[theory_87]

Was wondering how is the project coming. Any update?

 

[error401]

Just a quick status update:

I just finished building up my prototype (finally!) and it seems to work almost as advertised! There is a pretty annoying bug - the pot is wired in reverse so when you turn the amp on it blasts your ears out. I also think the gain is too high and will probably reduce the gain for the recommended configuration.

I used AD825 and AD8066 as suggested by FallenAngel and in the past 5 minutes I've been listening to it I really like the sound. I haven't populated the class-A bias CCS yet as that'll take a bit of time to tune properly. I don't expect any problem with it though.

I'll try to get some photos of my prototype and that parts placement diagram I keep promising up this week. Prototype boards should get in the mail in the next day or two.

 

[theory_87]

Quote:

Originally Posted by error401 /img/forum/go_quote.gif Just a quick status update: I just finished building up my prototype (finally!) and it seems to work almost as advertised! There is a pretty annoying bug - the pot is wired in reverse so when you turn the amp on it blasts your ears out. I also think the gain is too high and will probably reduce the gain for the recommended configuration. I used AD825 and AD8066 as suggested by FallenAngel and in the past 5 minutes I've been listening to it I really like the sound. I haven't populated the class-A bias CCS yet as that'll take a bit of time to tune properly. I don't expect any problem with it though. I'll try to get some photos of my prototype and that parts placement diagram I keep promising up this week. Prototype boards should get in the mail in the next day or two.

Very nice to hear that. Can't wait for the group buy

 

[pinkfloyd4ever]

YYEEAAAHHH...guess I need to actually get off my @ss and order those parts then, huh?

 

[Coreyk78]

Just found this thread, looks like an interesting project. Subscribing and looking forward to a group buy.

 

[pinkfloyd4ever]

what's a good, small 3.5mm stereo jack that's available from Digikey? All the mini jacks I've used in the past are bigger than I want in this amp.

 

[error401]

My favourite is CP-43502PM-ND, but maybe you think that's a bit large? You could use one of the many board-mount styles and just tack-solder to the pins. It's a bit of a pain, but they have smaller mounting hardware and maybe that's better for you.

 

[pinkfloyd4ever]

any conclusion as to what value we should use for the R7s yet? Or should I still order several values from 2K-9K? What opamp/buffer combos are the rest of you prototypers using so we don't have a bunch of the same configs?

 

[error401]

JFETs have fairly wide specifications on the Idss parameter that largely controls the size of the resistor. It seems that they just can't be easily manufactured to tight tolerances on this. You'll pretty much have to tune the resistor value for each individual JFET, even in the same batch they will vary fairly widely. Part of what I wanted out of the prototype was to get a sampling of resistor values and currents in the field to see how I might be able to tighten up the recommendation. It's currently a guess based a bit on the spec/theory and a bit on simulation results.

You should aim for around 1mA through them I figure. If you can get the CRDs specified in the original PPAS BOM, use those instead and jumper the R7 positions.

 

[linuxworks]

I didn't see this until recently.

I'm in on testing or GB or whatever. looks cool

 

[pinkfloyd4ever]

ooo mouser still has 2447 of em. Damn they're kinda expensive compared to the MMBF jfets. $1.83 apiece*7=$12.81...oh well, you'd think I'd be used to it by now. Why doesn't mouser ship to canada? Want me to get you some error? Do I need to do something special to use these or are they a drop-in replacement? The datasheet says to "short lead 2 to lead 3 via circuit board trace." And as for the R7 positions, I've never jumpered SMT resistor pads, I guess just use a tiny length of wire?

 

[error401]

Only substitute the class A bias JFETs! The CRDs are unnecessary in the power supply ones, so Q1 Q2 Q5. Use SST502. Mouser does ship to Canada, it's just a bit more expensive and slower than DK. I find for small orders DK is still cheaper, but when the order is bigger Mouser's lower per-unit prices win out (if they actually have what you need, DKs selection of ICs is much better).

Right now R7 sits between lead 2 and 3 of those JFETs, so you need to short that position if you use the CRDs. Just use a lead clipping or something, the only tricky part is that it's pretty much impossible not to melt the solder on both pads at a time, so be prepared for that.

 

[pinkfloyd4ever]

ahh crap see what happens when you let people like me who don't know what they're doing on the prototype team

. Prob didn't help that I've been pretty sick all week and def shoulda been asleep when I posted that...oh well. Good news for my wallet though.

 

[error401]

Tonight I got around to populating the CCSs on my prototype, so that marks the completion of the first build on this new board.

First some notes:

1. Overall construction was fairly easy. The board is dense, but there's enough room to work if you leave the TH parts till last. The hardest parts are the SOT-23 JFETs, but they're not too bad.
2. The JFETs I received (ON Semi MMBF4392s) were closely matched, and worked out to almost exactly 0.5mA (the target) with the first R7 value I tried - 6.34K - in all positions. Perfect! No rework! Start with this value and measure the voltage across it, then use Ohm's law to calculate the current. Swap resistors as necessary to close in on the target (~0.25-1mA). It's a log curve though, so don't try to make a linear approximation of the value you need to swap in. I'd like to hear what people's experiences are with this; if everyone's getting around the same current I'll just specify a value and be done with it.
3. As noted above, the volume control is wired in reverse on the prototypes. This is fixed in the artwork, so any production boards won't have the defect. The solution will make the wires a tight fit in the Hammond case, but it shouldn't be a problem. For those building prototypes, this can be fixed (albeit painfully) by cutting all the traces to the pot endpoints and rewiring them. I made some images: Front Back
4. The gain of 10 in the default BOM seems much too high to me, but it may be largely due to the reversed pot wiring. For low impedance phones, change R4 for a lower gain.
5. The LED current with the recommended 10K resistor seemed much too low at my 18V power supply. I swapped it for a 3.32K and it's still quite dim with the LED I used.
6. The amp is well behaved; there is very little turn on/off thump and no DC offset, and no hint of oscillation that I can detect.

Some pictures:

Front:

Proto boards are in the mail, a placement guide is on the way as soon as I have the time. Unfortunately Altium doesn't seem to have a way to easily unhide all the silks and print them, so it'll take a bit more time than I was hoping.

Oh, and I've updated the BOM a bit. You can find it here.

 

[theory_87]

nice to see that it working. Can't wait to built one