[H22]

Well, I was going to wait till I had this fully assembled and tested, but I changed my mind....
 
Here are a couple of pics of an amp I am building, Circuit boards came today and I started putting it together.
 
Most of the SMD parts are populated, the pic showing most of the through hole stuff was done while i was test fitting everything.
 
thought I would post a bit of a challenge, see if someone could guess what design this amp is based on.
 
the only clue I will give is that it is based on a DIY amp, i just wanted to make a smaller version. Those are nichicon "fine gold" caps, however I will not use those for the first build, I have some silmic II caps that will be used. The smaller board is one of two that will be mounted upright, and the whole thing will be mounted (more like squeezed) in a slim hammond case with a li-po battery. The board size is, overall, smaller than a credit card, about 1/8 inch wider and 3/4 inch shorter in length. The whole amp will be a little bigger than a mini3, but with a lot more packed into the case. Lets just hope it sounds as good as its bigger brother......

 
 
 

 

[Avro_Arrow]

Looks like a mini PPA

 

[H22]

Yep, you nailed it

 

[H22]

So far the build is going ok, I have populated and installed the opamp and buffer daughter boards. I also put the rest of the opamp power circuits together. So far so good, everything tests ok and I was able to adjust the opamp bias to 1ma.

 

[H22]

Well, Avro_arrow has a keen eye. This little project started because I wanted to make a PIMETA style amp that was a little smaller and had would fit in a nicer aluminum case. I have built 2 of them, both are great little amps, but I didn't particularly like the "standard" plastic case, and I wanted to see if I could try to modify the design a bit to get a better sound.
 
I also have a PPA v2 that I built about a year ago, and love it. The PIMETA is a great little amp, but cant hold a candle to the PPAv2. Of course, My PPAv2 has really nice components and uses an o11 for a PS, but the topology is fairly similar between the two. Both use an op amp voltage gain stage with a multi-loop feedback to a buffer stage. One of the big differences between the two is the fact that the PPAv2 has isolated power rails for the op amps. One of the first things I noticed about the PPA was the dramatic improvement it channel separation over any other amp I have heard, even the PIMETA. After looking at the schems, and seeing how similar they were, I thought it would be interesting if I could add the op amp isolation stage to the PIMETA design and see if it made a difference. However, once I started laying out the board, I decided to see if I could just use the PPA buffer stage as well.
Turns out that with a bit of SMD and ingenuity, the PPA buffer stage didn't take up much more room than the PIMETA buffer stage. So then the scope of the project changed, and got quite a bit more challenging. I decided that I wanted to build a portable amp that was as close as I could get to my desktop PPA, and compromise as little as possible.
 
What I ended up doing was replacing all of the resistors with 0805 SMD, except the large output resistors. All of the TO92 stuff was replaced with SOT23, except the virtual ground TLE devices. I did have to compromise on the main power rail caps a bit, but as can be seen by the photos, they occupy almost half of the board space. My desktop PPA has ~2000uf of capacitance using elna silmic II caps. This design will allow for 1000uf with silmic II's, or up to 1800uf with nichicon "fine gold" caps.  Other than that though, this little board is as close as i could make it to the original PPAv2 design, so hopefully it will preform very similar to my desktop unit.
 
Opamp rolling will not be easy with this design, as they are SOIC devices and are all 3 mounted to a "daughter board", but i plan to try a few different boards for fun anyway. Right now I am using the same opamps as are in the full sized unit, OPA637 for the L&R and OPA627 for ground.
 
Originally I had planned on a 14.8 volt 860ma li-po battery to power it, however I cannot get those cells anymore, so for now it will be powered by a 11.1volt 900ma pack, the voltage is not up where i would like it, but the 3 cell (11.1v) pack allows me to use a case that is 20mm shorter, so it is a much more portable amp. The 11.1v 900ma pack should be able to provide around 8 hours of life for the amp with nominal bias settings.

 

[H22]

Doesn't appear there is a whole lot of interest in this little project

, but i am slowly making headway

.
 
I ordered the wrong output resistors, they were the correct value, but the lead size on the resistors are too big to fit in the holes on the board

so I had to solder them on top of the holes, a pain but only temporary till i get the correct ones.
 
Once that was done, I finished putting it together and tried to set the buffer bias, this turned into my first problem to debug as the right channel was basically shorting across the 2.2 ohm output resistors. I was able to find a bad solder joint on that channels biasing circuit and was able to sneak the soldering iron in and fix it.
 
Next issue that came up was really bad op-amp oscillation on power up. If I touch the board on the right spot(s) they will settle down and the DC offset voltage at the output is very low (~6-12mv), and the amp sounds very very good, at least until the op-amp(s) start oscillating again and I have to fiddle with the board to get it to stop again.
 
So I  took a look at the board layout, the schems, the op-amp data sheet, and the parts recommendation guide and came up with a few reasons this could be happening;
 
I don't have an ideal board layout for the op-amps, its not terrible, but not ideal. to make the board small enough I couldn't get the op-amp input resistors, or the decoupling caps (C5)  as close as I would have liked to the requisite pins on the op-amp. They still quite close, and using the original PPAv2 board as a guide, total trace length is not that much different. However, I should have paid more attention to the filled V-ground plain on the board, it may be too close to the input resistors on the op-amp side. I also note that R8 and C5 are designed to form a low pass filter for the op-amp power, I got tunnel vision on this point, I read the parts selection guide where it stated that .1uf was the smallest to do any good, so I designed it for that sized cap because I could get a wima polypropylene  to fit nicely, but I glazed over the fact that doing so would raise the frequency of the intended filter by a factor of 10 if I left R8 at 10 ohms. So now, if I want to fix it I have to either go with a lesser quality polyester or (egad) ceramic cap, or replace R8 with a 100 ohm resistor. Not sure I like either of these, but i think changing the cap might be the lesser of the two evils.
 
My choice of op-amps was not ideal either (tunnel vision again), I wanted to make this amp as close to my original PPAv2 as possible, so i spent $$$ on OPA627/37 units, however, these amps minimum voltage level is around +/-4.5v, so absolute minimum voltage at the power rails is 9v. With the rail isolation circuit it seems to be that minimum battery voltage is closer to 9.5 volts. Nominal voltage on a 3 cell li-po pack is 11.1, however minimum voltage is anywhere from ~7.5 to 9 volts, so I cannot take full advantage of this battery with these op-amps. The oscillation problem gets worse as voltage drops, it is still there even with a fully charged (12.6-12.9v) battery pack, but the amp gets more and more unstable as the pack drains.
 
So, I think changing the op-amps is the first order of business, I will try the AD8610 to start with as that is the recommended amp, and I think i might even have some. I also want to try some others that are more forgiving of lower voltages, but that will be down the road a bit. I am hoping that the oscillation issues go away with a better suited op-amp, and as it seems to be more stable at higher voltages I'm hoping this will be the case. After seeing it assembled, There appears to be room enough on the backside of the op-amp board to add some chip style decoupling caps, not sure this would help but worth a try on a future board spin (if there is one).
 
This morning I started a road trip for work, so I thought I would do a long term test. As per normal, as soon as I turned it on it was oscillating like mad, If I touch the bottom of the board at the input and output connectors (that seems to be the best place) it stops and I started listening. I was driving all day so I was able to get a good idea for how it sounded, thankfully the op-amp oscillation only popped up a hand-full of times and was usually easily fixed buy just touching the board near the op-amp connection or at the input/output connectors. I was using my Iphone 5 for the input, and my M50's on the output side  of things.
 
The good news is it sounded great, it seemed to have all the things that I loved about my full sized PPA, great power and separation, extremely great detail everywhere, and oh yea, lots of power

.
I was also pleased to note that even though i had the buffers biased to 20ma each, the board never got very warm. I have been worried since i started this project that with such a small layout and everything being so close, that heat would be a big issue, however now I am fairly certain that it wont be.
 
I was also able to get 6 hours of play time through it, although the last 45 min or so I had to trim the volume down a tad (just a tad) to keep the op-amps happy, then it started clipping and oscillating and generally being a PITA, so I shut it off and measured 11.01 volts on the battery pack, hmmm, that should still be enough,....but guess not.
 
I am open to ideas or suggestions, I am still quite a bit of an amateur at these things, if anyone has any thoughts other than mine on these op-amps. I will be happy to post a jpeg of the circuit board layout, as well as provide the Express PCB files if anyone is interested in looking at them. The schematic is basically identical to the PPAv2, the only thing I did was add connectors to mount the op-amps and most of the buffer circuit to daughter cards, and I changed pin outs where needed to  use SOT 23 components.
The resistors values are all the same as the original PPAv2 schem, I used 1000uf for the power rail caps (10x100), .1uf and .01uf wima PP for C2, 100uf (2x50) for C4 and .1uf for C5.
 
Oh, and for Q3 i used 512-PN4391 (50ma), my thought was these would be good for just about any op-amp, but i might have been better off with 4392's in that spot.
 
joe

 

[i_djoel2000]

Dude, your project is really interesting!
 
looking forward to the finished prototype in that nice hammond casing 

 
btw, you got PM

 

[H22]

Project has stalled a bit with the holidays, I have de-soldered the opamp daughter board, and just ordered a few other opamps that will hopefully work better.

I also ordered enough components to assemble another unit, this will give me 2 to play with and AB test.

Next step will probably be another board spin, I have already started on that, I re designed the opamp daughter board to include the bypass caps, cleaned up the layout quite a bit, and actually was able to skim about 1mm off the width of the board ( this is huge). No guarantee the revised layout will work any better, but on paper it should. Hopefully I can make the current board work, if it won't then I will try the new one, if that doesn't work I will seriously start considering scrapping the whole project. I hope I will not have to do that!

 

[H22]

Ok, another update, I am still waiting on the new opamps, I ordered 3 sets, all jfet type, with voltage ratings more suited to this application. I scrounged up a few 1uf SMT film caps from a former project, and installed theon the back side of the board. Kind of a cobbled together fix but it seems to have helped a lot with the oscillation issue. I still think that to use the 637/27 opamp combo, i will need to add a cell to the battery pack.
 
The new board design looks a lot better than the current design, cleaner layout, revised power and ground planes, I am waiting on parts to show up before i order the new board. I am considering putting some RF chokes in the feedback loop, (alla Mini3), any thought on this? I am planning on putting them just before the headphone output connector, I have room to either tobmstone them, or to mount them normally, the tombstoning would make for a tidier board.
 
Listening to it right now, and have to say it is sounding very promissing, very open and clear. immaging is great, the detail is excellent, and i think is getting better with use.
 

 
Joe

 

[FallenAngel]

Try to stabilize with PPA parts without output restrictions like resistors or RF chokes to see if you can make it run normally. Board design is a major part, just try to make it stable.
 
So far, it looks pretty awesome, just keep at it!

 

[ppl]

Change the OPA 637 to OPA 627 note if stability returns if so you can continue to use the 627 or redo board layout. The pop upon start up is typical of the 627/637 devices . Also higher gain setting and larger value Op Amp feedback resistor , say maybe 1.5 megs. Also the value of resistors going to the buffer inputs along with layout contribute to stability..
Good luck with this project, It is great to see interest in my old designs.

 

[H22]

Wow, thanks guys, I appreciate the advice.

 
Quote:

Change the OPA 637 to OPA 627 note if stability returns if so you can continue to use the 627 or redo board layout

 
I thought about this, I would need to order a couple more 627's.
 
 
Quote:

Also higher gain setting and larger value Op Amp feedback resistor , say maybe 1.5 megs. Also the value of resistors going to the buffer inputs along with layout contribute to stability..
Good luck with this project, It is great to see interest in my old designs.

It has the 10x gain, actually a seems a little high for my current headphones. R6 is 499k, changing it to 1.5 meg sounds like a great idea, i_djoel2000 also recommended adding the Pimeta caps (4.7-10pf c6) in parallel with R6, that might also help, just not sure if it will degrade the overall design.
 
I have made a little more progress with this amp, I added a cell to the battery pack to bring the voltage up to 16 volts, this seemed to have the most dramatic effect on the amp. Now it will oscillate once it is turned on, however if i turn the volume knob up a little it settles down and will stay stable thereafter. It sounds simply stunning, I have the buffer bias at 30ma and the opamp bias set at 1.5ma.
 
I will try some of these fixes, I also have quite a few new op-amps to try. I am very excited about this, now that i am making progress and it is sounding so good, i really don't mind the trial and error so much.
 
Oh, now that it is stable i was able to get a good reading on the output offset voltage, its about 2-3mv.

 

[ppl]

The OPA637 is not unity gain stable,Thus you can not use a cap around the feedback loop or the Op-Amp. This trick will work with the OPA627 because it is unity gain stable. The cap puts the opamp into unity gain at high frequencies.

 

[H22]

Quote:

The OPA637 is not unity gain stable,Thus you can not use a cap around the feedback loop or the Op-Amp. This trick will work with the OPA627 because it is unity gain stable. The cap puts the opamp into unity gain at high frequencies.

Ah, that makes sense, I will try a larger 1.5 meg resistor for the op-amp feedback, I could also change the resistors going to the buffers, right now they are 1k, Should I go with a higher or lower value resistor there?

 

[H22]

I didn't have any 1.5 meg resistors, i did have some 1 meg, so i put those in the r6 position (replacing the 470k units) didnt seem to help much, the power on thump does seem to be better, however it will still oscillate until the volume knob is turned up about 25%, at witch point it calms down and stays that way.