The Objective2 (O2) Headphone Amp DIY Project

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.
 
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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.
 

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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.
 
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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.
 
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.
 
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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.
 
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.
 
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@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.

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.

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.
 
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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.
 
@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.
 
if you only have a fairly modest need for the cells like i did (I bought 18 x ANR26650m1 cells), not sure what the limit is without becoming an oem, i bought them directly from A123. Its not the cheapest price, but guaranteed legit, well packed and the packing is legal, so easy to hot-knife it to reuse yourself for storage, or shipping. its come in handy for flying with all the regulations that come with lithium cells now.

as a bonus, that way you have a direct line for any questions you might have and if things escalate you have a point of contact. there are a couple other options without resorting to ebay, which i would never do. they also sponsor a radio control battery brand, A123 racing funnily enough and their batteries use the m1 cell, you can break it down. there are a couple of US RC retailers that sell the single cells under that model name, just perhaps steer away from the asian RC stores like hobby king, some will be legit as manufacturing goes on there, but you just dont know, although the price is good and i did get some of the smaller capacity cells there, but ive also heard of fakes.

one last one, not sure if its still like this, but black and decker, or ryobi use the m1 in their cordless drills and other electric power tools and you can remove the casing to get access to them,

if you are keen on getting some from A123 direct let me know, i'll dig out the link to the oem page where you buy them, its not obviously placed. you'll need to have a company name though to place the order. actually i just had another look, looks like you can still do it, but definitely only as a lead up to full production through a company name (same process really). you can still by the a123 racing packs though and good hobby stores have them. i got mine about 12 months ago now. looks like they have just released an SLA replacement, like car battery size nanophosphate at 12v and 4.6wh. a couple of those for a power amp would be sweet.
 
Thanks for all the info qusp. I'm especially interested in SLA replacements but not for audio. But I don't want to get too far off topic here :)

If anyone has any other questions, comments, criticisms, suggestions, etc. for the O2 please share them? I certainly get things wrong, or may fail to realize there are better ways to doing some things. Ultimately my goal is to raise the bar for how much performance you can get at a nearly Cmoy price. So anything that improves the performance and/or appeal while retaining the bargain price should be on the table.
 
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no problem, good luck!! well worth the effort, i dont forsee having to replace my batteries until there is something better and the extra cells have actually turned out really handy for quickly knocking up a power supply for testing something in situ without changing the layout or wiring.

too large for a regular pocket size portable amp though; my battery takes up about 50% of the unit and adds a good chunk of weight, the 4 cell pack turned out ~25 x 110 x 65mm
 
Thanks agdr. I'll try to figure out if there's room. If so I'll add them as an option. I'm not sure if you read my paragraph on dissipation in the Circuit Description section of last article? I did the math and made several measurements of power-in vs power-out. I also measured case temps. For anything but sinewave testing it's a non-issue unless perhaps the amp was operated in an extremely hot ambient environment, on AC power, into a worst case load at clipping, etc.

@idjoel2000 I won't be offering anything myself as this is entirely a non-commercial project for me. I'm hoping someone else will have the bare PCB for sale around the end of the month (ready to ship) or the first week of September. Kits would likely a come a bit later based on demand. Pre-assembled boards are another possibility. Please see the first two sections of the O2 Details article for more.
 
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&pid=explorer&chrome=true&srcid=0B52Awjeyc5zKNjQyOTlhMjAtZTNmOS00ZTNlLWEyMzgtZTQ0YzUwN2ViOTc4&hl=en_US

I get the same message. I'm using Firefox, and I'm not from China.
Any chance of you posting the schematic here?
 
The Google problems are just weird. I just tried the link above and it works fine. And it's working fine for most everyone else.

In the article I've also added larger versions of the schematic, board drawing, and a screenshot of the main part of the BOM. Just click on the pictures in the article and a new window will open with the full size image. They're nowhere near as sharp as the PDF, but they're legible.

@agdr - I did think about vertical, and it can be a useful last resort in some cases. But, in this case, I'm sure others would complain or criticize the O2 for having vertical components. Vertical axial parts are also a bit less robust and DIY friendly (they can short out against each other or other parts if not properly bent/installed, the polarity is less obvious for diodes, etc.). If you're really bothered by the Vgs issue and plan to build your own O2 you can certainly add your own gate protection.

At this point I'm trying to avoid significant changes to the PCB unless there's a *really* good reason. Otherwise I'm going to have to do yet another prototype spin of the board before releasing it, build another amp, all the testing, etc. And that's going to add a lot of work, more expense, and a few weeks of delay. To try and make room for the gate protection means pushing several other components around, including some fairly critical power supply and grounds tracks.
 
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Another long term reliability suggestion - DIP8 heat sinks for U3 and U4. I've got a bunch here from Arrow. Mouser and Digikey don't have stock, but Jameco sells them too.

I like the idea, but unfortunately the heatsinks won't fit on the current design and I doubt I could make room no matter what. But if I do a desktop spin of this board without the batteries, I'll make sure there's room. If nothing else they will help the amp better survive sustained sine wave testing.

They're also available from Newark/Element 14 and they also carries the Box Enclosures cases. So someone could order the heatsinks with the enclosure and save paying $8 shipping for $1 worth of heatsinks.

My only slight concern is I assume the metal bit on the bottom is in contact with the PCB? The anodizing is an insulator, and production PCBs will have solder mask, but are there any sharp edges that facedownward? If so they might over time cut through the thin anodizing and solder mask and could short against traces routed under the chip. It's difficult to do proper power rail routing and bypassing of DIP8 chips, and get all the signals in and out, without using that space for traces.