M³ Project Announcement - Page 20

In retrospect, I don't think my experience with my passive is applicable and I probably should have kept my mouth shut. As long as you're using a switch with good contacts, you should be alright. Adding a resistor will lower the sensitivity of the phones and make the noise less audible, but then you're just asking for more power from the amp, less headroom, more heat, and an extra resistor to muck up the sound.

- Eric

I don't remember it either, but I agree that morsel is hot stuff!

Just a couple of thoughts on the layout thus far. It's looking pretty nice so far, I think.

Regarding the TLE, I prefer the rear location, removing the front location. I also would think it would be desireable to utilize the DIP-8 TLE, the cost is negligible, and there can be performance improvements. I'm not so sure that those improvements could be notable in this application, but if the cost is essentially the same either way, why not have the option.

As for the bass boost, I'm okay with either method of connecting the right channel. I'm more inclined to favor the wire, though. If you don't want boost, then you don't have the crossed traces in place.

Main power caps: again, either way works... but I do think that you have more options with 18mm caps. Either many smaller caps or two-three big caps should offer similar performance. You can make the case shallower with many smaller caps, presuming the heatsinks are not taller than the 18mm caps anyway. If the minimal heatsink spec is 40mm or taller, then might as well stick with the 18mm caps.

Regarding the gain, I'm glad to hear that you are trying to reduce it. If I had to pick one universal gain for any and all apps, it would be more like 6 or 6.5. I use 5 to 7 most often.

As for the C5 caps. I definitely prefer a pair per channel. I think you have plenty of room for them, also.

Finally, although you don't seem to have a maximum board size target or chassis in mind, it seems like a large amount of extra space to the right, especially considering that there are potential issues elsewhere (bass boost routing). I'd like to see one of two things-

Either keep the big board, and add the pads in the rear for the Dact attenuator, keeping the Alps up front...

or move both existing pots to the left by about half their width (heat from the sink should not be an issue, it's close already); move the front right mounting hole to where the version nomenclature is; drop the traces to the rear and have pads for some star-quad (including the shield) right at the back of the pot; move Q5+/- Q6+/- C4 group to the front a little bit, affording more clearance for a mounting hole at the right rear; slide the power cap bank a bit towards the front affording more clearance for the left rear mounting hole.
This should allow you to take the width down to about 5" (maybe), or certainly 5.25", and you could also shave off 0.125" to 0.25" from the rear without too much trouble.

It seems somewhat frivilous to have such a wide board only to accomodate traces to a front-mounted pot... you still need to cut and solder leads to run to the board either way. I have tried both locations on the PPA, and they both work very well if you are careful with your wiring. Don't run a long straight wire to the pot, hugging a power supply along the way, for example.

But of course the extra board cost from having it as it is would not be huge, and I'm not complaining, mind you. It would offer some further flexibility in fitting the amp and/or psu to a chassis.

Nice work, morsel and amb! Take your time and do it whichever way works the best.

Hi folks, what morsel posted above are two alternatives of the M³ pcb layout for your comment.

The first one is 6"W x 7"L and has the input pads at the right (which corresponds to the rear of the case). The input traces then run the length of the board under a ground plane and "protected" by ground guard traces to the volume control pot. This layout has the advantage that the length of hand-wiring from the RCA input jack to the board is minimized, and the board would provide optimum traces to the volume pot.

The second one is 5"W x 7"L and includes some of Voodoochile's suggestion. It dispenses with the full length input traces on the board. The input pads are adjacent to the volume pot, which means that the hand-wiring from the RCA input jacks must be routed to this location. To minimize crosstalk and interference we recommend separate shielded coaxial wiring per channel be used here. The upside to this layout is the reduction of the board width by one full inch, which lowers the board cost a bit and potentially makes case selection easier. Note that there is less room over where the M³ logo is, and this is where the headphone output jack would normally be situated. If two jacks are desired and the LED is to be mounted here too, the space will be tight unless a wider case is used. Also note that the entire amplifier block has been moved up by 50 mils so that the big 18mm capacitors at the top are now closer to the edge of the board.

We would like to hear your opinion about these two layouts and which one you prefer. We are going to base the first engineering prototype on one of these so speak up now or forever hold your peace .

i do like the fact the second one gives more options for enclosures. is it possible to not use or just jumper across the bass boost pot ?

Please have large holes for bass-boost pots!

I generally like to have bass-boost just to counteract the low frequency roll-off in the 'phones frequency response. This requires relatively large capacitors, to the point where I need to run wire to them since they don't fit on the board.

Would it be possible to ask for some extra pads with larger holes within the capacitor footprint to allow running wires? On the PPA I had to drill out the holes to run my wires.

The difference is one inch. So the minimum width for the case is 6.25" for the first one vs. 5.25" for the second to allow for the enclosure wall thickness and a hair of tolerance.
Yes, of course. The bass boost is entirely optional. You can jumper around it, use the pot for continuous adjustment, or use a switch for simple on/off.

How big are the caps you plan to use? Note that our layouts have six holes for each of the Cbb capacitor so you could fit a variety of different sized cap in there.

thats cool. ive just been offered a 6.5" wide extruded case thats why i vote smaller

If the inside dimension of the case is a bit over 6" wide x 7" deep then both designs would fit. The case will have to be tall enough to clear the heat sinks and the tall capacitors. The minimum heat sink we spec'ed out is 1.5" tall, and you must add board thickness plus standoff height and give a little extra for breathing room (for a total minimum of 2" H). Also, the enclosure should have ventilation slots because this amp will run warm.

Edit: BTW, we are targetting a Par Metal series 20 case, in a custom size; either 5.5"x8"x2" (WxLxH) or 6.5"x8"x2" depending on which design we go with. You are of course free to choose other cases as long as it all fits.

oh im fine on everything except ventilation. ah well may as well use it for ppa 2.0 and just get a nice 1ru rack enclosure for mmm

Lookin good! I can't wait to build this now. I wanna use a black beauty pot for bassboost. Mmmmmm stepped bass boost.

JV.

That was fast. Looks pretty nice to me. As for the mention of having the HP jack where the logo and revision nomenclature is located, consider that you could also shift the volume pads back to the right a bit, with the wiring pads behind it, and then place the mounting hole for the PCB in between the two pots, under the HP jack. The you could have two options for placement of the jack within the scope of the board's width.

As for wiring from RCAs to the PCB, small coax would work; I think that star quad would also work well. With the star star quad, I'd use two center conductors to wire the + and - signals from one channel, and solder the shield only to the RCA end's - tab. As for coax, Canare GS6 is great stuff, and nice and limber. The usual video options like L5CFB are great also, but pretty stiff.

Just to reaffirm, I didn't have problems with the long side traces, and would not mind spending an extra ~$4 on a bigger board, but it just seemed like overkill when you have to run wires from the RCAs to the PCB already. I could see running the traces if you wanted board-mounted RCA blocks for inputs, but thankfully that is not the case! Until someone makes a heavy-duty RCA block, that is a cheesy route for certain.

Another tiny tweak- how about wiggling the three C5+ caps a whisker towards the rear, just so the clearance from the C5+ caps to the OPAs equals the clearance from the C5- caps to the C6 caps? Or vice-versa.