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Old 24th May 2003, 05:10 PM   #1
sobazz is offline sobazz  Denmark
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Default P3A considerations

I need an amp for a first time speaker project. So far the P3A has been chosen. It seems to be suitable for the speakers and possibly to build for a newbie.

The amp must be compact and able to output around 30-35w continously into a 8 ohm load. This translates into a 25v rail voltage.

Given these criteria I decided to build to seperate mono amps. I should be able to stuff a proper transformer (toroidal 100-120VA) and the amp section into this nice case:

Click the image to open in full size.

I guess the heat dissipation from the output transistors is rather moderate at this voltage. Will I be able to use the cases as heatsinks by simply mounting the transistors directly on the sides?

I've been looking at different toroidal transformers, but haven't yet decided. It must rated 100-120VA and have a diameter of no more than 90mm. Any suggestions? The transformer must be available in Europe.

Does anyone in here use the above case and/or know the internal dimensions?

Thanks


Regards
Soeren B.
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Old 24th May 2003, 05:15 PM   #2
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Where does the case come from, and do manufacturer/supplier the not supply dimensions for it? Ayway, it looks nice, but I'm not sure about finding a transformer of that rating that will fit in, have you had a look at RS, or Farnel yet?
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Old 24th May 2003, 05:38 PM   #3
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Hi, I had a look about on RS, and although I couldn't find a toroidal model with would fit in the 90mm space with suficient power rating, I did find a 150VA open frame model which measured only 100 X 82 X 82 mm which should fit (provided the case is tall enough). anyway, hope that is of some help, and if you could still et me know where thcase is from I would be very greatful.
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Old 24th May 2003, 05:45 PM   #4
sam9 is offline sam9  United States
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Just guessing, but based on my experience building a modest power P3A, --- anything big enough to holdthe PCB and transformer, etc will probably be ok for a heatsink -- provided it is fairly heavy aluminum. The picture you show is encouraging since it has fins and looks like a casting. Somewhere on the Rod Elliot website, he has some formulas to calculate thermal resistance using the surface area. You might look at those - you would have to do a lot a measuring and some arithmetic but nother exotic.
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Old 25th May 2003, 08:41 AM   #5
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Hi!

I used this case (one size smaller) for my GainClone (which now lies unused upon my carpet in front of my P3A ), they can really provide some cooling. But one drwaback is that there are no holes in the case to make air ventilation possible, so you will suffer from heat buildup.

I don't know though, if you can find a suitable transformer. The toroids from RS Components are probably not usable, since they waste space for being potted...

Maybe use a "standard" transformer, not a toroid? Maybe out of an old disfunct amp?

Try it... I can not rely on such a case as a heatsink for my P3A... 42 V rails, 6 ohms load, MJ15003 / 4 transistors...


Bye,

Arndt
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Old 25th May 2003, 03:28 PM   #6
sobazz is offline sobazz  Denmark
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The case is sold at Conrad.com (data sheet here). No internal dimensions are given. Generally this is one of those data sheets that do not serve a purpose.

Farnell sells a toroidal transformer from Avel Lindberg, which is rated 100VA and has a diameter of 91mm. I cannot find this trafo on AL's website. Instead they have a product range called Y23, which is even more compact (100VA: diameter 85mm; 120VA: diameter 89mm). I should be able to fit into those cases. But who sells this Y23 series?

Cradle22, how serious is this heat buildup? Will it be a problem? Should I be able to use a 22vac type (corresponding to approx. 30vdc per rail) giving 50w? Remember, there's only one channel in each case. How powerfull is your GC?
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Old 25th May 2003, 04:29 PM   #7
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Hi, I know this is a bit of an indirect way of getting the internal dimensions fo the case, but the case is designed to hold eurocards, therefor, it must have an internal width of 100mm. So from this, you can tell that each wall has a thinkness of 5mm, and the internal dimensions of the model you are looking at must be:

162 X 100 X 70 mm (the ends are only 1.5mm thick).

Hope this helps you out a bit, as it should mean you will be able to use larger transformers.
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Old 25th May 2003, 09:14 PM   #8
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Hi!

Internation relationships have their benefits...

I have the conrad catalogue (and my GC case) lying in front of me, and the one you depicted on your picture must be the biggest one available, which (external measurements) is 165 * 110 * 105 mm big (length * width * heigth), this means 163 * 100 * 95 mm internal measurements (judging from my own, smaller case, but the thickness of the material doesn't vary).

I would say that using one case for one side would be OK (I thought that you meant implementing both channels into one case including transformer, whcih would have meant pushing your luck...)

Using a case, and a single trabsformer for each channel should be possible. I mounted both OPA548 in the smallest version of those cases, with external transformer, and I run them at 35 V rails (25 V transformer). I did not mount them to the sides, but to the top of the case, and did not try anything else. With my setup, the case only gets hot when pushed really hard into my 6 ohms speakers. I would say 30 V rails, with one case per channel, at normal listening levels (that means not constant PA action...), at loads > 4 ohms you are on the safe side. Just use standard mica washers with heatsink grease (I used Arctic Siver III from computer supplies, don't know if that makes such a difference).

You can also try to press the transistors (which ones to you inted to use) to the case (with a c-clamp) prior to drilling, and therefor try finding the best place to mount the transistors.

And please tell me the outcome of your experiments! I really like those cases (first saw them on the "GeenKlone"-project), and the smallest one is rather empty with my stereo GC inside (about 35 % space used)...

Bye,

Arndt
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Old 25th May 2003, 10:40 PM   #9
paulb is offline paulb  Canada
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Quote:
Originally posted by Cradle22
I used this case (one size smaller) for my GainClone (which now lies unused upon my carpet in front of my P3A ),
Heh heh heh.
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Old 25th May 2003, 11:52 PM   #10
JoeBob is offline JoeBob  Canada
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Heh heh heh, indeed. Is the trend dying?
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