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Old 4th April 2006, 03:53 AM   #1
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Default First amp build, first post...

Iíve been hanging out on the forums for awhile reading up on all kinds of neat projects. I have learned a tremendous amount on here, this is my first post. I am itching to attempt my first amp build. Iím a senior in college and am about to graduate as a mechanical engineer. My soldering skills I feel are up to the task, and I have basic electronic skills. My current amplifier is a Dynaco ST-150. In the future I plan on building some type of speakers that have quite a few drivers in them. That is still in the plans, but I will need something bigger than my Dynaco to power them. Iím not a ďcritical listenerĒ. I like clean sound but also like it loud. Anyway, here are my thoughts on this amp project:

To start with, I was looking at the Hx200 from Amplabs (http://www.ampslab.com/hx200.htm). My plan is to buy two of these so that I will have a stereo amp.

The power supply that Amplabs recommended was a 600 watt unit from Avel (might go with one slightly larger, see below). I was considering running a power supply for each channel but figured it isnít worth it monetarily. Also from Amplabs, I am looking at their 60,000 uF capacitor kit. Heatsink, case, etc. have not been thought about yet.

Overall I expect this project to be around $500 for a 330 wpc amp (I plan to build 4 ohm speakers). Now I ask for input. Has anyone heard these amps? I like the way my Dynaco sounds so if they sound the same or better then I would be satisfied. Thanks.

Vic
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Old 4th April 2006, 09:12 AM   #2
ilimzn is offline ilimzn  Croatia
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200W RMS, let alone 330W RMS out of two pairs of MOSFETs? That's stretching it 'a bit'...
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Old 4th April 2006, 02:03 PM   #3
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....does not look funny....
SOA limit of the MosFets is already touched with ideal 4Ohm load and assumed heat sink temperature of 25įC...
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Old 4th April 2006, 03:45 PM   #4
CBS240 is offline CBS240  United States
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...toss 40-60 degrees onto that 4 Ohms and 'PoP... fzzzzzz'
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Old 4th April 2006, 10:48 PM   #5
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Default Some like it hot

Hi Vic,
The previous comments about heat are quite right. I think you may benefit from a little more explanation.
The rule of thumb is that whatever the average speaker power (incorrectly, but popularly called RMS power) the amplifier dissipates half of it. The best efficiency of a class A/B is 70%. If you want 330W each channel will need to be able to dissipate some 165W. Since each channel has 4 mosfets each will need to dissipate about 40W.

Now, it is all about temperature of the silicon. The IRFP240 can handle a silicon (or junction) temperature Tj of 150C max. Any more and they expire - very quickly. You can calculate the temperature from a simple formula similar to ohms law. Replace current with power, resistance with thermal resistance and the voltage change is the temperature change.

If you use a really big heatsink like the one shown on the Amplabs website it will probably have a heatsink to air thermal resistance of about 0.5 C/W. Each transistor will have about 1.5 C/W between silicon and heatsink (when the insulator is in place). So the heatsink needs to get rid of 165W and its temperature will therefore be 165 x 0.5 = 83C above air temperature, provided the heatsink is mounted with fins vertical and free air flow below and above. Let's say your room is never more than 25C, the heatsink temperature will reach about 110C. Each transistor dissipates 40W so its silicon temperature will be 40 x 1.5 = 60C above the heatsink temperature. So Tj for each of the 4 transistors will reach 170C.

So, if you run these amps continuously at rated power with the heatsinks shown the transistors will exceed their Tj max and will fail and may take your speakers with them. Furthemore, the lifespan of transistors is reduced considerably when they run near their max temperature. You shopuld really plan to keep Tj below 100C if you can.

There are two main ways to achieve this. 1) Keep the average dissipation of each amp less than 90W or 180W average speaker power, or 2) Use a special fan or liquid cooled heatsink to keep the heatsink temperature no higher than 40C. Or some combination of the two.

This advice may seem like overkill and might be depending upon how close to the maximum average power output you'll be using and for how long. However, I would advise you to take this seriously if you want to have a reliable system.

You can find the IRFP240 datasheet via Google.
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Old 5th April 2006, 04:26 AM   #6
CBS240 is offline CBS240  United States
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Simple disign rules like these based on real numbers are the backbone of DIY because a lot of people don't have a big stash of cash and have to make do with what they have, which my not be excactly like the design recomendations, but must be DIY enhanced.
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Old 5th April 2006, 04:51 AM   #7
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Have a look at my amplifiers.
Much better than Ampslab.
Uses the same devices but more of them and a very proven design Hundreds of them sold over 12 years.

The NX150 would be a very safe design for you to start with.
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Old 5th April 2006, 09:35 AM   #8
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Greg's amplifier kits would be a better value too.

Eight of the same outputs and only USD $232 for a pair. Will use a lower voltage transformer and smaller heatsinks for the same output power due to the different design.

Click the image to open in full size.

http://members.dodo.com.au/~gregball/guru_006.htm
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Old 6th April 2006, 08:44 AM   #9
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That was a great reply Traderbam. I really appreciate how far into detail you went.

I had not thought of the thermal aspects of this a whole lot. From what you are saying, neglecting the heat added from the transistors (ie an amp that has an infinite number of transistors) the maximum power output of an amp would be 300 watts in a room that is 25C to not get the Tj over 100C (.5C/W * 300w / 2 + 25C = 100C). My question to this is how do companies produce the monster amps, such as some of the big monoblocks, that produce much more than 300 watts?

The Saint, how much is your NX150 complete kit unassembled?

djk, Gregís kits look interesting too but where do you see the price for a pair of them for $232? I see the kit being $320 + $10pp.


Thanks for the help so far guys!

Vic
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Old 6th April 2006, 09:09 AM   #10
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Hi Vic

The NX150 for a pair of kits unassembled will cost $190.00 Australian
Plus $20.00 postage and Handling.
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File Type: jpg nx150_1.jpg (40.4 KB, 488 views)
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