Mauro Penasa's INV GC/REF GC group buy.

[BrianDonegan]

Thank you Mauro. That is much easier to look at.

I could easily get the T packages (actually less money), but would need to add in an insulator. I could cut up some Berquist sheet, but that would likely erase any savings. I'll check it out. K10 akpton based pad has worked very well for me with my 4780's as well as MT-200 BJTs.

As for the heatsink, this is from the add:

.375 c/w per 12 inch section or better

.75 c/w per 6 inch section or better

1.5 c/w per 3 inch section or better

So I think a 4" piece will put us in the right ballpark.

[Russ White]

Thanks for the corrections Mauro.

The reason I prefer the TF package for the LM3886 is simply that I usually have my metal cases connected to ground. since the heatsink is directly connected to the case if you do not isolate the T package you will get a nasty short from V- to ground and quickly destroy your chip, and possibly other components (like the PCB itself). Because of this you almost always have to isolate the T package. The addition of the isolator mitigates/eliminates the advantage of the T package over the TF for heat disipation, as the added thermal resistance of the isolator has to be accounted for. All of that, and installation is not as simple. That is why I think for most (if not nearly all) people the TF package is the more logical choice.

As long as you use a good application of thermal grease between the heatsink and the LM3886 the TF package should work out even for high loads with adaquate heatsinking (which is why I want the 4" cut).

Cheers!
Russ

[GaryB]

Brian,
Thanks for all you hard work putting the kit of parts together. Can I suggest one minor adjustment?

I was looking through the BOM and noticed you seemed to be paying a premium for the 0.47ohm 5W resistors. Looking through the Mouser catalog I noticed that there seem to be equivalent Ohmite parts for considerably less money. Did you look at Mouser part number 71-CW5-0.5 ? Or if you do the equivalent at Digikey you can find the following: 0.47W-5-ND or CWB-.51CT-ND or CWD-.50CT-ND
Any reason these wouldn't do equally well?

Thanks,
---Gary

[BrianDonegan]

Quite correct. I did actually look at these. There were two problems when picking which one to use (this was actually the hardest item to pick out).

First, was size. I needed a 5W wirewound that would fit the footprint on the board (a 4w footprint according to the original BOM).

The second was that it needed to be round, to accomidate the inductor wrap. Turns out we don't need this. Think it was too much drugs or something

So, I will be updating this part for a cheaper model, likely an Ohmite rectangular, or maybe the Yageo you mentioned ($0.41 each).

The price of these has always bothered me. I'll have a correction out in a little bit. Thanks!

[BrianDonegan]

Also, I see that we have a few people opting for heatsink. Also, it seems that the better choice would be the 4" version which would result in a more robust amp. Would the people who chose 3" mind if we went for all 4"? I'll contact the supplier and see if he minds all the cutting we would be asking for.

[earsandeyes]

4" is fine with me
I will change the wiki
E&E

[mauropenasa]

hi Brian,
I have the guilt of it has not specified well in the BOM the characteristics of res. 0.47ohm 7(or5)W. If you wants be sure in 100% you suggestion to use a type as what I have used, as Mouser 280-CR5-0.47 5W 5% Cement 22*10mm. This type of res has excellent characteristics for this application. The PCB has a sketch for 4W but have left the spaces about this type.

Excuse my for non timely correction...

Ciao

Mauro

[BrianDonegan]

Actually, perfect timing! I have a lot of work to do today (real job) and was worried I woudn't have to look around. I'll update the BOM and cost this afternoon/evening. Looks like these are $0.31 each.

Thank you very much Mauro.

[Russ White]

Originally posted by BrianDonegan

Thank you very much Mauro.

Indeed! Thank so much Mauro! This is going to be a great GB.

[BrianDonegan]

Just in...

Heatsinks will be $7.07 each. Note that the 4" pieces would weigh about 1.5 pounds each. A consideration for shipping.

This is based on me buying 24" of heatsink and having it cut up (6 pieces total). If there aren't orders for that many, I will take the balance. If there are more, well, we'll see what happens...