A few PIMETA build questions (New: troubleshooting questions)

i'm putting together a BOM for my first PIMETA with a TREAD power supply, and i want to make sure i order the right parts and connect everything properly, especially since this will be my first experience with building a separate power supply. as such, i have a few questions for those who have experience with these two projects.


1) per other members' recommendations, i've decided to use single LMH6321 buffers for R/L/G in lieu of single/stacked BUF634s. based on all the info i can find in the forum archives, it seems that all i need to do to adapt the 6321 to be BUF634 pin-compatible is to omit R11 and clip pin 1. is this correct? i think i read something about adding an input resistor as well, but i can't remember where i read it.

2) will this adapter work well with the TREAD board, regulated down to 24V to supply the PIMETA? should i use a similar 24V adapter instead? will the heatsink recommended by tangent work with this configuration?

3) i'd like to use a Hammond enclosure rather than the recommended Serpac H-65/67. i want a case that will accomodate both the TREAD and PIMETA boards. any recommendations from people who have done the same thing?

4) tangent suggests placing a diode in-line with one of the power connections, rather than in the crowbar configuration, if the PIMETA will be used exclusively with wall power. he says that this allows the use of a much smaller diode. how much smaller are we talking? i picked out this one from the same line of Vishay Schottkys as the crowbar diode he recommends - is it appropriate, or would a different type/value work better?

5) what is the big advantage of the ALPS RK27 over the 097 or the Pana EVJ? just better tracking? smoother movement? since this will be a non-portable PIMETA, i'm not concerned about the space it occupies. in everyone's opinion,is it worth the extra $12-13?

6) if anyone wants to just take a quick look at my BOM and see if there are any glaring mistakes, i'd be very appreciative.


thanks for any help you can give!

interesting - this is from a thread entitled "Tweak Output Stage of PIMETA":
there seems to be some discrepancy as to which pin(s) must be clipped/bent for proper function of the LMH6321. any idea what the deal is with that? it sounds like there may be reasons to clip both pins 1 AND 2.

as far as the gain, i'm glad you mentioned that - i took the stock resistor values from the schematics and completely forgot to tweak the gain. i'll probably build the amp with a gain of 5 or 6, and leave room for tweaking by installing the resistors in a DIP socket.

the input caps i was planning on ordering just to have them around, in case i need them for some particularly high-offset source in the future.

thanks for the advice, and for all of the opamp suggestions. i do plan on rolling a bunch of chips through the PIMETA, but i figure i'll give my wallet a breather in between the purchase of the main parts and the purchase of all the other candidate opamps and tweak parts. the 8620/8610 combo sounds like one of the better performers, so i figure it's a good place to start.

EDIT: if i'm reading the data sheet and other people's posts right, it looks as though pins 1 (error flag), 2 (current limit), and 5 (gnd) can and should all be clipped when using the LMH6321 in place of a BUF634.

thanks for the input. i've switched from Schottky to 1N4001 - i guess in the (fairly unlikely) event of reverse voltage, i'll suck it up and pop in a new diode.

as far as the LMH6321 goes, has anyone had success using it as a buffer with pin 2 (current limiting) connected? it sounds like ATAT recommends against it for his PPAS boards, but i'm not entirely sure what the effect of connecting it would be.

also, any input on an ideal enclosure for PIMETA+TREAD, a good unregulated adapter for the TREAD@24V, or opinions re: RK27?

hmm, good to know... i didn't realize they sold pre-mounted 8620s and 8610s. however, i think i might circumvent the brown dogs entirely and just use Sijosae's SO-8 to DIP-8 technique (bending the end pins outward and soldering directly into a DIP socket). $0.20 for a DIP socket sure beats $2.39 for a brown dog adapter, and i don't see any reason why the two would behave differently from one another, assuming i can manage to bend the pins to the appropriate angles without snapping them off.

ok, another noobish question:

i've just realized that i don't know precisely how to mount my PCB(s) inside the case that i've ordered for my PIMETA. all the DIY projects i've done to this point haven't required me to mount the PCB to the bottom of the case. what's the best way to do it?

i see that hammond sells plastic, self-adhesive standoffs that can just be stuck to the bottom of the case. do those work well?

EDIT: another random one - anyone know of a nice panel-mount LED holder that will "frame" it on the front panel and hold it in place?

so you just used the tube washers as standoffs, drilled holes in the bottom of the case, and threw some nuts on the outside ends? these specifics are what i was hoping to get at. i guess i'd like to avoid drilling through the case if possible, but it sounds like that's probably the best and most secure option, right?

unfortunately, i believe the PIMETA board was designed with the Serpac H6x cases in mind, and there doesn't appear to be a Hammond case that fits the width of the board exactly. of course, i'm also mounting a TREAD within the same case, so that's kind of a moot point.

another quick question: i've received my LMH6321s from Digi-Key, and they're packaged in a bag that says the contents are both static- AND moisture sensitive. anyone know why these buffers are moisture sensitive, and whether this should be a worry during/after assembly of my PIMETA?