoke instability and haven't even seen any hints.

paulb 4th August 2011 06:51 PM

I can't seem to access the documentation package. Google reports "This service is not available"

qusp 4th August 2011 06:56 PM

cool ok, fair enough, at a glance thats how it looked, maybe there is something up with either the pdf, or the viewer on google docs at my end because it wouldnt redraw when i zoomed in, just remained kinda half blurry and aliased, thus not being able to read part numbers or have any patience at all haha. i'll download it and have a look tomorrow. yeah was just asking, given your history i couldnt picture you publishing anything unstable for public scrutiny, but given the inability to read it i thought id ask.

ah IC, had a bit more success, i was reading it wrong, so the pot is wired as 2 shunt volumes in (well on the edge of each) the feedback, loop, but with no feedback currents flowing through the resistive element (only returns),

haha nope right in the middle, just normal pots haha

I withold further comment till i can download it, cant read ****.

RocketScientist 4th August 2011 06:57 PM

Quote:

Originally Posted by paulb (Post 2660786)
I can't seem to access the documentation package. Google reports "This service is not available"

Hmmm... way to go Google! I've had random problems with Google Docs which is sort of amazing when you consider they want companies to run their entire businesses on Google Docs and Google Apps.

But, that said, I just checked and it works for me. Here's the link:

https://docs.google.com/viewer?a=v&p...iOTc4&hl=en_US

qusp 4th August 2011 07:28 PM

1 Attachment(s)
yeah its being really buggy, could be safari or something, dunno, this is what it looks like here and it doesnt improve when i zoom

no excuse for not seeing the separate local loops though, guess i was just seeing what i expected to see, some global feedback.

RocketScientist 4th August 2011 07:30 PM

When I use the Google Docs viewer it takes a second or two after you zoom in to download the hi-res version. Perhaps Australia is connected to the USA via dialup? ;)

agdr 4th August 2011 07:35 PM

2 Attachment(s)
Quote:

Originally Posted by RocketScientist (Post 2660754)
And the half-wave AC supply might seem "cheap and dirty" but properly engineered it works as well as any other AC power supply in this application.

I was wondering about that and just (LT) Spiced it. Linear Tech's model base is pretty devoid of negative regulators so I've substituted in a couple of 100mA units and lowered the load.

Looks good, but 1.9A diode charging current and that's with reduced load... would 1N5402 be more appropriate or am I missing something?

Good work, BTW. :) I'll have to echo qusp's comments about not particularly agreeing with your method or agenda, especially as relates to AMB's mini^3, but you've clearly put in a boatload of work here in actually making an amp and writing it up, rather than just reviewing others. I'm impressed that you open source licensed it, too. Looks like a good solid original build.

RocketScientist 4th August 2011 07:47 PM

Quote:

Originally Posted by agdr (Post 2660844)
Looks good, but 1.9A diode charging current and that's with reduced load... would 1N5204 be more appropriate or am I missing something?

The 1N400x diodes are rated at 30 A of peak current for a half wave cycle (8.3 mS) and don't even get lukewarm in this application under worst case conditions. I don't think it's a problem unless I'm missing something? The diodes are running at 1.9/30 or 6% of their rated current.

qusp 4th August 2011 07:51 PM

Quote:

Originally Posted by RocketScientist (Post 2660840)
When I use the Google Docs viewer it takes a second or two after you zoom in to download the hi-res version. Perhaps Australia is connected to the USA via dialup? ;)

hehe, nope, its still like that now, i'm on a 10mbit cable line too, i refreshed this page to see this post, but the google page is still craptastic

RocketScientist 4th August 2011 07:57 PM

If anyone else is having trouble with fuzzy Google Docs results please let me know and I'll have to give my friends at Google a call. Several good friends work for them and even if it's not their responsibility they usually know who to prod. Google employees tend to be a rather proud bunch and quick to defend their honor.

agdr 4th August 2011 08:06 PM

Quote:

Originally Posted by RocketScientist (Post 2660859)
The 1N400x diodes are rated at 30 A of peak current for a half wave cycle (8.3 mS) and don't even get lukewarm in this application under worst case conditions. I don't think it's a problem unless I'm missing something? The diodes are running at 1.9/30 or 6% of their rated current.

30A non-repetitive peak. I just found my mistake - forgot to add series resistance to the voltage source. That drops it down below 1A peak.

qusp 4th August 2011 08:06 PM

1 Attachment(s)
I have a boatload of windows open in safari which does tend to bog it down memory wise, but all the same there does seem to be something a miss.

just went to download it and couldnt (see below), looks like theyve got some honorable defending to do

RocketScientist 4th August 2011 08:17 PM

Quote:

Originally Posted by agdr (Post 2660882)
30A non-repetitive peak. I just found my mistake - forgot to add series resistance to the voltage source. That drops it down below 1A continuous.

It's the duty cycle that ultimately matters. The "non-repetitive" issue, AFAIK, is mostly related to thermal issues. And if the diode is staying very close to ambient what matters are what sorts of other harm peak currents can cause. If the 1N400x can handle peaks of 30A without such harm, brief repetitive spikes of 1.9A should be a walk in the park.

But, it's good to know it's theoretically below 1A :). I never attempted a simulation on the power supply. The only simulation I ran for the entire O2 was was the power management circuit. I've mostly not had good luck with simulation representing the real world due to all the things it normally doesn't account for. And when I try to crank in all the parasitics etc., almost without fail, it fails to converge at all. But I'm sure others are better at it than I am. I'm a math and hands on guy mostly.

RocketScientist 4th August 2011 08:23 PM

Quote:

Originally Posted by qusp (Post 2660884)
I have a boatload of windows open in safari

Well THERE'S YOUR PROBLEM! Try using a well designed browser, like Chrome, Firefox or IE9 ;) Safari is largely a half baked cousin to iTunes. Seriously, it loses nearly every browser comparison lately.

agdr 4th August 2011 08:50 PM

Diodes are so cheap I always spec them so that repetitive peaks stay under the rating. Probably over-enginering, but it has never done me wrong. :)

I've actually tried spice-ing the comparator circuit too, but no luck. I'm sure its on my end though. I've subbed in a LT comparator and I don't have models for the mosfets. Time permitting I may try adding the appropriate models in if I can find them.

A few months ago I was pondering a dual-battery protection circuits and was thinking about comparators on the rails. Never did design anything. But I more or less decided that any such circuit would be best as "supervisory", powered by a separate supply with a separate battery. Seemed that collapsing rails during a fault condition would produce random results with the protection circuit unless powered by something else.

But it looks like you've used comparators good down to 2V, and the LED reference is probably around 2V - haven't looked it up yet - so that likely long before the rail caps have discharged in a fault condition the protection circuit would have kicked in. I'm assuming you probably have designed in some hysteresis so the protection circuit doesn't bounce around the trip point. Haven't read through your ckt description yet.

Also during my past ponderings it seemed like a latching protection circuit would be a good idea, something that alerted the user a fault had occurred by a LED and required manual (power cycle) action to reset. Given that if the protection circuit ever operated then by definition it needs user attention..

bat A low
bat B low
bat A and B low
bat contact A disconnect
bat contact B disconnnect
both batt contacts disconnect

Earfanatic 4th August 2011 08:52 PM

The O2 is very interesting. Well done.
And thanks for the answer in the other thread too.

RocketScientist 4th August 2011 09:00 PM

@agdr: I've tested the power management circuit every way I know how and, it took a few tweaks, but it works very well. It shuts down around 13- 14 volts (6.5-7 volts per battery) so there's no issue of the circuit itself not having enough power. I have a bipolar bench supply and have fed the O2 even conditions of extreme battery imbalance and it shuts down flawlessly every time. The trick is minimizing any transients at the headphone output due to asymmetrical rail conditions and I think I've done that. Yes it has hysteresis.

Please let me know if you have further comments after you read through the Circuit Description at the end of the article? It's always good to have another set of eyes or two checking for errors and if anything needs correcting I'll fix it ASAP.

agdr 5th August 2011 02:45 AM

I just made it back to the computer. :) Well congratulations on the protection circuit. Sounds like a very good design. A good protection circuit is really the special sauce to making a dual battery design safe and reliable.

I'm most useful with power supplies and other things close to DC. :) Hopefully someone here will eventually run a group buy of O2 boards.

RocketScientist 5th August 2011 03:46 AM

Quote:

Originally Posted by agdr (Post 2661194)
I just made it back to the computer. :) Well congratulations on the protection circuit. Sounds like a very good design. A good protection circuit is really the special sauce to making a dual battery design safe and reliable.

I'm most useful with power supplies and other things close to DC. :) Hopefully someone here will eventually run a group buy of O2 boards.

Thanks. I agree about dual battery designs.

There are already some places interested in selling the bare PC board for a very reasonable price (hopefully < $10). So a group buy isn't required. But I'll be publishing the gerber/drill files so if a group buy works out cheaper, that's fine too.

Because the enclosure isn't available from Mouser, that would be a great item to include in any group buy to cut down on the total shipping costs. The same is true for the metal volume knob from DigiKey and perhaps the batteries purchased in a large lot from All-Battery (eBay) or elsewhere. If a group buy included the options of a PC board, enclosure, volume knob, batteries and perhaps even the FPE front panel, that might substantially broaden the appeal? Everything else is available from Mouser (as is a plastic volume knob).

The latest revision prototype boards are due to arrive next week. Once I build one up and torture it with the dScope, etc, I'll know if I can release the board design into the wild--including the vendors interested in offering them.

Also, as a general FYI to everyone, I've substantially revised the first two articles. If you haven't looked at them in a while there's quite a bit of new info. I've also tried to trim some of the "fat" and redundant content. For example I'd left the output stage details out of the Design Process article and have added quite a bit there. And I've also added new measurements to the first O2 Headphone Amp article.

qusp 5th August 2011 11:48 AM

yeah i ended up just using ltc2935 for battery supervisor hanging off one of the 4 x 3v3 cells (nominal 3v3, 3v6->3v range) the one with the largest current draw and sensing its own supply; at 3.15v, as the PFO pin is pulled low, an led on the front panel lights for low battery via an xor gate that sniffs the pin and opens the gate, which flows current via a transistor with beta 10; given just enough ic to produce 10ma with RLED chosen to make sure 10ma flows over the led, at 3v the RST pin is pulled, which shuts down all the regulators into a low current SHDN state and an LED (via an identical mechanism but with different RLED and threshold to cope with a dropping VCC) and lights to say 'charge me now'

the led remains lit until I switch the relays, disconnecting the battery so i can connect to the external high current battery balancer/rapid charger (its a 2300mah 14v LiFePO4 battery). the LEDs are indicators only, not directly powered by the monitor and the regulators are effectively off in shutdown. the relays are normally closed types, so even if i miss all the signals the relay coil will eventually drain the battery to the point where it can nolonger stay open, so i'm never in a position where there is no charge to ready it for charging.

the battery is still a pretty safe voltage when the RST pin is pulled, but after 3v it starts to fall more steeply. this is a balanced, DC coupled DAC/Amp unit, with each channel of the amp section powered by a bipolar LDO regulator from LT and a differential opamp, so i have little to worry about wrt offset, the load will just center itself around the middle of the rails and will never swing close enough to the rails for me to have anything to worry about, plus even if it did it would be a common mode error and thus deleted at the driver; plus if there is some sort of disconnect on one phase, the signal wouldnt be complete, because the headphones dont know what ground is

all these regs are insanely efficient, never needing more than the dropout to survive, but not what you would call cheap.

yeah Safari sux, used to be much better, i run both chrome and firefox too, but i had safari open today as it has a plugin i needed and didnt get around to doing anything else. that being said, that big honking error message has nothing to do with the browser.

kboe 5th August 2011 01:12 PM

Cool project. I'll be watching for the final product.

RocketScientist 5th August 2011 02:24 PM

@qusp, there certainly are ways to do Li-xxx batteries relatively safely. But even LiFePO4 has issues. I had a defective LiFe prism cell puff up and explode before it was ever used, charged, or connected to anything. It didn't catch on fire, it just built up pressure internally from a manufacturing defect and made a mess. I did some searching online and it turns out that's not uncommon. Many have blamed it on abusing the battery but it happens even with brand new unused batteries. Apparently if the wrong impurities get into the the wrong places when they're made they turn into ticking time bombs.

And I'm not aware of any HPA suitable through hole charger controller/power management IC for a bipolar Li-xxx supply. They're all surface mount.

What's really weird to me is FiiO essentially cloned the AMB Mini3 with their new E11 amp. Except instead of a 9 volt battery they used LiPoly and a DC-DC converter. The could have very easily made it a bipolar DC-DC converter as in the E9 and the amp would have 0 volt referenced inputs and outputs--i.e. a real ground. Instead, however, they used a single DC-DC converter and the same inefficient expensive OPA690 virtual ground op amp that seriously degrades the Mini3's performance. Why?

khaos974 5th August 2011 03:07 PM

Unsurprisingly Google Docs is another inaccessible website here, nayway, it's not like buying the components in China would have been easy anyway. And sending them from the US would make it way more expensive than it should be.

I'll take a look again later.

RocketScientist 5th August 2011 04:17 PM

@khaos974, You mean Google Docs is censored across the board in China? I don't know where they ship from, but Mouser has a Chinese branch: Mouser China

Google Blogger has restrictions on maximum image size, but I've made some changes and if you click on the schematic, board drawing, and/or BOM images in the article you'll get much larger versions. They're fuzzy but legible. The PDF from Google Docs is much better but if that's not working for some of you, I've tried to provide another option.

khaos974 5th August 2011 04:49 PM

Yes, Google Docs and Google sites and probably Google + are inaccessible from China.

agdr 5th August 2011 05:57 PM

2 Attachment(s)
I just got the power management circuit to simulate! The LED was drawn backwards on the schematic PDF I downloaded yesterday. Just took a look at the Google site and see you found/fixed that in the latest drawing.

Here is a failure voltage drop spice sim on the negative rail with 8.4V constant on the positive rail. Negative rail goes from -8.4V to -3.2V at 3 seconds in. The converse parameters with the drop moved to the positive rail with the negative rail held constant produce similar results.

I've subbed in an LT 1716 for the comparators from the LT spice model base. It is rail to rail, over the top, and good from 2.7 to 44v. Yours is better suited with the 2V min supply but this does the job for sim. IRFP240/IRFP9240 subbed in for the mosfets - so in real life they would gate-source overvoltage short when operating. :D

The legend is on the top. Blue -vbatt is the negative rail drop at 3 seconds in. Orange +vout is the positive rail shutting down, while light magenta -vout is the remainder of the negative rail shutting down.The green trace for the output of comparator 1 is kind of hard to see, but the other blue trace is the output of comparator 2 toggling from high to low. You can see the bright magenta voltage divider sample voltage crossing over the grey LED reference voltage at about 3.05 seconds, with the comparator(s) toggle happening just a hair after it.

RocketScientist 5th August 2011 06:07 PM

@agdr Thanks for the sim! If I'm reading your graph correctly, you show about a 60 mS lag between when each crosses 3 volts? I don't think that's true in real life but I'd have to double check. It turns out that's the critical parameter. Below that is where the audio op amps can become unstable. As long as each rail is above |3| volts the entire audio circuit doesn't care how unbalanced the rails are.

agdr 5th August 2011 06:13 PM

You are right about the lag, but I've subbed so many parts in here just to fit the LT Spice model base that the real thing will likely be different.

Pretty impressive design, though! It does work for a fault in either rail. I haven't tried a combined rail fault, but just logically since its all between the rails that is just a change in point of reference. Should also work just fine.

RocketScientist 5th August 2011 06:20 PM

Thanks for the compliments. What ultimately matters is how long the period is between the op amps going unstable and the 220 uF caps gasping out their few remaining electrons into the headphone load. And your sim shows both rails are way down by the time that happens.

It's also worth pointing out such a rapid change can only happen in the real world if a battery is somehow disconnected in use. And for anyone building the full amp with the B2-080 case, that's not very likely to happen if they secure the batteries properly. The batteries would just gradually die which is a less stressful condition for the circuit to manage.

I measured the actual transients at the headphone outputs and worked backwards from there to optimize the power management circuit (without going overboard in terms of cost and board space--it could certainly be improved with those constraints removed).

It's much like the rest of the amp. I tried to aim for "well past good enough" and that was especially obvious with the power management circuit. The goal is not to damage any headphones if there's a battery problem and, from my tests, I've met the goal. I really tried to keep the O2 as "minimalist" as possible to keep the price, size, and DIY complexity all to a minimum.

agdr 6th August 2011 04:02 AM

2 Attachment(s)
Here is a suggestion. I'm kind of a reliability freak and I just know what is eventually going to happen to the Vgsmax = 25V mosfets over time operating at Vgs=24 +/-10%. I see some of the Sanyo datasheets actually putting it in writing these days, to the effect of "operating the device near max parameters, even if not exceeding, can result in shortened life. Contact your representative".

I would suggest adding 15V zener clamps between gate and source. That would allow the use of more standard - and available - Vgsmax=20V parts, plus would keep the Vgs a good 5V under the max. Still works, as the plot shows.

True some current would now flow through the zener, adding to battery drain, but it is only around 11uA for batteries or 33uA for the 12V rails. I'm not certain how much of an issue zener noise injection into the rails would be in this configuration - something to consider.

Another interesting way to handle the Vgsmax issue is make the mosfets double as cap multipliers when operating. Swap the two mosfets (n channel for p), flip them around to be source-follower, then swap the positions of r6/c3 r7/c4, swap the comparator outputs going to the gates, then adjust up the capacitor and down the resistor to maintain the same rc constant. A gate-source zener would still be needed since when the comparators toggle the output caps would still be charged up to the rail, with the other rail now on the gate. May also be an issue if the amp output is shorted with exceeding Vgsmax.

agdr 6th August 2011 04:38 AM

Whoops - brain hiccup in the above about the cap multiplier option. That isn't going to work with the mosfet Vgs threshold dropping the output voltage down significantly. They would have to be BJTs and that would introduce significant base current draw. Nevermind...

So the sole suggestion then is the addition of the 15V gate-source zeners in the existing design.

RocketScientist 6th August 2011 05:52 AM

Quote:

Originally Posted by agdr (Post 2662353)
So the sole suggestion then is the addition of the 15V gate-source zeners in the existing design.

Yeah, that's bothered me a bit as well. But, technically, the MOSFETs I've specified are within spec. The diodes drop the 12 volt rails a bit which further helps. And there's nothing to cause transients much above the rail voltage.

The other good news is I've run MOSFETs rated at 20 Vgs(max) with zero problems--both P and N channel. For the first prototype that's all I had in my parts collection so that's what I used and they worked flawlessly. That might be just luck, but it's at least encouraging the Vgs(max) spec doesn't seem super critical.

The reality is the PCB layout is really tight. The PCB drawing doesn't show the routing, but when you consider all the traces, there's just isn't much room without making other compromises. Besides zeners, a resistor divider will also work and part of the dividers are already in place.

I'll look at the layout again and see if I can find some room. But, at least based on everything I know so far, it's not likely to be a real world problem even if it's not ideal. With the diode drops, and regulator tolerances, the design is still within spec.

agdr 6th August 2011 01:00 PM

I was wondering if there would be enough space for 2 more diodes. I'm sure the circuit will work fine in the short term - maybe even go years without a problem - it is just a long term reliability thing. Hopefully you can find a way to wedge in the diodes, or resistive divider. The divider is a good idea. Hadn't thought of that.

RocketScientist 6th August 2011 01:56 PM

I have a suspicion because the gate current is limited to 88 microamps worst case by the large 270K resistors the gates might be protected from damage even if the junction does start to break down. Power MOSFETs, except for ESD, are rather beefy and robust devices. That gate area is relatively large. But I don't know enough about the semiconductor properties to know for certain if 88 microamps is too low to damage the gate. But that would help explain why the circuit worked flawlessly even with 20V parts.

It's also worth noting C16 and C21, in combination with the 270K resistors, form a low pass filter that will tend to isolate the gates from any spikes or brief transients.

agdr 6th August 2011 02:20 PM

Good question. Seems that even just static voltage near or above max will eventually do semiconductor parts in. But the gate filtering is bound to help.

I think part of the problem is "binning" during post fab test by the manufacturers. There is just no margin left there anymore in any parameter. All the "better stuff" gets moved to a higher grade part classification. Unfortunate that there are not Vgsmax=30V mosfets. That would be enough headroom. Maybe there are - I'm pretty out of date on what is out there anymore.

qusp 6th August 2011 02:42 PM

Quote:

Originally Posted by RocketScientist (Post 2661689)
@qusp, there certainly are ways to do Li-xxx batteries relatively safely. But even LiFePO4 has issues. I had a defective LiFe prism cell puff up and explode before it was ever used, charged, or connected to anything. It didn't catch on fire, it just built up pressure internally from a manufacturing defect and made a mess. I did some searching online and it turns out that's not uncommon. Many have blamed it on abusing the battery but it happens even with brand new unused batteries. Apparently if the wrong impurities get into the the wrong places when they're made they turn into ticking time bombs.

oh for sure, anything that can put out 14v x 140 amps (1960W and probably more into a short) and that can be unstable under some conditions, is a power that must be respected. i find it quite disturbing the trend of people on this forum and other audio forums just using trickle chargers for them. just because its proven to be pretty safe chemistry and nobody on here has had one explode in their gear from doing it yet, doesnt mean it wont happen. even just briefly accidentally shorting/grazing the terminals of a single 3v3 cell while soldering a pack together was enough to scare the crap out of me.

I went to great lengths to keep this as safe as possible, since it will go into my coat pocket sometimes. they are just such fantastic/pretty much perfect power sources, with that sort of current, low noise and output z of less than 10mOhms i couldnt resist, considering the needs of my portable are around 300-350ma i didnt really have many choices if i wanted to be able to carry it and have it last more than 1 or 2 hours

I use legit A123 cells only, they are the only brand of lifepo4 i have any real experience with and theyve been flawless, theyre rep rides on reliability due to the sector they are in and they charge a premium for it.

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And I'm not aware of any HPA suitable through hole charger controller/power management IC for a bipolar Li-xxx supply. They're all surface mount.
yep, the only things on my board that are through hole are the relays, the toslink connector, pot for the ADC/volume, plus one of the connectors and only those because i already have the parts from previous projects. the regs i'm using are not even supposed to be hand soldered, but i made it work with a bit of creativity with the pads and vias. i dont really understand why everyone is so scared of smd tbh, i find it easier.

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What's really weird to me is FiiO essentially cloned the AMB Mini3 with their new E11 amp. Except instead of a 9 volt battery they used LiPoly and a DC-DC converter. The could have very easily made it a bipolar DC-DC converter as in the E9 and the amp would have 0 volt referenced inputs and outputs--i.e. a real ground. Instead, however, they used a single DC-DC converter and the same inefficient expensive OPA690 virtual ground op amp that seriously degrades the Mini3's performance. Why?
yeah i dunno, seems strange to me also, solves a lot of problems when you can just treat everything as if it was a regular bipolar layout, grounding is just so much cleaner and you dont ever have to worry about a situation where the signal produced cannot be sunk properly, because by its nature if its been sourced it can be sunk.

Arius 6th August 2011 04:21 PM

Very nice work and excellent documentation. Thank you for sharing!

For the objectives stated, the part/design choices are well thought out.

Unfortunately, the same objectivity will put off a lot of the readers (to their own loss). Their idea of DIY involves changing regulators, caps, opamps, resistors, connectors, wires and reporting on the perceived improvements.

RocketScientist 6th August 2011 04:36 PM

@agdr there are some 30 V MOSFETs but they tend to be expensive high voltage versions with unacceptably high Rds(on) in this application.

@qusp thanks for your added comments. I agree they make nice low impedance power sources. And I think the round cells are far more stable than the flat prism LiFe cells. I'm curious if you have a solid source for real A123 cells? From what I understand nearly all the ones on eBay are Chinese clones (some of which apparently work reasonably well). And the manufacture supposedly refuses to sell them to distributors as they're not protected cells and they don't want the liability.

@Arius, thanks. I agree it puts some off. In the comments to the first article it didn't take long for some to post comments about upgrading the op amps, power supply, connectors, etc.


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