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Group Build: Dynafet


luvdunhill

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I'm liking it just fine with 2 ohm emitter resistors.

Vbe multiplier just fine with 10k resistors and 20k pot.

a resistor in series with the 20k pot is a really good idea.

I'm not using the input bias trimmers. As long as the voltage

at the output of the opamp is less than +/-2V no need to

trim.

working on a custom fully machined heatsink now.

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Kevin: as it is now I've got a solder jumper to ground the middle feedback point (just before the diamond buffers). Under what circumstances is that necessary? Do you think it should be kept as a solder jumper, exposed as a 1x2 pin header, or just removed?

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Kevin: as it is now I've got a solder jumper to ground the middle feedback point (just before the diamond buffers). Under what circumstances is that necessary? Do you think it should be kept as a solder jumper, exposed as a 1x2 pin header, or just removed?

Kevin discussed this in posts #1001 and #1076. I did as well in a few more posts.

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I am finished tinkering now. The board is biasing properly with the parts Kevin mentioned above (10k resistor, 20k pot), and has amazingly stable offset without the servo. Yea it's gonna be a little hotter than the Dynahi. The one FET board transplanted into my amp with only two outputs is a little warmer than 2 Dynahi boards with all 16 outputs.

In the mean time, I will continue to tinker with my own proto board and see if there is anything I can come up with to reduce the bias and make things not so hot.

On the biasing resistor values Marc wanted, I have 480 on the positive rail and 520 on the negative. Marc had 470 and 510 so basically the same thing. Should you choose to leave the board and values as-is, then 620ohm paralleled with a 10k pot will be, to me, perfect. The board Marc sent has 681 and 10k and is very workable and has good range.

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Jacob:

The changes look fantastic! I just collected all my notes and here are the remaining suggestions that I have:

1.) On C1 can you enlarge the holes to 1mm diameter?

2.) In post #884 I suggested a change to the silk for the dual devices. It would be great if you could implement this, as this part of the assembly is very confusing. I'm sure someone will screw this up. I'd also add the 2SJ109, 2SK389, 2SA1349, 2SC3381 nomenclatures on the silk as well, per my the mockup part I included in post #884, as you have plenty of room. Trust me, when people are installing the devices, they will thank you profusly for this change.

3.) Along these lines, can we label the positive and negative power inputs? For some reason I blindly assumed the positive was on the left... Glad I double checked it in the end!

4.) Furthermore, can we add the cap ratings as well to the silk? It's hard to tell without really understanding the layout that the top two caps need to be >=63V caps and the bottom for need to be >=35V caps.. Again, I made this mistake, and I'm glad digger945 pointed it out early :)

5.) For the four bottom caps, I think if you separate them out a bit, you might be able to cram in slightly larger diameter caps. I guess what I'm asking, is it possible to make the bottom four caps 18mm diameter as well?

6.) I'm not sure what your DRC rules are set to, but the output pad seems very close to the FET pads. In looking at the copper, if you moved the output down a bit, you could perhaps get another fenestration to connect from the power planes to the output FETs, and that might be worth it. I'm not sure, but it might be worth looking at.

7.) Would it be possible to go back to the prototype buffer layout where none of the transistors are facing each other? I'm just concerned because these get so hot that the BJTs will start to runaway and the buffer become unstable. I liked the prototype layout where there was two rows and they were all lined up. I think you could even space the two rows apart a little more to help with cooling, and if this is possible, which it looks, I'd definitely try and do this. If you want to face some of the transistors together, I'd recommend the CCS transistors (i.e. the three pair that are not a part of the buffer) as this could potentially help the offset be even more stable. For example, I'm not sure if the BJT facing the LED is the best idea, but who knows. Maybe at minimum you could turn these two to face each other?

8.) Pars recommended a flush socket part in post #886 that may be worth looking into supporting. Looking into it, it's an easy change I believe... along these lines, I'm assuming it was too hard to enlarge the outer pads on the resistor footprint? You mentioned that you might be able to do this in post #893. I'm assuming this wasn't possible? Even the smallest amount of room would be greatly appreciated, as it's a very tight fit and many of us are hoping to use PRP resistors, which due to uneven tolerances will be even tougher to fit than the Dales.

9.) I noticed that all the holes on the top of the board for mounting the FETs and the board itself almost fit a #6 screw... maybe it's a M5 hole? Anyways, could we enlarge these a tiny bit so #6 would fit without "threading" the through-hole plating?

10.) You might want to make the screw on the center pot point to the right of the board, as that way the value will be visible when it is installed and hooked up to the heat sink, since this is a rather important value for determining the bias range. I had the hardest time trying to read this value once it was installed on my heat sink :palm: Obviously people can swap the part themselves, but the silk is a good reminder :)

11.) I'd line up R57-R58 with R7,R8,R10,R12 and line up R1-R4 with R59, if possible. I like resistors in nice neat rows I suppose :)

Finally, what PCB fab are you considering? I'd sort like black solder mask, with gold plated pads, with 2 oz copper :) Depending on where you get the boards made up, these aren't really all that expensive. Check Phoenix Gold for example, who even offers colored solder mask and gold plated finish for free for larger orders, and at a nominal cost for smaller orders. I wasn't so impressed with the silk screen from PCBFabEx, but everything else was top notch... However, I'm not sure that their quantity pricing is very competitive, compared to some of the other places, hence the question... oh, and black boards FTW.

Edited by luvdunhill
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