jsteiger

Re: gar2520 build thread
« Reply #40 on: July 06, 2009, 10:45:15 PM »
Isn't the potting supposed to act like a radiator, dispersing the heat across a larger surface, allowing it to dissipate faster? Shout me down if I'm wrong... I may be miss-informed.
I don't really know. You may be right. I do know that Scott Liebers doesn't think to much of it. I also have been told that John Hardy had issues with the clear that he used for a while. Was too hard or something. It sure did look cool.

They insulate pipes for opposite reasons though. I know it's not insulation on a DOA but a little illogical none the less.

Maybe John will enlighten us a bit. I'm sure he knows.

Jeff
Visit the CAPI store at http://www.capi-gear.com

CAPI is not affiliated with, endorsed by, or sponsored by API.


Re: gar2520 build thread
« Reply #41 on: July 07, 2009, 12:16:05 AM »
Potting compound serves a number of purposes:

1. For some products, it hides the details of the actual circuit. In some cases, the circuit is unique and the "intellectual property" deserves protection. In other cases, the circuit is just a scam, and the reality of the scam needs to be hidden. The circuit of the 990 was published by Deane Jensen, so I chose to pot the original 990 product in a clear epoxy to hopefully show off the quality of components and layout, etc. It certainly has been a conversation piece. One of my friends saw a clear 990 for the first time, way back when. He stared at it for some time, shortly after smoking a big joint. Finally, he turned it upside down and right side up, then said "It would be great if it snowed inside".

2. The potting compound can provide structure to hold things together. In the 990C, there are way more than 100 holes in a p.c. board that is roughly 1 square inch. Copper traces and pads are very small, so the added strength and structure of the potting compound is important to make it reliable.

3. The potting compound can help to thermally couple parts together. Different compounds have different levels of thermal conductivity, so performance will vary.

4. The potting compound can help to conduct heat away from the components that generate excess heat. Again, the net result will vary depending on the thermal conductivity of the specific compound.

5. The potting compound has a "dielectric constant" that can affect the performance of the op-amp. Two conductors that are separated by a dielectric (insulator) of air will perform differently from those with a dielectric of epoxy, or silicone, or whatever. Different compounds will have different dielectric constants.

6. The potting compound has an expansion rate known as the thermal coefficient of expansion. This is the rate of expansion per degree of temperature increase.

7. Potting compounds have a hardness rating, typically rated in "Shore D" or "Shore A" units.

Generally speaking, potting compounds expand at a faster rate than the components of an op-amp (or any circuit). If the expansion rate of the compound is sufficiently higher than that of the components, and the temperature change is high enough, and the compound is hard enough, the electronic components can be damaged as a result of the build-up of stress. I had a rash of failures in the clear 990 op-amps in early 1981 due to 1N914B diodes being pulled apart on a microscopic basis as the modules heated up. One or more diodes became open-circuit. Of the eleven 1N914B diodes in the original 990 circuit, certain ones becoming open-circuit because of thermal expansion stress would have no effect on the basic performance of the 990. Other 1N914B diodes would cause a catastrophic failure. I changed diode suppliers. I also changed the curing procedure for the epoxy to hopefully reduce the buildup of stress by using a lower curing temperature for a longer time period.

The 990C that I make has an aluminum potting shell. The two output transistors which create most of the heat in a 990 are bonded directly to the wall of the aluminum shell for maximum heat conduction. Aluminum has a much higher thermal conductivity than most epoxies. The heat from the transistors is quickly conducted away from them, distributed evenly around the entire surface of the shell, and dissipated into the surrounding air. The only way to improve the dissipation would be to add fins, making it a much larger package, or providing some type of air circulation. Both are possible, but, within the limits of the package format, the aluminum shell is the best approach.

Epoxies would have to have a large amount of thermally conductive filler material added to them to come anywhere near the thermal conductivity of aluminum. There would be so much filler that the epoxy would be very thick and hard to work with. The dielectric constant would probably be a lot higher. It would also be so hard that it might cause stress fractures in components due to the differences of thermal expansion.

I use a soft silicone encapsulant. It is sufficiently soft that it does not build up a significant amount of stress on the components as things heat up and cool down. The aluminum shell keeps the heat evenly distributed around the entire circuit, and dissipates the heat as effectively as possible. Components that need to be thermally coupled are located next to each other. The even distribution of heat by the aluminum shell keeps the thermally coupled parts at as closely matched a temperature as possible. The silicone has a much higher temperature rating than most epoxies. It is more expensive, and requires special procedures, but I think it is the best way to go for this particular situation. The 990C package is not as fun to look at as the original clear 990 package, but it is better in terms of basic performance.

Trade-offs. Life is full of them.

John Hardy
« Last Edit: July 07, 2009, 01:00:41 AM by John Hardy »

Brolik

Re: gar2520 build thread
« Reply #42 on: July 07, 2009, 12:36:37 AM »
Thank you Mr Hardy. That was one of those posts you could dig a lifetime for.

gemini86

Re: gar2520 build thread
« Reply #43 on: July 07, 2009, 02:37:53 AM »
Browsing this forum...It's like stealing books! Thanks for that in-depth JH!
- Rodney

"...you better call Kenny Loggins, 'cause you're in the danger zone."

Tubemooley

Re: gar2520 build thread
« Reply #44 on: July 07, 2009, 04:37:29 AM »
That sums it all up right there. Nice work John. Here at my dayjob, we have a machine which mixes the two parts of epoxy right there at the dispenser gun tip. My guys go thru a gallon of potting compound each day. We manufacture industrial instrumentation and sensors. There are many, many different formulations of potting compound. John has mentioned most all of the important aspects. The potting compound is wicked stuff. It creeps into every friggin' crevice. It can cause problems you would have never dreamed of. We use products from Epotek. They are located right here in Massachusetts. I know one of the applications experts up there. She knows all their products very well. DW.

Tubemooley

Re: gar2520 build thread
« Reply #45 on: July 07, 2009, 04:54:35 AM »
I just went back and re-read this entire thread. Friggin' awesome. We're all insane. Getting killer audio by doing it ourselves. 15-cent resistors. 50-dollar Cinemag trannie's. A bunch of bench time. Priceless. DW.

martthie_08

Re: gar2520 build thread
« Reply #46 on: July 07, 2009, 06:12:29 PM »
ok, I found some time tonight and finished two gar2520:



they took me about 45 min each to complete, quite packed and fiddly, but the instructions are good. They work and sound great - have to do more A/B tomorrow but first impression is spot on!

Thanks for doing this kit! cheers, Marten

gar381

Re: gar2520 build thread
« Reply #47 on: July 07, 2009, 10:15:10 PM »
martthle_08

Looks REAL GOOD !! :)  Are you SHURE I didn't make those????!! ;D

GARY
RETIRED.....  from 51yrs in Show Biz

gar2520, gar1731,garAM10,gar918 DOAs at  http://capi-gear.com/catalog/index.php?cPath=71_72
NEW Sphere section  http://gar381interests.com/gar381interests.html

gemini86

Re: gar2520 build thread
« Reply #48 on: July 07, 2009, 10:15:47 PM »
I just went back and re-read this entire thread. Friggin' awesome. We're all insane. Getting killer audio by doing it ourselves. 15-cent resistors. 50-dollar Cinemag trannie's. A bunch of bench time. Priceless. DW.

15 cents resistors? Too spendy for my budget... lol.
- Rodney

"...you better call Kenny Loggins, 'cause you're in the danger zone."

Biasrocks

Re: gar2520 build thread
« Reply #49 on: July 24, 2009, 04:40:49 PM »
I've assembled my first 2520 and I'm having a bit of a problem.

R1, R9 the 56R resistors are getting very hot along with Q7.

Q7 tests out okay with no shorts. Somethings drawing a ton of current through those resistors.

I suspect a solder bridge somewhere but can't seem to track it down.

Any clues where I might look.

Update:

Got two up and running successfully. They sound very nice in my SCA A12's, much smoother and richer than the stock opamps.  8)
Took about an hour and 1/2 to put each one together, going slow and checking every solder point under a magnifying glass. I guess I'll ohm a good one out and compare it to the non-working one and hopefully figure out what the problem is.

Mark
« Last Edit: July 25, 2009, 10:59:46 PM by Biasrocks »
http://SharktankPro.com

"I'd rather use an SPX90 than a UA plugin....." Joe Barresi


gar381

Re: gar2520 build thread
« Reply #50 on: July 25, 2009, 02:01:19 PM »
Hi Mark

Sounds like a possible solder bridge to me as well. I would do a direct side by side
Comparison with the one that works under a magnifying glass or better yet a magnifying light.
Compare each hole one at a time with the working one.  Look for solder bridges or component
leads that are bent funny and touching something that they shouldn't.  Look on the component
side for part leads that are touching each other as well.

Glad you like them in your A12s :) :)  I hope you can find your problem with the non-working one.

GARY
RETIRED.....  from 51yrs in Show Biz

gar2520, gar1731,garAM10,gar918 DOAs at  http://capi-gear.com/catalog/index.php?cPath=71_72
NEW Sphere section  http://gar381interests.com/gar381interests.html

flintan

Re: gar2520 build thread
« Reply #51 on: July 26, 2009, 10:51:27 AM »
The Build PDF manual is good but here a few notes that might make the kit a little less confusing.

Stupid question but..where do i find the manual, or a schematic, or partslist? I just ordered the PCB's and want to make shure i have all the needed parts.

Edit: Nevermind, got the e-mail now with all the info..  ::)
« Last Edit: July 26, 2009, 10:58:18 AM by flintan »

plumsolly

Re: gar2520 build thread
« Reply #52 on: July 26, 2009, 06:48:09 PM »
Warning: Apparently, if you accidentally step on these it can damage the pins. Gary, you might include this in the next revision of the build docs, as it caught me off-guard.  ;) Luckily I had some extra pins around. Best, Ben

jsteiger

Re: gar2520 build thread
« Reply #53 on: July 26, 2009, 06:59:27 PM »
Warning: Apparently, if you accidentally step on these it can damage the pins. Gary, you might include this in the next revision of the build docs, as it caught me off-guard.  ;) Luckily I had some extra pins around. Best, Ben
Oh boy Ben!  :o  I hope you had shoes on or it was laying pins down. Ouch! 
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CAPI is not affiliated with, endorsed by, or sponsored by API.

plumsolly

Re: gar2520 build thread
« Reply #54 on: July 27, 2009, 04:11:54 AM »
no shoes but pins down thankfully - got 2 built today but have yet to test them - need some flux cleaner. I'm excited to get some 2503's to finish my 2aca. Best, Ben

Holger

Re: gar2520 build thread
« Reply #55 on: July 31, 2009, 04:03:46 PM »
Received mine today and finished one. It took some  80 min incl. 2 cigarettes to build one   ;)
Thanks for the wonderful documentation.

Re: gar2520 build thread
« Reply #56 on: August 01, 2009, 07:53:28 AM »
Where can I find the documentation for this? I have searched but came up with nothing.

Thanks!

/Dusty C

jsteiger

Re: gar2520 build thread
« Reply #57 on: August 01, 2009, 09:02:13 AM »
Where can I find the documentation for this? I have searched but came up with nothing.

Thanks!

/Dusty C
Dusty, I usually send out a .zip file right after I ship the PCB's or kits. Did I forget to send them to you? If so, shoot me a quick email and I'll take care of it.

Best, Jeff
Visit the CAPI store at http://www.capi-gear.com

CAPI is not affiliated with, endorsed by, or sponsored by API.

Re: gar2520 build thread
« Reply #58 on: August 02, 2009, 06:34:55 AM »
jsteiger: No, You did not forget me. I was just curious about the design and thought I might have missed a build thread or something. But perhaps I'll order a couple when my ship comes in :)

Best Regards
/Dusty C
« Last Edit: August 02, 2009, 07:47:56 AM by Dusty Circuit »

gar381

Re: gar2520 build thread
« Reply #59 on: August 02, 2009, 03:22:30 PM »
Dusty

gar2520s are not expensive and sound VERY NICE !! :)

Give one or two a try.  you will not be disappointed !



(Shameless Plug) ;D

GARY
« Last Edit: December 30, 2009, 07:17:16 PM by gar381 »
RETIRED.....  from 51yrs in Show Biz

gar2520, gar1731,garAM10,gar918 DOAs at  http://capi-gear.com/catalog/index.php?cPath=71_72
NEW Sphere section  http://gar381interests.com/gar381interests.html


 

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