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Old 19th November 2008, 03:55 PM   #1101
amp_guy is offline amp_guy  United States
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try the enermax enlobal computer fans , they have magnetic bearings . they are almost dead silent.
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Old 19th November 2008, 07:04 PM   #1102
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I agree there are no absolute rules regards how hot things get and I certainly followed the conservatice approach.

This also makes excelent reading as well and goes into the various factors to be considered http://sound.westhost.com/heatsinks.htm

The aim ultimately is maintaing a good operating temperature for the Mosfets (OK stating the obvious here!). But what is the maximim or optimal operating temp for the mosfets we are using in the Mini-A? As per the article the temp range the transistor operates at varies device to device and the effect of the heat varies also. It has been said somewhere in this forum that the mosfets (in the Mini-A at least) operate better at lower temps and this seems to be born out by the article.

I also note in the article the effect of the type of insulator used and how the transistor is mounted makes a large difference. It reads that typical sil-pad insulators have a thermal resistance around 1C/watt thus if you are running at 25watt/device there is a 25C differential between heatsink and fet (eg my sinks are running at 40C at the moment and my fets are at 65C). So if you use thin mica insulators rather than sil-pad your heats sinks can effectively run 10-15C hotter for the same mosfet temp.

I have loked at the datasheet for the IRF240 and fail to understand the operating temp limits. If anybody understands this I would dearly like know the best operating range for these devices.

Cheers
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Old 19th November 2008, 07:20 PM   #1103
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Let it run hot, but have spare transistors ready for ten years down the road. I think if you're wondering if it might be too hot, then it's probably OK (although something one has to learn to accept compared with other cooler-running amps).

To me, hanging a fan on >afterward< seems an admission of defeat or an indicator that the builder didn't know what they were doing. A fan should be designed-in from the beginning using a different style of heat sink (like a chimney or tunnel) and take advantage of the possible space savings and styling options this opens up (like NP's A75, for example.)
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Old 19th November 2008, 07:45 PM   #1104
amp_guy is offline amp_guy  United States
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Quote:
Originally posted by Tosh
Let it run hot, but have spare transistors ready for ten years down the road. I think if you're wondering if it might be too hot, then it's probably OK (although something one has to learn to accept compared with other cooler-running amps).

To me, hanging a fan on >afterward< seems an admission of defeat or an indicator that the builder didn't know what they were doing. A fan should be designed-in from the beginning using a different style of heat sink (like a chimney or tunnel) and take advantage of the possible space savings and styling options this opens up (like NP's A75, for example.)
exactly but my hindsight is allways my clearest vision.
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Old 19th November 2008, 08:13 PM   #1105
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hah!

Quote:
To me, hanging a fan on >afterward< seems an admission of defeat or an indicator that the builder didn't know what they were doing.

you got that right


Fran
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Old 19th November 2008, 08:19 PM   #1106
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http://www.enjoythemusic.com/magazine/manufacture/0808/

Quote:
At Pass Labs the bias is set to the value which raises the heat sinks 25 – 30 degrees C. above ambient temperature. The result is a heat sink which you can put your hand on for about 10 seconds or so.
And that is for a commercial product where long life and low maintenance is almost as important as sonic performance. It is probably worth noting that the IRF-240 (these are fairly typical) has a maximum junction temperature of 150degC.

If you read the Zenv6 article Nelson talks about 4A bias and 40V rails giving stunning performance, but his heatsinks not coping with the thermal load. I'd suggest this is the key - keeping the heatsink temperature to a level which you decide is acceptable at the bias current and rail voltage which you want to run the amp.

If it sounds good and the heatsinks aren't getting much higher than 60degC then that appears to be a nice compromise.

I'm using a pair of Aleph 30 monoblocks built on the brian gt boards. They were running at 52degC in a 22degC room last night. Unfortunately the hot summers mean it's not realistic to bump the bias or rails higher than the present levels.
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Old 19th November 2008, 08:19 PM   #1107
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Default the last amp you'll ever build?

Sometimes you just build with whatever you have on hand to see how it sounds. I don't see anything wrong with using a fan after the fact. After all, heatsinks don't grow on trees (yet)
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Old 19th November 2008, 09:45 PM   #1108
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ok:

voltage across the resistors is an average of .375V (range 0.360 to 0.395). divided by the resistance is 1.97 which to me is fairly close to 2A - voltage is 12V so each mosfet is putting out 24W, or 48W per side.

Do those figures add up?

This is the detail on the heatsinks:

* HEAT SINK, 150MM, 0.75°C/W
* Thermal resistance:0.75°C/W
* Mounting type:Adhesive Mount
* Length:150mm
* Height, external:25mm
* Width, external:200mm
* Depth, external:150mm
* Length / Height, external:25mm
* Material:Aluminium
* Surface finish:Black Anodised
* Fixing style:Glue bond



What do you guys think? the consensus seems to be that cooler would be better, but I should get away with it. The actual heat off the amp isn't an issue, just whether I am doing harm running it like this. FWIW, I think I have the IRFP140N (red stickers from group buy)


Fran
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Old 19th November 2008, 09:54 PM   #1109
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Thanks Spzzzzkt. I saw the max junction temp in the datasheet but I was unsure how that would relate to the actual temp you would read on the IRF240 body.

Looks like I have been rather conservative with my heatsinks but I had 6 of them anyway and ended up using 4. I think clearly that 6 would not have worked in the A75 project I bought them for years ago.

On a related subject I have a question around rail voltage and bias current. Having read that 25-30watts/mosfet is a good range to aim for I ended up with 23v and 1.12amps. Question is - is it better to have higer voltage rail or higher bias current in achieving the 25-30 watts? Are there different advantages to each assuming an 8 ohm load?
I kinda understand that higher bias current relates to how much class A the amp runs - am I right and if so what is the relationship betweem the bias current and number of watts in class A?

Cheers
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Old 19th November 2008, 10:24 PM   #1110
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Quote:
The aim ultimately is maintaing a good operating temperature for the Mosfets (OK stating the obvious here!). But what is the maximim or optimal operating temp for the mosfets we are using in the Mini-A? As per the article the temp range the transistor operates at varies device to device and the effect of the heat varies also. It has been said somewhere in this forum that the mosfets (in the Mini-A at least) operate better at lower temps and this seems to be born out by the article.

In regards to Pass designs you'll do better reading what Nelson has to say. Rod Elliot says of the Zen amps "Having looked at the original and many of the "improvements" currently on the web, I did a few tests of my own and frankly, found the amp lacking in the fidelity department. Hi-Fi this most certainly is not. " Not exactly a person who's advice I'd look to on optimal operating conditions for a Pass design

Have a look at the Pass article I linked to in my last post. NP explains very clearly that it is bias current which is the most important factor in Class A operation:

Quote:
Higher bias doesn't just move the Class A transition to higher ground – it has a profound influence on the amplifier at all power levels. It lowers the distortion at low levels as well as high levels, as seen in the distortion versus power curves for an amplifier with the bias set at different levels.

In Fig 3 we see the distortion of an output stage operated without feedback driving 8 ohms from 0.10 watts up to 20 watts. The top curve with the highest distortion has a bias of 0.016 amps. The next lower is 0.08A, followed by 0.16A, 0.32A, 0.64A, 1.28A, and the lowest distortion curve at 2.56 amps. What we see clearly is that higher bias lowers the distortion at all power levels, and that the distortion is inversely proportional to the bias current.
Given that I=V/R raising rail voltages will increase the bias. Once you have your rails set it's usually easier to swap resistors.

Heat sink temperatures are a limiting factor not a determinate of performance in themselves. The more heat dissipation you build into the amp the higher the bias current you can run and the lower distortion you'll get.


cheers
Paul
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