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Old 1st December 2004, 06:42 AM   #101
Jennice is offline Jennice  Denmark
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Terry,

Don't worry about having one heat sink (side?) smaller than the other. With 200W per channel, you'll need all the heat sinking you can accomodate.

If it's rated 200 W /ch, and you'll use it for music, I would probably count on 100W sinking ability per channel. Yes, class B is inefficient, but then again, most of the time you won't uss more than 20% of the power available if you want good head-room for music transients. Short transients will be handled by the heat capacity of the heat sink material. Long term power will be handled by the heat sink thermal rating.

Anyway... considering that there will be thermal impedance in the transistor casings and mounting insulation (don't forget that part!) I would not recomment a heat sink temperature above 50deg. Celcius. Given a 25C room temperature, you'll need to sink 100W with a 25degree temperature rise. Thus, your heat sinks should have at least 0.25K/W rating or better (meaning: lower value) per channel.
This will easily fill the entire side of your amp. (and yes, they're a convenient way of making the sides of the enclosure).

In conclusion, your 2- or 3ch. considerations may be decided simply by the heat sinking capability of your enclosure, and not by the fear of complexity of the PSU.

If the amp will be placed in a location with good air circulation at all sides, you may consider using either the front or rear of the enclosure as location for the 3rd channel heat sink area.

On another note:
I'm glad if you find my notes useful. I think it's the first time (I'm aware of) that the document has come to use as a whole. I doubt that I have time at the moment to change it, but let me know if you come across serious errors or things that need improvement.

Jennice
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Old 1st December 2004, 11:24 AM   #102
mAJORD is offline mAJORD  Australia
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Hey Terry,

Ive just finsihed building 2 p101 boards for my amp prject, im runing them monoblock with 2 transformers like so:

Click the image to open in full size.

if anyone can comment on that as a layout , would be GREATLY appreciated, (mostly in ref to keeping the 0.1r resistors with only 4 caps)




Terry, if ur having any probs and need me to compare anything with my boards to yours etc (voltagesmeasurements, etc ) , pls dont hesitate to ask, my boards are up and running on my test (single) power supply , you shouldnt have any dramas tho.

mine are the 2 FEt versions at the moment due to budget, ill be adding the extra 2 fets later on.

do you have Rev a boards that require the mod? , make sure u dont forget it if so


goodluck with all.
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Old 1st December 2004, 11:51 AM   #103
Jennice is offline Jennice  Denmark
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mAJORD,

Are you using 3 caps per rail or per transformer ?
If you're using 4 per transformer, I wouldn't suggest the resistor. (unless you're using massive caps).

Regarding the fuse, I would consider to move it before the switch.
This way, swich arching accidents and possibe failure will be less dramatic.

Also, have you considered a soft start circuit? It'll be easier on all your PSU components, and you can rate the fuse nearer it's intended value, and still prevent it from blowing when turning the PSU on.

Jennice
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Old 1st December 2004, 05:04 PM   #104
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Hi mAJORD,

Thanks for the reply and the offer. I will be taking you up on that.

I wasn't planning on using two trannies unless I use all three PCBs. Rod Elliot seemed to think one was sufficient for stereo. O showed somewhere earlier in this thread how he suggested I hook up two trannies for the three channel amp. To go mono block I would need three trannies to go three channel. I don't want to get that aggressive yet.

I do have the Revision A boards. They came with the traces already cut and I just had to add the jumpers.

I went ahead and ordered all twelve MOSFETs from Tech-DIY. Cost me $102 including shipping. They were all matched.

May I ask what rail voltages you are using with your trannies?

Thanks

Hi Jens,

I have built a soft start circuit for my mp, but have been told I may not need it due to my trannies being IE as opposed to Toroid. What do you think?

As far as the heat sinks go, if I'm reading correctly, with the Conrad heatsinks I will need a 12" X 4" X 2" heatsink for each PCB. I had better stick to stereo if that is the case.I don't think I want to have a much bigger case than that. I don't really like the idea of running a third heatsink across the front or back. I was hoping to mount two boards on one side and one on the other. I didn't realize I would need that much heatsing for three channels.

Blessings, TErry
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Old 1st December 2004, 06:04 PM   #105
Jennice is offline Jennice  Denmark
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Hi Terry,

Could you give a name/type of the Conrad heat sink you have in mind?

As for the soft-start, I would surely use it.

Unless your transformers are much smaler than I recall, your switch, your capacitors & bridge-diodes, as well as your mains fuse will have a tough life ( = shorter life).
Your mains switch will be easy/cheap to replace, but the caps won't.

How much capacity did you plan on using?

Jennice
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Old 1st December 2004, 06:40 PM   #106
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Hi Jennice,

To get to the rating you suggest I would need the MF30-151.5. You can see it here.

Looks like they will cost $50 each.

My transformers are 64 volt rails @ 400mV.

I plan to use These caps for this amp. I had panned to use 6 or 8 of them but am open for suggestions.

Thanks, Terry
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Old 1st December 2004, 06:58 PM   #107
Jennice is offline Jennice  Denmark
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Hi Terry,

I don't understand this one:
>>My transformers are 64 volt rails @ 400mV.<<

anyway, with any transformer suited for a 2 x 200W amplifier, I would definitely use a softstart circuit. Especially with the kind of capacity you'll be using. I would use at least 4 caps per rail (8 in total).

As for the heat sinks: Yes, they're outrageously expensive and make me feel like a in the wrong business.
The ones in my previous amp project (the one I use for playing music these days) are 420mm wide, 200 mm high and 42mm deep, and rated 0.25 K/W. Either mine are rated conservatively, or yours are optimistic, but I agree on your choice. They look huge, but make excellent sides of your amp. With right-angle aluminum profiles, you'll be making yourself a sturdy amp frame/enclosure, with reasonable overall cost.

Jennice
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Old 1st December 2004, 07:06 PM   #108
Jennice is offline Jennice  Denmark
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Terry,

Have you seen this section on their heat sink calculations?
http://www.conradheatsinks.com/technical_details.html

This may explain why your smaller heat sink is rated better than mine!
The problem with their specification is that you do absolutely not want a 80 deg. Celcius heat sink temperature in such a project!
Thus, your heat sinking ability will be reduced somewhat, but you should still be able to get a decent result. Heat sinking and capacitors are two things that one can never get too much of.

Jennice
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Old 2nd December 2004, 12:58 AM   #109
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Terry,

Just another note about Conrad Heatsinks (and no I don't work for them) $50 seems to be alot of money but when you factor in how much it is going to cost you to build the sides anyway, and believe me when I say the finish of these heatsinks is nice enough that it would be a shame not to use them as the sides. They are very high quality and I wouldn't be affraid to shell out twice as much for them. Hopefully they don't read this I have to buy seven of them.

I haven't completly read the notes Jen put on here, buy the way nice work, thanks, but looking at the construction notes Rod has, he suggests a heatsink with a minimum of 0.6deg C/W, with this I was going to go with the MF20-75 here http://www.conradheatsinks.com/produ...350.html#MF25. I think I better go and read Jen's notes some more.
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Old 2nd December 2004, 04:30 AM   #110
mAJORD is offline mAJORD  Australia
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Terry:

MY Transformers are 300Va each, 40-0-40 , giving + - 56v rails, (approx) , thisis the max voltage for a 2 fet version.


Jennice:

4 caps per transformer, (note ive chopped off 2 of the 6 in the orignal single tranny version)

so i figure thats 2 per RAIL, they are 4000uF each, 70v 4.6amp ripple rating.
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