Interest Check: PPAS redesign? - Page 3

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RCR123A may be an option, but you'll need to figure out a charging scheme, and a fireproofing scheme (heatwrap of some sort like the lipo packs are). They are small, higher voltage, and would take up less space than AAA in the C802. In turn they cost more, and can be a fire hazard.

booster + AA is cheaper and takes less space.

I'm not opposed to either methods, but would prefer the LiPo primarily because there are no popular DIY amps that are spec'd with a LiPo PS section. I know you can always make your own.

All the questions/issues essentially come back to what are your goals and targets. If you can itemize a list for yourself and prioritize accordingly, you'll find something that accomodates what you want.

things like:
- voltage and current and consequently power output targets
- buffer choices
- opamp choices
- stability
- improved layout
- target board size
- target case size
- power section
- etc.

what kind of phones will this amp be driving? mid-Z PK-1? low Z IEM's? high Z Beyer's/Senn's? or mid-Z but demanding stuff like Yamaha ortho's?

Good comments Filburt, thanks.

1. If this is going to be a completely new amp design, I will certainly consider proper bypass. I was shocked looking at the original PPAS board no to find any. If we're just copying the PPAS, well, maybe not, will have to see if there's room.

2. I think the PPAS layout is pretty good, but I could be missing your complaints.

3. Most opinions seem to prefer LMH6321 anyway. At the end of the day, the discrete output doesn't really 'cost' much, it'll be a bit larger, but cheaper and probably perform better. Plus it gives it more sex appeal

4. Such is the plan. I know it can work if done properly, but some don't seem to be convinced...

FallenAngel, I hadn't even considered that layout, how dumb of me. I don't really want to have to build a battery pack for the amp, it should be something off the shelf and easy to mount nicely.

Unfortunately I think RCR123A is slightly too large to fit. There is nominally 16mm of vertical space in the case, and they're specced at 16.5mm dia. Otherwise it'd be perfect, aside from the safety hazards.

I'm still thinking 2xAA is the route to go unless someone has some other LiPo cell recommendations. Unfortunately 3xAAA isn't really worthwhile since it's barely more capacity than 1x9V without the booster. Could look at e.g. digicam batteries, the only problem there is how to connect them, though soldering wires to the pads isn't a terrible option.

deciding the intended load can speed up the design process greatly. Opamp+ buffer is basically nothing new, just how you put the combo together, right? instead of make a great compromise to make one amp that "suit most phones", it is probably easier to creat two versions, one uses low supply voltage but have high current capability to drive IEM's, and one uses high supply voltge but needs less current capability for stuff like 770/580.

The C802 PDF says 20mm internal height, but still, too small to fit 3 across even if staggered. How about the 1455B802 or the 1455B1002? The B802 is a bit wider than the C802, but also thinner, for a sleeker look. The 1455B1002 is bigger in general, but not horribly so, about the size of an ipod touch in width and height but thicker.

I don't know if being the small is one of your targets or not.

There are options for this. Make the PS section separate, either through some molex connectors, ribbon cable, air wire, etc. Then you can either use a multiplier or a lipo section with the amp.

And, going to another discussion about this, we had crossed paths about DAC+AMP, something along the lines of Pico quality. If the PPAS is as good as people claim, then a good DAC section "plug-in" would essentially complete the package.

20mm height, but with a PCB in the lowest slot, it drops to 16.5mm available. We could consider a slightly larger case, that might be a good idea to fit more batteries, but I definitely want to keep it small enough to easily pocket. The Lisa III for example I don't really consider a portable amp. That particular one is definitely not tall enough though, with only 12mm available height when a PCB is installed. What do you think of the Bud Industries EX-4500, it's slightly larger in all dimensions than the Hammond and a bit more unruly, but it'd fit our needs well. It could easily accomodate 4xAAA or 3xAA, and is about 0.5mm too narrow to fit 4xAA. Personally I think it's a little big at 1.5" thick.

As far as intended load, I think the PPA design, if we take it pretty much verbatim (adding decoupling and whatnot) with a Sijosae buffer it should be suitable for most low to medium impedance phones and IEMs. I agree that it'd be tough to get a truly one size fits all amp, but the basic PPA design should be applicable to everything but the toughest cans I imagine. There's not much point in a lower voltage version I don't think, I figure at least 8V is needed to drive the opamps and buffers adequately, and anything more than 10-12V is probably impractical with the proposed power supplies so far.

I'm not sure a modular power supply approach is necessary, but I don't think it's feasible anyway. My plan is to use the area on the other side of the board from the batteries for much of the amplifier circuit. It would be tough to squeeze the amp in if it wasn't possible to do that.

wow, that's quite a bit bigger than the C802. It seems almost too big for a portable.

So you're planning on making the PCB take the entire case size and have batteries on it like the Mini3 v2? Otherwise I don't understand why it would matter where you put the PCB.

Are you looking for something in terms of the fit and finish of the Mini3? I always thought it was going to be a small board that slides into a slot and is held to the panels by the pot and doesn't take the whole case. If so, that's cool too. In that case, you could definitely put the PCB down at the very bottom (not in a slot). for a bit more area. This requires flush soldering for a good fit, a single sided PCB, and a small insulating layer underneath (business card, plastic, etc.).

Ugliest case I ever bought... and subsequently threw away.
the extrusion is actually pretty slick but the end panels are just raw stamped aluminum and those bezels ...

If you are going to go with a Larger enclosure I would suggest taking a look at the Box B1-080XX

Newark part numbers 26K8988, 26K8989, 26K8990, 26K8991, 26K8992 and 26K8993 (they are available in 5 colors)

Here is an example:



Not a whole lot bigger then the Hammond 1455C801 though.

Sex appeal, sure. Better performance? I'm not so sure on that one. Discrete doesn't automatically mean better performance. In fact, in this type of application, it can rather easily mean lower performance. I'm not saying it isn't conceivable or theoretically doable, but the degree of optimisation involved could stick the project in development hell or at least lead to disappointing or inconsistent results.

Boost converter should really be last resort for this kind of thing. You're inviting a lot of noise by doing it and you're going to have to be prepared to do some serious work down the pipeline to keep the rails clean, and I'm not sure you're anticipating just how much of a pain that is going to be. To be honest, this sounds like a piece of the design that could sit the project in the aforementioned development hell.

I'm not trying to throw water on the fire, but I am trying to be pragmatic about this.

MisterX: Thanks, that's kinda what I thought from looking at it, I think it's a bit too high for portable use. That Box Enclosures case looks very nice, too bad it's only available from Newark.

Filburt: I think maybe I'm underestimating and you're overestimating the problems with SMPS. I've used it before with success, but never designed it in this application. I really don't think it will be a huge problem though as long as I can design something that actually works out of the gate. IMO the noise problems with switchmode stuff are overstated, I've used switch mode supplies, both off the shelf wallwarts and onboard with a few different amp topologies and they've all sounded fine.

How about two 3.6V LiIon AA cells? That would limit the drivable cans slightly more, but should drive most cans pretty well. They're pretty easy to get, the holders are easy to get, and the battery life should be excellent. Will require careful charge controller design.

Regarding the discrete buffer - I actually tend to agree. I don't shy away from ICs and am not usually one to go discrete just for the sake of it. I haven't heard this amp though, and though it may be placebo, discrete buffers are generally claimed as an improvement. It's probably not too hard to put pads for an IC buffer if the discrete option doesn't work out or isn't desired.

Allied and Grainger offer them as well but they always seem to have limited numbers in stock.
You could always order them direct from the manufacturer

Box Enclosures - New Page

It's kinda strange though because it has always been cheaper for me to order them from Newark then to order them direct

I was going to suggest those earlier but as you pointed out even simple LiIon charge controllers are pretty complex... do you really want to go there if you can avoid it?

The only really tricky part is the temperature sensing. Everything else is pretty straightforward (ie. integrated into the IC). A $2 IC like bq2057 and some cheap passives is all it will take. The thermistor isn't really required, but it'd be pretty irresponsible to release a design that doesn't at least provision for it. Probably will take the form of thermal epoxying one to a cell and air wiring or such, I can't think of a clean way to implement it. I think a series resistor should be sufficient for current limiting. And lots of warnings to builders, since these things are pretty dangerous if shorted or reverse charged.

The only problem I have with Newark is that it's just yet another vendor to order from for a build, and more shipping to pay. It's already unfortunate that appropriate JFETs aren't available at DigiKey...