[Dougigs]

This isn't particularly pioneering stuff -- the best commercial battery-powered headphone amps use these cells; and since they offer about a tenfold increase over other options in terms of both charge-life and physical size, I'd like to make them available in the DIY world.

My situation is like that of many people on this forum: Between 2 and 8 times a week, I get on an airplane or a long train. I spend a lot of time in hotels in various places. I like to have an IPod and a good set of headphones, and an amplifier that will give me high-quality audio on the road. Like most people, I like to travel with a single bag, so everything is carryon. And like most people here, I already have a laptop in my bag, with its AC adapter (16V, 5A).

So right now I travel with a MINT. Great little amp. But I have to spend $30 a week on batteries. Or carry a charger, with various adapters to let me use it with varying voltages and styles of outlets -- more space and weight. Which seems silly, when I already have a source of regulated DC in my bag.

Right now we have two battery options:

1) a pocket-sized MINT, with 4-5 hours of battery time in $15 worth of non-recargable cells, or 2 hours of battery time in $50 worth of rechargable cells -- though you have to carry a separate recharger with you - or

2) something like a PPA -- a non-pocket-sized case carrying a very heavy quantity of batteries and a charging circuit, offering perhaps 20 hours of listening. But you have to carry a separate AC adapter to charge it (though it could easily be made to accept a laptop charger -- really, the issue is its size -- can you put it in your suit jacket without looking like you've got an uzi?)

So why not make something that's a) pocket-sized and b) has a battery life that will let you listen to Mahler's 8th in its entirety and c) recharges using an AC adapter that you already have on you all the time?

If you can name another technology that will offer this, let me know. I would say that my circumstances are similar to most heavy headphone users -- on the road a lot, so not able to get near real speakers; wanting to go light, but not give up on top audio quality. We have the means to do this now, so why don't we try it?

Take a look at the datasheets for the relevant chips and tell me what you think.

 

[Nisbeth]

Quote:

Originally Posted by Dougigs <snip> tell me what you think.

I think I would like to see a design for a mid- to high-end portable amp with batteries and charger that would fit in the smallest Hammond case. You have any ideas?

I'm thinking that a TPA6120 or an AD8397-based design would work if powered off two Li-ION Cells? The amp and charger should preferably be on a single PCB which would just slot into the case.

/U.


EDIT: I don't have the skills to do mjuch in the way of circuit design, but if anyone wants to give this a shot I would be happy to assist with ideas/concepts/critique/layout sketches.

</hijack>

 

[mono]

Quote:

Originally Posted by Dougigs This isn't particularly pioneering stuff -- the best commercial battery-powered headphone amps use these cells; and since they offer about a tenfold increase over other options in terms of both charge-life and physical size, I'd like to make them available in the DIY world.

Absolutely not a "tenfold" increase, unless again, you refuse to compare apples to apples.

Quote:

So right now I travel with a MINT. Great little amp. But I have to spend $30 a week on batteries.

No, you don't have to... everyone and their brother have been using rechargables for the past decade in such scenarios. Even then with the higher initial cost of NiHM rechargables, it still wouldn't cost $30. You are making a completely unrealistic and illogical comparison and it appears to be biased far beyond reason.

Quote:

Or carry a charger, with various adapters to let me use it with varying voltages and styles of outlets -- more space

You mean, less space, since the alternative can only be larger, as it removes no need, only further circuitry to implement it.

Quote:

My situation is like that of many people on this forum: Between 2 and 8 times a week, I get on an airplane or a long train. I spend a lot of time in hotels in various places. I like to have an IPod and a good set of headphones, and an amplifier that will give me high-quality audio on the road. Like most people, I like to travel with a single bag, so everything is carryon. And like most people here, I already have a laptop in my bag, with its AC adapter (16V, 5A). and weight. Which seems silly, when I already have a source of regulated DC in my bag.

Exactly. Your amp doesn't even need it's own power source in this scenario.

Quote:

Right now we have two battery options: 1) a pocket-sized MINT, with 4-5 hours of battery time in $15 worth of non-recargable cells, or 2 hours of battery time in $50 worth of rechargable cells -- though you have to carry a separate recharger with you -

... or you have the best and more popular option, do it like everyone else does. NiMH.

Quote:

2) something like a PPA -- a non-pocket-sized case carrying a very heavy quantity of batteries and a charging circuit, offering perhaps 20 hours of listening. But you have to carry a separate AC adapter to charge it (though it could easily be made to accept a laptop charger -- really, the issue is its size -- can you put it in your suit jacket without looking like you've got an uzi?)

You're trying very hard to invent a problem that doesn't exist. Define maximum runtime that is actually necessary inbetween charging opportunties, then define all potential charging opportunities. Use appropriate minimal adapters IF you want access to these alternate power sources for recharging.

Quote:

So why not make something that's a) pocket-sized and b) has a battery life that will let you listen to Mahler's 8th in its entirety and c) recharges using an AC adapter that you already have on you all the time?

None of which requires what you're doing unless you are 100% set on one particular case (dimensions). If that is the situation, then that choice necessitates an alternate battery at great cost increase, only minor potential for size reduction and a runtime that thus far, does not seem necessary. If you just "want" to do it, that's an entire topic onto itself, but technically speaking you are not solving a problem in general rather than ignoring the obvious solutions.

Quote:

If you can name another technology that will offer this, let me know. I would say that my circumstances are similar to most heavy headphone users -- on the road a lot, so not able to get near real speakers; wanting to go light, but not give up on top audio quality. We have the means to do this now, so why don't we try it?

You've already given up on top audio quality... A Mint is a great amp for it's size and cost, but it's not this ideal you're proposing it to be, and no $70 battery changes that. Having the means to do something means little. I have the means to build a bridge out of tortilla chips, but so far it hasn't happened.

Quote:

Take a look at the datasheets for the relevant chips and tell me what you think.

II think it's a grossly overpriced option that may have no realized benefit except in a very narrow application. You're talking about lugging around pounds of stuff of far larger volume then suggesting it matters whether you have +- 2 cubic cm of space or 6X the runtime you'd ever need instead of 3X inbetween each recharge opportunity. You are enchanted by the technology but not practically applying it yet.

For many people, the issues you've written about are not difficult to overcome nor expensive. Mainly the gain would be dependant on whether you fit it all in one of the two more popular cases, the smaller/smallest Hammond or Serpac H65-like types. That kind of targeted application might validate the idea more.

 

[Dougigs]

Nisbeth -- Yes, you're thinking of the same sort of shape and scope I am -- smallest possible pocket case, 2 chips on a tiny board for the charger/monitor. You'd only need one of those 11.1-v cells, since that's plenty of voltage for a number of very good amps using a number of good op amps and buffers.

I don't think it's necessary to design a whole amp with its own charger on one board. I think the design of tiny amps has really been mastered by others on this forum -- no point reinventing the wheel on the audio end or forcing people into one amp design.

I see the goal being to develop a tiny companion board that would make these new cells a safe and easy-to-use option for people with any amp (including a number of line-powered amps that have previously been excluded from portable use -- with 2 and a half amp-hours in a battery, you can take a lot of amplifiers on the road, if minimum size is not your bag).

So here's what I picture: a half-MINT-sized PCB containing one SMD charger chip and one SMD 8-pin discharge manager (both chips cost less than $4 each), a transistor and a few support components (maybe $20 for the whole board, max). It would have six wires: 2 to the battery, 2 to the + and - of the amp power input, and 2 to an AC adaptor socket that could take any 16v+ adaptor for charging. Those 2 chips, just using their basic datasheet schematics, would automatically handle the smart charging and charge-end cutoff, the low-voltage cutoff of the batteries, and the transition from charger power to battery power -- that's what they're designed for.

I've ordered an existing tiny 11.1-v li-poly charger ($15 US for the whole unit) to see how such things work in the real world.

We should start looking at datasheets for various charger chips -- I haven't quite found the one that's perfect for this application. That low-voltage cutoff chip I mentioned is exciting, if we can resolve that question about the 50khz oscillator first. Otherwise a simple comparator design would be better (Elektor published a nice one this year).

I'll have some breadboarding time in October, and some 11.1-v, 2400 mah cells to toy with. No point hurrying on this right now, but it'll be interesting to hear if there are others interested in working on such a PCB. Are there people on this forum with modern battery-design experience?

 

[Dougigs]

Here's the charger I'm using for prototype (note its size next to a 1/8" inline socket):

http://www.heli-fever.com/product_in...products_id=76

Here's where the 2400 mah, 11.1-volt li-poly cells can be had (along with many lesser batteries):

http://stores.ebay.com/RevolutionShop


I haven't seen a pre-made low-power cutoff unit -- because the RC helicopter folks prefer to have a gizmo that flashes a bright LED or beeps when you're getting close to that state (so their investment does not immediately do a Black Hawk Down on them, presumably). Anyway, it's the simplest link: Basically a comparator and a switching transistor.

 

[Nisbeth]

Quote:

Originally Posted by Dougigs Nisbeth -- Yes, you're thinking of the same sort of shape and scope I am -- smallest possible pocket case, 2 chips on a tiny board for the charger/monitor. You'd only need one of those 11.1-v cells, since that's plenty of voltage for a number of very good amps using a number of good op amps and buffers. I don't think it's necessary to design a whole amp with its own charger on one board. I think the design of tiny amps has really been mastered by others on this forum -- no point reinventing the wheel on the audio end or forcing people into one amp design.

My main reason for wanting to design everything on one PCB was simply that it is easier to mount in the small enclosure, but as I can see over at headwize that Tangent is (inadvertently) helping us with the amp section (

) your approach is probably better

/U.

 

[Born2bwire]

I don't really understand mono's opposition toward Li-Poly in amps. I think that Li-Poly are wonderful solutions. They can last for great lengths of time and can provide large amounts of current in a small package. A lot of cell phones and mp3 players are using these batteries so the whole package can take up little real estate. My only reservations are that there are necessary design constraints that need to be accounted for. Since there has not been a published circuit for the general head-fi community, it's not something that I would recommend for most DIY-er's here. I have thoroughly abused these batteries and never had any dangerous results (though I have not done abusive chargings), but I have found that it's easy to ruin a $50 battery, in multiple ways. It would be nice to see a small charging and discharging circuit put up on head-fi. It would just be a black box for people to incorporate a simple Li-Poly power source into an amp. Even building your own charging circuit is rather simple because we have been using pre-existing Lithium Ion charging chips for ours.

 

[mono]

Quote:

Originally Posted by Born2bwire I don't really understand mono's opposition toward Li-Poly in amps. I think that Li-Poly are wonderful solutions.

yes they are interesting technology beyond the details of significantly higher expense, risk, time to implement, and often unrealized benefit. That doesn't make them "always" bad, BUT, painting a picture that only mentions the positive aspects is pointless as we could do similar with even the worst battery alternatives.

Quote:

They can last for great lengths of time and can provide large amounts of current in a small package.

Show us a headamp that needs more current than other rechargeables (or often, even alkaline) can provide.

Quote:

A lot of cell phones and mp3 players are using these batteries so the whole package can take up little real estate.

Professionally designed products where they qualify chargers and batteries, the usage pattern, and as I've already written, they are targeting the use towards specific minimal sized product case. Simply claiming "it's good because some other devices use it" only makes sense if the target device actually has the same criteria _as_implemented_. It is possible to implement, but instead we're going over details backwards, ignoring the real reason "most" people would benefit, IF they would.

Cost vs risk vs benefit is a standard analysis done in any good design. if your use of Li-Poly doesn't do this, of course you won't understand. I am not simply suggesting "do not use them", but rather, there is no reason to use them unless the specific application benefits and needs them enough to offset the cost and risk.

 

[Born2bwire]

One example that I can think of off the top of my head is the Headroom Bithead, which uses four AAA batteries. A two cell 7.2V pack would be a smaller solution over these. But the main benefit here is that you can get a cell pack that can have high voltages and high mAh. A standard 9 V NiMH will have 250 mAh compared to an easy 1100 mAh for 11.1 V pack that costs about as much as two 9 V rechargeables. The advantage of having available products using Li-Poly is that there is a ready selection of IC's that cater to them. You can get a SOIC chip from TI for $3.50 that will charge 1, 2, or 3 cell Li-Poly pack off of a DC line. So all you would need is to have is a wallwart. There are similar chips for battery monitoring to turn off the system when the batteries have discharged enough. There was a chip that did both, charging and monitoring but it's being discontinued.

 

[mono]

Quote:

Originally Posted by Born2bwire One example that I can think of off the top of my head is the Headroom Bithead, which uses four AAA batteries. A two cell 7.2V pack would be a smaller solution over these.

So if you used a smaller battery you'd then take the case apart and grind it down to the smaller size?

Quote:

But the main benefit here is that you can get a cell pack that can have high voltages and high mAh.

It is a feature, that is "possibly" a benefit. Big difference. If the amp can use, and the cans need higher voltage it's important. These are not universal advantages and can't be argued as such. Rather, the implementation has to fulfill several criteria before it matters.

Quote:

A standard 9 V NiMH will have 250 mAh compared to an easy 1100 mAh for 11.1 V pack that costs about as much as two 9 V rechargeables.

Which, as I've already written, matters only if/when the specific case can only accomodate certain battery dimensions. Otherwise, why would anyone mention a 200mAH ~9V if they are trying to increase capacity?

It definitely does not cost "about as much". No contest there, a pair of 9V cost $10 and the Li cells all need additioal protection circuitry at further cost and space to implement.


Quote:

The advantage of having available products using Li-Poly is that there is a ready selection of IC's that cater to them. You can get a SOIC chip from TI for $3.50 that will charge 1, 2, or 3 cell Li-Poly pack off of a DC line. So all you would need is to have is a wallwart.

If you want to downplay the whole design, testing, space, etc, then it's an arugment made while ignoring most of the significant details.

Quote:

There are similar chips for battery monitoring to turn off the system when the batteries have discharged enough. There was a chip that did both, charging and monitoring but it's being discontinued.

Having a chip(s) available does not validate any particular implementation of Li cells, only making it easier to use them. Note that "most" devices using rechargable cells do NOT use them. Li cells are not some ultra-new technology as yet unknown to product developers, rather they are unnecessary and the benefits are outweighed in many uses.

Where they are used, it is generally only within the context of it being necessary to have least weight and size as a relative proportion of total product size. "Some" amps and case combinations may fit this description but many do not.

 

[Born2bwire]

http://cgi.ebay.com/New-Li-Poly-11-1...QQcmdZViewItem
http://froogle.google.com/froogle?q=9V+nimh+250+mah

 

[Dougigs]

See above -- precisely. Those $12-each nimh batteries will last under 4 hours in a reasonably equipped headphone amp (MINT in class A, PIMETA etc). So for a day's travel you'd need 2 sets of batteries.

The li-poly equivalent will cost about the same as those 2 batteries, and will last you 5 times as long. For the cost of your 2 sets of 9v, you can get a single cell that will last 10 times as long.

And those are the very top-capacity nimh cells. I know: I've got them. They are wholly inadequate for any real-world headphone-amp needs. Such amps only work if you're near a desk -- and if you're near a desk, you're going to be using a line-powered amp.

So we'd like to make a battery-powered amp that serves people well in the conditions where they need batteries -- i.e. when you're away from an outlet, and you've perhaps only got the clothes on your back, for many hours. In your scenario, you need to have your pockets stuffed with extra batteries. Well, I'd like to forget batteries forever. And with these things, you seal one inside your amp case, screw it shut, and don't think about batteries for 3 or 4 years or more. Just plug it into your laptop adapter every few days. You may enjoy your big pile of batteries; I'd rather live a no-battery life.

And yes, Mono, is is fun to play with new, promising technologies (or long-forgotten old ones). How else did we end up walking around with pockets full of video buffers, twisted-pair line drivers and radio-frequency instrumentation amplifiers plugged into our headphones?

 

[Dougigs]

Take a look at the MAX846A charger chip:

http://pdfserv.maxim-ic.com/en/ds/MAX846A.pdf

16-pin, hardly any external components needed, wide voltage range.

It doesn't handle undervoltage cutoff -- we'd have to use the 8-pin chip I mentioned above. But it does otherwise seem made-to-order for our application; let me know if I've missed something.

Maxim sells an evaluation board (2 by 2 inches, though you could saw it down to less than 1 x 1 if you cut off the connector-pad part of the board).

Otherwise I'm not sure where to source it, other than Maxim.

I might find a better chip... just starting to look.

 

[Born2bwire]

The lab I was with used TI's bq24702 chip. It's designed for laptops so it is a lot more complicated than we probably need. The big advantage of the bq24702 is that you can decide the charging voltage and such so you can use it in a system with a lot of cells. From a quick look over their product selection guides, I think the bq24105 looks promising. It has support for upto 3 cells so it should be good for the 11.1 V pack you had linked.

bq24702: http://focus.ti.com/docs/prod/folder...t/bq24702.html

bq24105: http://focus.ti.com/docs/prod/folder...t/bq24105.html

 

[mono]

Quote:

Originally Posted by Dougigs See above -- precisely. Those $12-each nimh batteries will last under 4 hours in a reasonably equipped headphone amp (MINT in class A, PIMETA etc). So for a day's travel you'd need 2 sets of batteries.

Why would I see above? You are arbitrarily concluding that it HAS to be your questionable cheap ebay pack vs the most expensive NiMH cell possible, and even then you ignore the added cost of the controller and time to implement it all. I never wrote to use "most expensive" NiMH, nor a 9V battery.

What did I write? I wrote about the application, the amp (and cans) voltage need, and the case size, not only as it pertains to what options will fit, but how much space your proposed soltion takes up after adding the control board. You cannot make a good battery choice if you ignore all the basic facts involved in (making a battery choice).

To clarify, I"m not 100% opposed to Li-poly packs when implemented properly. BUT, so far it isn't even clear that you need one at all nor that any theoretical gain is actually a realized gain except in the one lone contrast you make which is the worst choice and I still don't understand why you keep trying to make it.

Just because someone else used a 9V NiMH, that does not, even for one tiny moment, mean that it what you "need" to contrast. That was THEIR choice, it does not need to be yours, and it also does not mean you 're doing any kind of useful comparison when contrasting two alternatives when they're optimized for a different purpose. You make no meaningful comparison until you compare a NiMH solution optimized for your needs, instead of just the opposite.