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Old 24th July 2012, 08:21 PM   #201
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Hi Kaplaars,

One thing to bear in mind (and ignore this part if you know it) is that running the transistors hot actually reduces their capacity, ie the TO3 transistor that dissipates 200W can actually only do so at a temp of 20c, by 100c it is, for example, only a 100W transistor. This is called thermal derating and needs to be considered before choosing final settings.

Safe has a wide variety of definitions, it really depends on your assumptions and expectations. Are you talking about whether the amp survives a short on the output? or whether someone that stumbles and puts a hand on the amp then needs to go to hospital? Or what if there is a persistent very low impedance load (ask how I know ;-) on the output what happens when you continue to drive the amp, increasing the input to compensate for the apparent lack of volume...

I don't think I have a clear understanding of your goals, at one point you make it clear you don't need a lot of class A bias because you don't use that much power, later (IIUC) you are figuring the maximum bias the output stage, mounted on your heatsinks can dissipate before failing.

Stuart
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Old 24th July 2012, 08:27 PM   #202
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The last build I did was just after my daughter was born, so it was important to me that the amps be 'safe' in the domestic sense, ie

As as example of safety choices made in building a monoblock, fan cooled KSA50(75?):
- We used a normal 5A fuse and CL30 on the input to the 625VA transformer.
- We used resetting 85c cut-offs on the heatsinks, to protect against fan failures etc.
- We added basic speaker protection (on/off thump and DC offset detection).

The final temperature of the outside of the amp was determined by how fast we wanted to run the fans...we had the top and bottom plates remachined after we realised the fans were running faster (and noisier) than we liked in order to move the required air to keep the transistors at a reasonable temperature...

We kept the outside of the amplifier cases to ~45c and the heatsinks to 65, transistors were more like 95, IIRC. It took a number of iterations of settings and we didn't finish winding and potting the transformers until we were pretty clear on what the overall dissipation parameters could be (~200w, ~40V, ~2.5A) giving 100W class A / 4ohms.

HTH

Stuart
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Old 24th July 2012, 09:56 PM   #203
spurlte is offline spurlte  United States
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Stuart, I am sorry that I went with the 40v/1000kva transfos. They were on sale for $65 each--12v/1A and 18v/1A secondaries included. I went to the metal scrap yard and found really nice very heavy irregular shaped extruded aluminum where I will cut diagonal slots in the direction of the incoming air. Since this is my next step, my current concern is about making the heat sinks live to the TO3 transistors and is there a way to side step this setup. After understanding this, I think I will fire this baby up--with fire extinguisher not too far away.
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Old 24th July 2012, 10:31 PM   #204
tvrgeek is offline tvrgeek  United States
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So, we are supporting people here who are ripping off the hard work and reputation of legit' companies? Jims does good work. Usually the design is slightly modified. But he is still ripping off Krell, Music Fidelity, and others. It cost these companies a lot of money to develop the products and build their reputation. Heck, I'd love to have a KAS 100. Intellectual property theft is, well just plain theft. I don't care it is a 20 year old design. I want to see Jims stand up on their own. Build a board "inspired by" not ripped off. I know they can. We just have to stop buying stolen designs.
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Old 24th July 2012, 10:32 PM   #205
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Hi Stuart,

Thanks for your reply! Appreciate it :-) Yes, knew that. That is why I got worried because, temperature is pretty high. I had to check and wanted to know if it was OK. Off course, I want, and have to be operating in the SOA area. I am pretty curious to what temperature the original KSA-100 is operating. I almost can't believe it keeps temperature below 70 degrees with 0.625 mV bias.

My goals are fun and to learn But the tecnical goals are a bit variable I want to reach for the stars, but there are a lot of different, unknown, parameters involved. I have discovered most, but some are still unknown. One of this parameters was for example C/W value from the heatsinks. The heatsinks are surplus, without documentation. Therefore I had to determine the C/W-value. Now I have a rough estimate I can adjust bias to it. I think I will try to bias it to a KSA-50, so 442mV across the 1 ohm resistors. The PSU is capable of delivering much more current, but

What I did for safety are:
  • one main fuse for when the main leads go short circuit.
  • 4 secondairy fuses for when outputs go short circuit.
  • speaker protection (DC offset, on / off thump).
  • an other fuse with the primaries of the little transformer which is mounted at the powerboard for the speaker protection.
  • two thermostats (rated 75 degrees)
  • heavy 10 0hm / 10 W (replaced the previous used, lighter ones compairing to previous pictures) circuit breakers, so when there is a short circuit to GND, they don't act as fuse..
  • soft start PCB. Next to delaying power on, it switches the amp on with two heavy relays. This way I dont have to use a heavy switch (which can more easily fail than the relays) but a little momentary switch instead.
One big avantage of the current setup of the tunnel cooler is that it radiates very little heat to the enviroment. So all the heat will flow out of the tunnel instead of into the cabinet of the amplifier. But that has a price, temperature is getting higher at the outtake compaired to the intake of the tunnel.

I've did some further calculations, and yes, made a few mistakes in the post above :-)


My calculation of C/W seems to be a good estimate. When having an ambient temperature of 30 degrees and using 270 mV bias, the temperature would theoretically become: Tambient + C/W * bias * rail voltage * output devices --> 30 + 0.27 * 0.270 * 51 * 8 = 60 degrees @ the heatsinks. I think temperature is even a bit lower. But can hold my hands without a problem to the theatsinks, so it is most certainly not warmer than 60 degrees. So great news, the heatsinks actually do their job pretty well.

Tj would in this situation be: Theatsink + Tr * power dissipated --> 60 + 0,7 * 51 * 0,270 = 69,6 degrees. This is worst case scenario, temperatures are seldom higher than 30 degrees here in Holland and Tr is maximun value. Also I my calculations are without load, so when applying a load, there is a good chance temperature is a few degrees lower at my trannies.

I think I shift my goal to biassing it to KSA-50 bias, which is 444 mV. That would be more than enough, and I don't have to ruin my design So same calculation would give 30 + 0.27 * 0.444 * 51 * 8 = 78,9 degrees on the heatsinks, and Tj would be 78,9 + 0,7 * 51 * 0,444 = 94,75 degrees. I could not find a SOA-curve of the MJ15003/4 to check if they could deliver the current needed when operating at this temperature.

625 mV is too tricky: 30 + 0.27 * 0.625 * 51 * 8 = 98,9 degrees on the heatsinks, and Tj would be 98,9 + 0,7 * 51 * 0,444 = 121.2.

Hope my calculations are right :-)
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Old 24th July 2012, 10:51 PM   #206
Arick is offline Arick  Canada
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Since you are working so hard at calculating the temp, you might want to try this simulator from R-Theta. It is called R-tools.

R-Tools

Have fun,

Eric
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Old 24th July 2012, 10:57 PM   #207
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Haha NO WAY, there is a tool for it!?!? Thanks Eric! I am curious if the simulator gives the same values as I calculated. Will try it
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Old 25th July 2012, 12:46 AM   #208
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Quote:
Kaplaars, you didn't tell me the transformers are big as tug boats
I can tell you for sure your driver heat sink is way too small. Kaplaars is almost large enough. Those drivers run steamy hot when the amp is biased correctly andthey are actually an excellent sounding small amplifier in their own right. When I tested the origonal prototype boards about 5 years ago I found this out really quick...

Last edited by Mark Allen; 25th July 2012 at 12:48 AM. Reason: Posted photo wouldn't work.
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Old 25th July 2012, 01:48 AM   #209
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Ai Mark, I hope that Spurtles heat sinks will do. Mine stay very cool. Total dissipation is about 15 W.

I made by the way a little spreadsheet with Excel. It calculates automaticly if your trannies are operating in SOA You can download it from my ftp: http://michaelq.home.xs4all.nl/Proje...0derating.xlsx

Hope it could help somebody. I found out that even at 625 mV bias, I am still on the safe side, max power handling is still 113 W per device (if my spreadsheet is OK). When operating at 90 degrees Celsius, would that shorten lifespan of the trannies? It feels unnatural when operating at temperatures that high, have no intention to do that. But I am curious. From what I can make up of the datasheets, it should not be a problem.
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Old 25th July 2012, 03:20 AM   #210
spurlte is offline spurlte  United States
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Default heat sinks

Quote:
Originally Posted by Mark Allen View Post
I can tell you for sure your driver heat sink is way too small. Kaplaars is almost large enough. Those drivers run steamy hot when the amp is biased correctly andthey are actually an excellent sounding small amplifier in their own right. When I tested the origonal prototype boards about 5 years ago I found this out really quick...
Which heat sinks are you referring to? MJE15032/3? Klapaars and I had communicated earlier about these heatsinks and agreed the MJEs did not dissipate too much heat and the heatsinks I have will do the job. I plan to have the power supply in it's own case therefore seperating its heat from the amplifier's circuitry. If computers CPUs can use water cooling (I see r-tools.com program considers water cooling), why isn't it possible to cool off TO3s this way? I have seen whole working mother boards submerged in a fish tank with a circulating bath of some special liquid of some sort here in Chicago at a local computer shop I must get a picture of it for reference, but the liquid cooling type I am speaking of is are those attached directly to the computer CPU. What a mouthful.
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