A mini PPA v2,my first portable attempt. - Page 5

Quote:

Originally Posted by KimLaroux 

Looking at the spec you posted, I think it would be possible to power this from a USB port. One could use the space reserved for the battery for a USB DAC, and tap on the USB power from the DAC... All it needs is a DC-DC converter to boost the 5 V to 15 V, then a 7812 for a clean 12 V...

 

Now I have to convince myself that I don't really need such a thing. But I do. Gah!

that's not a bad idea, I have looked into a boost converter quite a bit, I quit looking into it because the efficiency losses involved. However, a USB powered unit would be really cool.

Most USB outlets are rated to 500ma correct? so there is 2.5w to play with. My 2 amps right now are  running nominal at 12 volts and 150ma, that's 1.8w.

So, even given the voltage drop through a LM317 and DC/DC converter efficiency loss, May actually be possible.

Third amp is a rounding success, got It together (mostly) and listening to it right now .

DC offset on this one is ridiculously low,  ~000.2 mV. I like the opa827's right from the get go. 

As I mentioned in the previous post, I have finished assembling my third set of boards.

I also made a few mods to the first amp I built, the one with the $$$ opa 627/37 combo. Unfortunately even with higher voltage, and a ~16khtz filter on the op amp power rails (installed 1uf caps in front of the 10 ohm resistors) it still suffers from oscillation.

I will continue to work with it, although de-soldering  the op amp board is taking its toll on the main board. Hopefully I can come up with a sollutoin.

As for the OPA-827 amp I just finished, It is not cased up yet, I am using it to play with different power rail caps, but i am listening to it.

It's still very early to start describing the sound, but I do like it and I can see why people liken it to the 627/37 amps. I havent A-B'ed it with the 1641 amp yet, but my initial impression is it is at least as detailed, maybe more, but seems smoother, possibly more "natural". The sound stage and immaging stood out for me as well, lots of "air" around and between the instraments.

So I can see why it is a popular op-amp.

I also did a quick low voltage test with this amp, as the 827's are rated at +/-4volts (8 volts total), and a 3 cell li-po pack will go down to ~7.5v, I powered it by 2 nominally charged cells that added up to 7.5volts. It preformed very well at this voltage, it did start to clip at very high (uncomfortable for me) volume levels, but I was very pleased with the sound at well below the rated min voltage, YMMV. 

Oh, I also built this amp with only a 5x gain, I like it much better. I think 5X-8X gain is probably a good range to stay in, unless one has very hard to drive headphones. The 10x gain is a bit overkill.

Looks like that case would be a good fit for this design. The 14mm is the big issue, the tallest components on the current board are the power caps, followed by the input/output connectors, and POTs. the daughter boards on the new revision wouldn't be an issue, they are shorter, and the input/output connectors could be changed out for shorter-wider units. Power caps could also be changed to suit. 14mm leaves 12mm to work with(pcb is 1.6, and need some room under it) . The issue would be batteries, Would have to come up with cells that were 6-7mm thick, and ~30mm wide and ~50mm long. This would allow for 4 cells (a good thing) but not sure about the amp-hour rating.

I am not going to change the current design to fit that case, but I will order a case to look at for future builds. That said, I think the current board could be put into that case without much work, might have to trim the input/output connectors a tad, they are 12.3mm.

Missed this part of your post, don't worry, a "supersmall, teensy, tiny, itty-bitty, macro,micro, nano" (had enough yet?) is already brewing in the back of my feeble brain, I'm thinking another daughter board for the op-amp power might better utilize space. I start running out of room for POT's and phone connectors though.....

however i want to get the current designs finished up before i tackle another revision.

And, as i said above, I think the upcoming boards will fit that case without much trouble at all.

joe

3rd amp is working now, this is the first one I assembled with the OPA627/37 op-amp combo.

I removed the op-amp board, and replaced said op-amps with 3 AD8610 units.

Amp is very well behaved and is quite stable.

Right now I have it set with .5mA op-amp bias, and 15mA buffer Bias. Amp current draw at idle is ~90ma, this would be a good combo for someone wanting longer run times on a battery pack. 

I haven't gotten it cased up yet, I blew a battery PCB up when I slipped with the iron .

Putting the battery pack together can be nerve racking to say the least. Working with live cells that can explode if shorted or punctured is not always fun.

So, will order another board or two and try again.

Hear are a couple of bitmap images of the boards i am designing, hopefully they will load..

This board is 57X100mm, will work with the hammond case. It will allow for a larger battery. Still working on this one, It will also have a dedicated power switch and charge connector on the rear.

This is the standard board, it is 99% done:

Both of these boards will be on one PCB. Both have provisions for through hole and SMD caps.

Good question.

A fully charged li-po  cell can vary from 4.1 to 4.3v, so figure about 4.2 nominal full charge voltage. So for 3 cells a 12v charger wont quite cut it.

A 15-18 volt would probably be better, but keep it under 20v as the battery management PCB warns against anything over 20 volts if memory serves me. 

I purchased a Mean Well 15v 400ma thinking it would charge the battery in ~ 3 hours, actual charge time is quite a bit more than that so i may need to get a higher capacity charger. Care must be taken though, each cell has a recommended and a max charge rate, it would be unwise to try and charge a cell pack to fast.