PPA last call
Jul 17, 2003 at 3:34 PM Post #16 of 69
Quote:

Originally posted by tangent
Which part of the screw is 3mm? The shank, or the thread diameter, or the desired hole size, or...?

Have you tried these screws in the META42 board's crossfeed mounting holes? These are the same size holes as we're currently using on the PPA board. The crossfeed mounting holes have been on the META42 since November and I have heard no complaints about it so far.


The shank is 3mm in diameter. I can't get them to fit in a META v 2.1 board. The standard hole size is 3,2 mm (exactly 1/8" or 125 mils) for an M3 screw to fit through. Enlarging the holes would be a nice thing (for the imperial-unit challenged
tongue.gif
) If this is impossible for some reason, there _is_ an M2,5 (diameter 2,5 mm) but they are much harder to get hold of.

/U.
 
Jul 17, 2003 at 5:05 PM Post #17 of 69
Quote:

is would be sort of cool if the cross feed board had the ability to plug into the PPA like a video card would into slots in the PC.


That wouldn't be much better electrically than using pins, but it does solve some of the "where to put the board?" issues. The resistors could go towards the connector edge, and the caps would protrude from the board above this, high enough up that they would float over other components.

It's probably too late to do something so drastic, though. Maybe for version 2?

Quote:

That would sure clean up a lot of wire!


No, it only removes 6 short wires. You still need 12 long wires to go to the front panel and back, for the switching setup. You can't run those as traces across the board for the same reason you can't run the bass boost traces to the front of the board.

Quote:

The cap pads - I don't really have suggestions,


We don't do that much on the PPA because we have some very specific ideas of what type of parts go in which positions, and currently have had no reason to wish for other parts in those positions. The only place I can think of where you have multiple sets of pads is for C6, where you could use either a silver mica (big) or a ceramic (very small). If you can come up with a similar argument for the other positions, we'll consider adding pads. Just adding pads because we can is the wrong reason.

Quote:

Enlarging the holes would be a nice thing


I guess we can do this. It makes #4 screws a little sloppy in the hole, but I imagine a lot of times you're going to be countersinking this hole anyway to allow the use of flat-head screws so the hole is really just there to be a starting point for the countersink bit.

How wide are the heads of flat-head 3mm screws?
 
Jul 17, 2003 at 5:47 PM Post #18 of 69
Neophyte, I actually wanted to use SIP headers to interface with the daughterboard much as you just suggested, but the idea was not very popular with the rest of Team PPA, and we had trouble finding a source for SIP pins of sufficient length, so we settled on the standoff holes. Good thinking, though! As Tangent says, perhaps in v2.

Squalish, I appreciate your interest and effort, but I'm not going to add holes for caps that don't exist or are inappropriate to use. The multiple holes for the input coupling caps on the META42 serve their purpose because there is such a wide variety of caps that can be used in that position. On the PPA the electrolytics can't be made bigger because of their proximity to other components, and pointless to make smaller. There are plenty of good choices ranging from 1000uF Panasonic FC to 220uF Elna Silmic or Cerafine. The PCM5 200 mil standard film cap spacing can be populated by Wima, BC, etc. C3 can be either leaded or SMT 1210. C6G can be 100, 200, or 300 mil lead spacing. If there are specific caps you want to see holes for, let me know.

Regarding the hole diameter issue, I will ask machinist binary_digit to do some measurements and fit tests with #4 and 3mm screws in 110 and 125 mil holes. Kurt said he would check this out as well, but he does not get home until late this evening.

If the #4 screw is snug or tight in a 110 mil hole and fits comfortably in a 125 mil hole, than changing the hole size is a no brainer. If it is wobbly loose in a 125 mil hole then I might be more tempted to suggest 3mm screw users ream the hole wider, but I'll bet this is not the case.

Nisbeth, you mentioned the 2.5mm screw, and I thought of that too, but the real question is: Which screw size mates with European standoffs?
 
Jul 17, 2003 at 6:04 PM Post #19 of 69
the holes on the META take a 2.5mm (M2.5) screw quite happily but are too small for 3mm screws... this was a problem as 2.5mm screws only come up to about 20mm in length, whereas 3mm screws can be purchased in much bigger lengths...

edit: the M3 flat heads i've got here have a head diameter of 6mm... they're the ones from Rapid Electronics so most European DIYers should be able to get them

g
 
Jul 17, 2003 at 8:43 PM Post #21 of 69
Quote:

I actually wanted to use SIP headers to interface with the daughterboard much as you just suggested, but the idea was not very popular with the rest of Team PPA


He didn't suggest using SIP headers. As I understand it, he suggested a vertical-mounting slot of some sort. This fixes the physical strength issues I brought up, and because the board would be vertical the physical compatibility issues would be lessened. I envision a board with the resistors right above the gold fingers, the switch pads above that, and the caps above that. This puts the caps high enough up that they're floating above the 600 mil level, and everything below that is small stuff.

The problem with this idea is that I would have to make a radically different crossfeed board, adding gold fingers and probably making it wider than high to fit in most case arrangements. It's just too late to talk about doing something so drastic. I also wonder if it would be worth the cost: slot connectors and gold fingers can't be inexpensive additions.
 
Jul 17, 2003 at 9:08 PM Post #22 of 69
some edge connectors are real cheap, but most of them are quite large:

36 way £.48 from Rapid

however, RS had a 6 way but that was £3(?!)

as for the fingers, are they not just extensions of the traces on the PCB? or are they properly gold flashed??

g
 
Jul 17, 2003 at 9:12 PM Post #23 of 69
just a thought, how about an IDC type connector? the crossfeed board (or any other daughterboard for that matter) could be mounted on screws, and then an IDC ribbon from the daughterboard to the PPA itself? Only problem would be the quality of the IDC interconnect, but if it's good enough for high MHz signalling in noisy PCs, it should be fine for low noise audio transmission (don't shoot me now
biggrin.gif
)

g
 
Jul 17, 2003 at 9:27 PM Post #24 of 69
Quote:

are they not just extensions of the traces on the PCB? or are they properly gold flashed??


You can do it as you suggest, but solder doesn't make a good contact material, either from a hardness or corrosion perspective. It's much better to do gold plating over copper, something that only some board houses can do. Also, you have to route the board edges so that it fits neatly in the slot, and ensure that the solder mask doesn't cover the fingers. It's a lot of work and expense to avoid adding 6 wires.

Quote:

Only problem would be the quality of the IDC interconnect, but if it's good enough for high MHz signalling in noisy PCs, it should be fine for low noise audio transmission


They're only trying to recover ones and zeros from those signal lines, and they have a span of several volts between valid signal levels. Audio is in about the same ballpark peak-to-peak, but you want a -90dB or better noise floor and 50dB channel separation. That means your lowest signal level is 1/30,000 that of your highest level, and adjacent signals cannot be allowed to effect changes greater than 1 part in 300 in each other. Do worse than either of these two rules, and it's no good. And, if you can't exceed these two specs, you don't have much call for labeling the amp "high end".
 
Jul 18, 2003 at 12:38 AM Post #26 of 69
Quote:

Originally posted by tangent
No, it only removes 6 short wires. You still need 12 long wires to go to the front panel and back, for the switching setup. You can't run those as traces across the board for the same reason you can't run the bass boost traces to the front of the board.


But you can run the traces to the front, remember?
wink.gif


Seriously, though, put the crossfeed card at the front of the board and put in holes for a boardmount rotary switch...

Damn, and I thought the PPA wasn't supposed to include everything up to and including the kitchen sink...
 
Jul 18, 2003 at 11:38 PM Post #28 of 69
Tangent wants Cross-Feed. Myself i have not heard an X-feed i like. thay all introduce tone colorations that wipe out any inprovement in imaging Crossfeed provides. However aos's Daughter Board DAC is interesting.
 
Jul 19, 2003 at 2:56 PM Post #30 of 69
That would be sweet. Would it be possible for HDCD support? Or is that just too much?
 
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